[ovs-dev,v6] backtrace: Extend the backtrace functionality

Use the backtrace functions that is provided by libc,
this allows us to get backtrace that is independent of
the current memory map of the process. Which in turn can
be used for debugging/tracing purpose. The backtrace
is not 100% accurate due to various optimizations, most
notably "-fomit-frame-pointer" and LTO. This might result
that the line in source file doesn't correspond to the
real line. However, it should be able to pinpoint at least
the function where the backtrace was called.

The usage for SIGSEGV is determined during compilation
based on available libraries. Libunwind has higher priority
if both methods are available to keep the compatibility with
current behavior.

The backtrace is not marked as signal safe however the backtrace
manual page gives more detailed explanation why it might be the
case [0]. Load the "libgcc" or equivalent in advance within the
"fatal_signal_init" which should ensure that subsequent calls
to backtrace* do not call malloc and are signal safe.

The typical backtrace will look similar to the one below:
/lib64/libopenvswitch-3.1.so.0(backtrace_capture+0x1e) [0x7fc5db298dfe]
/lib64/libopenvswitch-3.1.so.0(log_backtrace_at+0x57) [0x7fc5db2999e7]
/lib64/libovsdb-3.1.so.0(ovsdb_txn_complete+0x7b) [0x7fc5db56247b]
/lib64/libovsdb-3.1.so.0(ovsdb_txn_propose_commit_block+0x8d) [0x7fc5db563a8d]
ovsdb-server(+0xa661) [0x562cfce2e661]
ovsdb-server(+0x7e39) [0x562cfce2be39]
/lib64/libc.so.6(+0x27b4a) [0x7fc5db048b4a]
/lib64/libc.so.6(__libc_start_main+0x8b) [0x7fc5db048c0b]
ovsdb-server(+0x8c35) [0x562cfce2cc35]

backtrace.h elaborates on how to effectively get the line
information associated with the addressed presented in the
backtrace.

[0]
backtrace() and backtrace_symbols_fd() don't call malloc()
explicitly, but they are part of libgcc, which gets loaded
dynamically when first used.  Dynamic loading usually triggers
a call to malloc(3).  If you need certain calls to these two
functions to not allocate memory (in signal handlers, for
example), you need to make sure libgcc is loaded beforehand

Reported-at: https://bugzilla.redhat.com/2177760
Signed-off-by: Ales Musil <amusil@redhat.com>
---
v2: Extend the current use of libunwind rather than replacing it.
v3: Allow vlog_fd to be also 0.
    Return the backtrace log from monitor in updated form.
    Return use of the "vlog_direct_write_to_log_file_unsafe".
v4: Rebase on top of current master.
    Address comments from Ilya:
    Address the coding style issues.
    Make sure that the backrace received by monitor doesn't
    have more than maximum allowed frames.
    Change the backtrace_format to accept delimiter.
    Remove unnecessary checks in the tests.
v5: Rebase on top of current master.
    Address missed comment from v3:
    Reaname the vlog_fd to vlog_get_log_file_fd_unsafe to reflect that
    this is really unsafe.
    Return the ovsdb backtrace to correct position in log and
    keep the previous format.
v6: Rebase on top of current master.
    Adrress comments from Ilya:
    Some missed style comments.
    Fix the compilation failure on Windows.
    Fix BSD test failure.
---
 include/openvswitch/vlog.h |   3 +
 lib/backtrace.c            | 128 ++++++++++++++++++++++++++-----------
 lib/backtrace.h            |  58 ++++++++++-------
 lib/fatal-signal.c         |  53 +++++++++++++--
 lib/ovsdb-error.c          |  12 +---
 lib/vlog.c                 |   7 ++
 m4/openvswitch.m4          |   9 ++-
 tests/atlocal.in           |   2 +
 tests/daemon.at            |  50 +++++++++++++++
 9 files changed, 245 insertions(+), 77 deletions(-)

On 5/30/23 09:34, Ales Musil wrote:
> Use the backtrace functions that is provided by libc,
> this allows us to get backtrace that is independent of
> the current memory map of the process. Which in turn can
> be used for debugging/tracing purpose. The backtrace
> is not 100% accurate due to various optimizations, most
> notably "-fomit-frame-pointer" and LTO. This might result
> that the line in source file doesn't correspond to the
> real line. However, it should be able to pinpoint at least
> the function where the backtrace was called.
> 
> The usage for SIGSEGV is determined during compilation
> based on available libraries.

The usage for SIGSEGV?  This sentence looks strange.

> Libunwind has higher priority
> if both methods are available to keep the compatibility with
> current behavior.
> 
> The backtrace is not marked as signal safe however the backtrace
> manual page gives more detailed explanation why it might be the
> case [0]. Load the "libgcc" or equivalent in advance within the
> "fatal_signal_init" which should ensure that subsequent calls
> to backtrace* do not call malloc and are signal safe.
> 
> The typical backtrace will look similar to the one below:
> /lib64/libopenvswitch-3.1.so.0(backtrace_capture+0x1e) [0x7fc5db298dfe]
> /lib64/libopenvswitch-3.1.so.0(log_backtrace_at+0x57) [0x7fc5db2999e7]
> /lib64/libovsdb-3.1.so.0(ovsdb_txn_complete+0x7b) [0x7fc5db56247b]
> /lib64/libovsdb-3.1.so.0(ovsdb_txn_propose_commit_block+0x8d) [0x7fc5db563a8d]
> ovsdb-server(+0xa661) [0x562cfce2e661]
> ovsdb-server(+0x7e39) [0x562cfce2be39]
> /lib64/libc.so.6(+0x27b4a) [0x7fc5db048b4a]
> /lib64/libc.so.6(__libc_start_main+0x8b) [0x7fc5db048c0b]
> ovsdb-server(+0x8c35) [0x562cfce2cc35]
> 
> backtrace.h elaborates on how to effectively get the line
> information associated with the addressed presented in the
> backtrace.
> 
> [0]
> backtrace() and backtrace_symbols_fd() don't call malloc()
> explicitly, but they are part of libgcc, which gets loaded
> dynamically when first used.  Dynamic loading usually triggers
> a call to malloc(3).  If you need certain calls to these two
> functions to not allocate memory (in signal handlers, for
> example), you need to make sure libgcc is loaded beforehand
> 
> Reported-at: https://bugzilla.redhat.com/2177760
> Signed-off-by: Ales Musil <amusil@redhat.com>
> ---
> v2: Extend the current use of libunwind rather than replacing it.
> v3: Allow vlog_fd to be also 0.
>     Return the backtrace log from monitor in updated form.
>     Return use of the "vlog_direct_write_to_log_file_unsafe".
> v4: Rebase on top of current master.
>     Address comments from Ilya:
>     Address the coding style issues.
>     Make sure that the backrace received by monitor doesn't
>     have more than maximum allowed frames.
>     Change the backtrace_format to accept delimiter.
>     Remove unnecessary checks in the tests.
> v5: Rebase on top of current master.
>     Address missed comment from v3:
>     Reaname the vlog_fd to vlog_get_log_file_fd_unsafe to reflect that
>     this is really unsafe.
>     Return the ovsdb backtrace to correct position in log and
>     keep the previous format.
> v6: Rebase on top of current master.
>     Adrress comments from Ilya:
>     Some missed style comments.
>     Fix the compilation failure on Windows.
>     Fix BSD test failure.
> ---
>  include/openvswitch/vlog.h |   3 +
>  lib/backtrace.c            | 128 ++++++++++++++++++++++++++-----------
>  lib/backtrace.h            |  58 ++++++++++-------
>  lib/fatal-signal.c         |  53 +++++++++++++--
>  lib/ovsdb-error.c          |  12 +---
>  lib/vlog.c                 |   7 ++
>  m4/openvswitch.m4          |   9 ++-
>  tests/atlocal.in           |   2 +
>  tests/daemon.at            |  50 +++++++++++++++
>  9 files changed, 245 insertions(+), 77 deletions(-)
> 
> diff --git a/include/openvswitch/vlog.h b/include/openvswitch/vlog.h
> index e53ce6d81..481e1c0f0 100644
> --- a/include/openvswitch/vlog.h
> +++ b/include/openvswitch/vlog.h
> @@ -148,6 +148,9 @@ void vlog_set_syslog_target(const char *target);
>  /* Write directly to log file. */
>  void vlog_direct_write_to_log_file_unsafe(const char *s);
>  
> +/* Return the current log file descriptor. */
> +int vlog_get_log_file_fd_unsafe(void);
> +
>  /* Initialization. */
>  void vlog_init(void);
>  void vlog_enable_async(void);
> diff --git a/lib/backtrace.c b/lib/backtrace.c
> index 2853d5ff1..3a61e5591 100644
> --- a/lib/backtrace.c
> +++ b/lib/backtrace.c
> @@ -32,12 +32,27 @@ VLOG_DEFINE_THIS_MODULE(backtrace);
>  void
>  backtrace_capture(struct backtrace *b)
>  {
> -    void *frames[BACKTRACE_MAX_FRAMES];
> -    int i;
> +    b->n_frames = backtrace(b->frames, BACKTRACE_MAX_FRAMES);
> +}
> +
> +void
> +backtrace_format(struct ds *ds, const struct backtrace *bt,
> +                 const char *delimiter)
> +{
> +    if (bt->n_frames) {
> +        char **symbols = backtrace_symbols(bt->frames, bt->n_frames);
> +
> +        if (!symbols) {
> +            return;
> +        }
>  
> -    b->n_frames = backtrace(frames, BACKTRACE_MAX_FRAMES);
> -    for (i = 0; i < b->n_frames; i++) {
> -        b->frames[i] = (uintptr_t) frames[i];
> +        for (int i = 0; i < bt->n_frames - 1; i++) {
> +            ds_put_format(ds, "%s%s", symbols[i], delimiter);
> +        }
> +
> +        ds_put_format(ds, "%s", symbols[bt->n_frames - 1]);
> +
> +        free(symbols);
>      }
>  }
>  
> @@ -47,23 +62,14 @@ backtrace_capture(struct backtrace *backtrace)
>  {
>      backtrace->n_frames = 0;
>  }
> -#endif
>  
> -static char *
> -backtrace_format(const struct backtrace *b, struct ds *ds)
> +void
> +backtrace_format(struct ds *ds, const struct backtrace *bt OVS_UNUSED,
> +                 const char *delimiter OVS_UNUSED)
>  {
> -    if (b->n_frames) {
> -        int i;
> -
> -        ds_put_cstr(ds, " (backtrace:");
> -        for (i = 0; i < b->n_frames; i++) {
> -            ds_put_format(ds, " 0x%08"PRIxPTR, b->frames[i]);
> -        }
> -        ds_put_cstr(ds, ")");
> -    }
> -
> -    return ds_cstr(ds);
> +    ds_put_cstr(ds, "backtrace() is not supported!\n");
>  }
> +#endif
>  
>  void
>  log_backtrace_at(const char *msg, const char *where)
> @@ -77,41 +83,85 @@ log_backtrace_at(const char *msg, const char *where)
>      }
>  
>      ds_put_cstr(&ds, where);
> -    VLOG_ERR("%s", backtrace_format(&b, &ds));
> +    ds_put_cstr(&ds, " backtrace:\n");
> +    backtrace_format(&ds, &b, "\n");
> +    VLOG_ERR("%s", ds_cstr_ro(&ds));
>  
>      ds_destroy(&ds);
>  }
>  
> +#if defined(HAVE_UNWIND) || defined(HAVE_BACKTRACE)
> +static bool
> +read_received_backtrace(int fd, void *dest, size_t len)
> +{
> +    VLOG_WARN("%s fd %d", __func__, fd);

This log is a bit strange.  It's a debug log printed at WARN level.
It was WARN before, but maybe we should change it to DBG?

> +    fcntl(fd, F_SETFL, O_NONBLOCK);
> +    memset(dest, 0, len);
> +
> +    int byte_read = read(fd, dest, len);
> +    if (byte_read < 0) {
> +        VLOG_ERR("Read fd %d failed: %s", fd, ovs_strerror(errno));
> +        return false;
> +    }
> +    return true;

This fucntion will return 'true' if the read buffer is empty.
For exmaple, this is causing 'SIGSEGV detected' message after
the process dying from SIGABRT.  Which is incorrect.
The old implementation handled a zero bytes read correctly.

> +}
> +#else
> +static bool
> +read_received_backtrace(int fd OVS_UNUSED, void *dest OVS_UNUSED,
> +                        size_t len OVS_UNUSED)
> +{
> +    return false;
> +}
> +#endif
> +
>  #ifdef HAVE_UNWIND
>  void
> -log_received_backtrace(int fd) {
> -    int byte_read;
> +log_received_backtrace(int fd)
> +{
>      struct unw_backtrace backtrace[UNW_MAX_DEPTH];
>  
> -    VLOG_WARN("%s fd %d", __func__, fd);
> -    fcntl(fd, F_SETFL, O_NONBLOCK);
> -    memset(backtrace, 0, UNW_MAX_BUF);
> +    if (read_received_backtrace(fd, backtrace, UNW_MAX_BUF)) {
> +        struct ds ds = DS_EMPTY_INITIALIZER;
> +
> +        ds_put_cstr(&ds, BACKTRACE_DUMP_MSG);
>  
> -    byte_read = read(fd, backtrace, UNW_MAX_BUF);
> -    if (byte_read < 0) {
> -        VLOG_ERR("Read fd %d failed: %s", fd,
> -                 ovs_strerror(errno));
> -    } else if (byte_read > 0) {
> -        VLOG_WARN("SIGSEGV detected, backtrace:");
>          for (int i = 0; i < UNW_MAX_DEPTH; i++) {
>              if (backtrace[i].func[0] == 0) {
>                  break;
>              }
> -            VLOG_WARN("0x%016"PRIxPTR" <%s+0x%"PRIxPTR">\n",
> -                      backtrace[i].ip,
> -                      backtrace[i].func,
> -                      backtrace[i].offset);
> +            ds_put_format(&ds, "0x%016"PRIxPTR" <%s+0x%"PRIxPTR">\n",
> +                          backtrace[i].ip,
> +                          backtrace[i].func,
> +                          backtrace[i].offset);
>          }
> +
> +        VLOG_WARN("%s", ds_cstr_ro(&ds));
> +
> +        ds_destroy(&ds);
>      }
>  }
> -#else /* !HAVE_UNWIND */
> +#elif HAVE_BACKTRACE
>  void
> -log_received_backtrace(int daemonize_fd OVS_UNUSED) {
> -    VLOG_WARN("Backtrace using libunwind not supported.");
> +log_received_backtrace(int fd)
> +{
> +    struct backtrace bt;
> +
> +    if (read_received_backtrace(fd, &bt, sizeof bt)) {
> +        struct ds ds = DS_EMPTY_INITIALIZER;
> +
> +        bt.n_frames = MIN(bt.n_frames, BACKTRACE_MAX_FRAMES);
> +
> +        ds_put_cstr(&ds, BACKTRACE_DUMP_MSG);
> +        backtrace_format(&ds, &bt, "\n");
> +        VLOG_WARN("%s", ds_cstr_ro(&ds));
> +
> +        ds_destroy(&ds);
> +    }
>  }
> -#endif /* HAVE_UNWIND */
> +#else
> +void
> +log_received_backtrace(int daemonize_fd OVS_UNUSED)
> +{
> +    VLOG_WARN("Backtrace using libunwind or backtrace() is not supported.");
> +}
> +#endif
> diff --git a/lib/backtrace.h b/lib/backtrace.h
> index 5708bf9c6..9ccafd6d4 100644
> --- a/lib/backtrace.h
> +++ b/lib/backtrace.h
> @@ -36,41 +36,53 @@
>   *       log_backtrace_msg("your message");         <-- with a message
>   *
>   *
> - * A typical log will look like the following. The hex numbers listed after
> - * "backtrace" are the addresses of the backtrace.
> + * A typical backtrace will look like the following example:
> + * /lib64/libopenvswitch-3.1.so.0(backtrace_capture+0x1e) [0x7fc5db298dfe]
> + * /lib64/libopenvswitch-3.1.so.0(log_backtrace_at+0x57) [0x7fc5db2999e7]
> + * /lib64/libovsdb-3.1.so.0(ovsdb_txn_complete+0x7b) [0x7fc5db56247b]
> + * /lib64/libovsdb-3.1.so.0(ovsdb_txn_propose_commit_block+0x8d)
> + * [0x7fc5db563a8d]
> + * ovsdb-server(+0xa661) [0x562cfce2e661]
> + * ovsdb-server(+0x7e39) [0x562cfce2be39]
> + * /lib64/libc.so.6(+0x27b4a) [0x7fc5db048b4a]
> + * /lib64/libc.so.6(__libc_start_main+0x8b) [0x7fc5db048c0b]
> + * ovsdb-server(+0x8c35) [0x562cfce2cc35]
>   *
> - * 2014-03-13T23:18:11.979Z|00002|backtrace(revalidator_6)|ERR|lib/dpif-netdev.c:1312: (backtrace: 0x00521f57 0x00460365 0x00463ea4 0x0046470b 0x0043b32d 0x0043bac3 0x0043bae2 0x0043943b 0x004c22b3 0x2b5b3ac94e9a 0x2b5b3b4a33fd)
> + * GDB can be used to view the exact line of the code for particular backtrace.
> + * One thing to keep in mind is that the lines in source files might not
> + * 100% correspond with the backtrace due to various optimizations as LTO etc.
> + * (The effect can be seen in this example).
>   *
> - * The following bash command can be used to  view backtrace in
> - * a more readable form.
> - * addr2line -p -e vswitchd/ovs-vswitchd <cut-and-paste back traces>
> + * Assuming that debuginfo for the library or binary is installed load it to
> + * GDB:
> + * $ gdb ovsdb-server
> + * (gdb) list *(+0x7e39)
> + * 0x7e39 is in main (ovsdb/ovsdb-server.c:278).
> + * (gdb) list *(+0xa661)
> + * 0xa661 is in commit_txn (ovsdb/ovsdb-server.c:1173)
>   *
> - * An typical run and output will look like:
> - * addr2line -p -e vswitchd/ovs-vswitchd  0x00521f57 0x00460365 0x00463ea4
> - * 0x0046470b 0x0043b32d 0x0043bac3 0x0043bae2 0x0043943b 0x004c22b3
> - * 0x2b5b3ac94e9a 0x2b5b3b4a33fd
> + * $ gdb /lib64/libovsdb-3.1.so.0
> + * (gdb) list *(ovsdb_txn_propose_commit_block+0x8d)
> + * 0x3aa8d is in ovsdb_txn_propose_commit_block (ovsdb/transaction.c:1328)
> + * (gdb) list *(ovsdb_txn_complete+0x7b)
> + * 0x3947b is in ovsdb_txn_complete (./include/openvswitch/list.h:321)
>   *
> - * openvswitch/lib/backtrace.c:33
> - * openvswitch/lib/dpif-netdev.c:1312
> - * openvswitch/lib/dpif.c:937
> - * openvswitch/lib/dpif.c:1258
> - * openvswitch/ofproto/ofproto-dpif-upcall.c:1440
> - * openvswitch/ofproto/ofproto-dpif-upcall.c:1595
> - * openvswitch/ofproto/ofproto-dpif-upcall.c:160
> - * openvswitch/ofproto/ofproto-dpif-upcall.c:717
> - * openvswitch/lib/ovs-thread.c:268
> - * ??:0
> - * ??:0
> + * $ gdb /lib64/libopenvswitch-3.1.so.0
> + * (gdb) list *(log_backtrace_at+0x57)
> + * 0x999e7 is in log_backtrace_at (lib/backtrace.c:77)
> + * (gdb) list *(backtrace_capture+0x1e)
> + * 0x98dfe is in backtrace_capture (lib/backtrace.c:35)
>   */
>  
>  #define log_backtrace() log_backtrace_at(NULL, OVS_SOURCE_LOCATOR);
>  #define log_backtrace_msg(msg) log_backtrace_at(msg, OVS_SOURCE_LOCATOR);
>  
>  #define BACKTRACE_MAX_FRAMES 31
> +#define BACKTRACE_DUMP_MSG "SIGSEGV detected, backtrace:\n"
>  
>  struct backtrace {
>      int n_frames;
> -    uintptr_t frames[BACKTRACE_MAX_FRAMES];
> +    void *frames[BACKTRACE_MAX_FRAMES];
>  };
>  
>  #ifdef HAVE_UNWIND
> @@ -88,6 +100,8 @@ struct unw_backtrace {
>  
>  void backtrace_capture(struct backtrace *);
>  void log_backtrace_at(const char *msg, const char *where);
> +void backtrace_format(struct ds *, const struct backtrace *,
> +                      const char *delimiter);
>  void log_received_backtrace(int fd);
>  
>  #endif /* backtrace.h */
> diff --git a/lib/fatal-signal.c b/lib/fatal-signal.c
> index bbb31ef27..f80f32182 100644
> --- a/lib/fatal-signal.c
> +++ b/lib/fatal-signal.c
> @@ -35,10 +35,14 @@
>  
>  #include "openvswitch/type-props.h"
>  
> -#ifdef HAVE_UNWIND
> +#if defined(HAVE_UNWIND) || defined(HAVE_BACKTRACE)
>  #include "daemon-private.h"
>  #endif
>  
> +#ifdef HAVE_BACKTRACE
> +#include <execinfo.h>
> +#endif
> +
>  #ifndef SIG_ATOMIC_MAX
>  #define SIG_ATOMIC_MAX TYPE_MAXIMUM(sig_atomic_t)
>  #endif
> @@ -94,6 +98,17 @@ fatal_signal_init(void)
>          inited = true;
>  
>          ovs_mutex_init_recursive(&mutex);
> +
> +        /* The dummy backtrace is needed.
> +         * See comment for send_backtrace_to_monitor(). */
> +        struct backtrace dummy_bt;
> +
> +        backtrace_capture(&dummy_bt);
> +
> +        if (!dummy_bt.n_frames) {
> +            VLOG_DBG("Capturing of dummy backtrace has failed.");
> +        }
> +
>  #ifndef _WIN32
>          xpipe_nonblocking(signal_fds);
>  #else
> @@ -181,7 +196,8 @@ llong_to_hex_str(unsigned long long value, char *str)
>   * library functions used here must be async-signal-safe.
>   */
>  static inline void
> -send_backtrace_to_monitor(void) {
> +send_backtrace_to_monitor(void)
> +{
>      /* volatile added to prevent a "clobbered" error on ppc64le with gcc */
>      volatile int dep;
>      struct unw_backtrace unw_bt[UNW_MAX_DEPTH];
> @@ -211,11 +227,10 @@ send_backtrace_to_monitor(void) {
>          /* Since there is no monitor daemon running, write backtrace
>           * in current process.
>           */
> -        char str[] = "SIGSEGV detected, backtrace:\n";
>          char ip_str[16], offset_str[6];
>          char line[64], fn_name[UNW_MAX_FUNCN];
>  
> -        vlog_direct_write_to_log_file_unsafe(str);
> +        vlog_direct_write_to_log_file_unsafe(BACKTRACE_DUMP_MSG);
>  
>          for (int i = 0; i < dep; i++) {
>              memset(line, 0, sizeof line);
> @@ -239,6 +254,36 @@ send_backtrace_to_monitor(void) {
>          }
>      }
>  }
> +#elif HAVE_BACKTRACE
> +/* Send the backtrace to monitor thread.
> + *
> + * Note that this runs in the signal handling context, any system
> + * library functions used here must be async-signal-safe.
> + * backtrace() is only signal safe if the "libgcc" or equivalent was loaded
> + * before the signal handler. In order to keep it safe the fatal_signal_init()
> + * should always call backtrace_capture which will ensure that "libgcc" or
> + * equivlent is loaded.
> + */
> +static inline void
> +send_backtrace_to_monitor(void)
> +{
> +    struct backtrace bt;
> +
> +    backtrace_capture(&bt);
> +
> +    if (monitor && daemonize_fd > -1) {
> +        ignore(write(daemonize_fd, &bt, sizeof bt));
> +    } else {
> +        int log_fd = vlog_get_log_file_fd_unsafe();
> +
> +        if (log_fd < 0) {
> +            return;
> +        }
> +
> +        vlog_direct_write_to_log_file_unsafe(BACKTRACE_DUMP_MSG);
> +        backtrace_symbols_fd(bt.frames, bt.n_frames, log_fd);
> +    }
> +}
>  #else
>  static inline void
>  send_backtrace_to_monitor(void) {
> diff --git a/lib/ovsdb-error.c b/lib/ovsdb-error.c
> index a75ad36b7..3966f421b 100644
> --- a/lib/ovsdb-error.c
> +++ b/lib/ovsdb-error.c
> @@ -139,16 +139,10 @@ ovsdb_internal_error(struct ovsdb_error *inner_error,
>          va_end(args);
>      }
>  
> +    ds_put_cstr(&ds, " (backtrace:");
>      backtrace_capture(&backtrace);
> -    if (backtrace.n_frames) {
> -        int i;
> -
> -        ds_put_cstr(&ds, " (backtrace:");
> -        for (i = 0; i < backtrace.n_frames; i++) {
> -            ds_put_format(&ds, " 0x%08"PRIxPTR, backtrace.frames[i]);
> -        }
> -        ds_put_char(&ds, ')');
> -    }
> +    backtrace_format(&ds, &backtrace, ", ");
> +    ds_put_char(&ds, ')');

Why printing an empty backtrace?

>  
>      ds_put_format(&ds, " (%s %s)", program_name, VERSION);
>  
> diff --git a/lib/vlog.c b/lib/vlog.c
> index 9ddea48b8..b2653142f 100644
> --- a/lib/vlog.c
> +++ b/lib/vlog.c
> @@ -664,6 +664,13 @@ vlog_direct_write_to_log_file_unsafe(const char *s)
>      }
>  }
>  
> +int
> +vlog_get_log_file_fd_unsafe(void)
> +    OVS_NO_THREAD_SAFETY_ANALYSIS
> +{
> +    return log_fd;
> +}
> +
>  /* Returns 'false' if 'facility' is not a valid string. If 'facility'
>   * is a valid string, sets 'value' with the integer value of 'facility'
>   * and returns 'true'. */
> diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
> index 14d9249b8..f4943b140 100644
> --- a/m4/openvswitch.m4
> +++ b/m4/openvswitch.m4
> @@ -359,9 +359,12 @@ AC_DEFUN([OVS_CHECK_DBDIR],
>  
>  dnl Defines HAVE_BACKTRACE if backtrace() is found.
>  AC_DEFUN([OVS_CHECK_BACKTRACE],
> -  [AC_SEARCH_LIBS([backtrace], [execinfo ubacktrace],
> -                  [AC_DEFINE([HAVE_BACKTRACE], [1],
> -                             [Define to 1 if you have backtrace(3).])])])
> +  [AC_SEARCH_LIBS([backtrace], [execinfo ubacktrace], [HAVE_BACKTRACE=yes], [HAVE_BACKTRACE=no])
> +   if test "$HAVE_BACKTRACE" = "yes"; then
> +     AC_DEFINE([HAVE_BACKTRACE], [1], [Define to 1 if you have backtrace(3).])
> +   fi
> +   AM_CONDITIONAL([HAVE_BACKTRACE], [test "$HAVE_BACKTRACE" = "yes"])
> +   AC_SUBST([HAVE_BACKTRACE])])
>  
>  dnl Defines HAVE_PERF_EVENT if linux/perf_event.h is found.
>  AC_DEFUN([OVS_CHECK_PERF_EVENT],
> diff --git a/tests/atlocal.in b/tests/atlocal.in
> index 859668586..18d5efae0 100644
> --- a/tests/atlocal.in
> +++ b/tests/atlocal.in
> @@ -2,6 +2,8 @@
>  HAVE_OPENSSL='@HAVE_OPENSSL@'
>  OPENSSL_SUPPORTS_SNI='@OPENSSL_SUPPORTS_SNI@'
>  HAVE_UNBOUND='@HAVE_UNBOUND@'
> +HAVE_BACKTRACE='@HAVE_BACKTRACE@'
> +HAVE_UNWIND='@HAVE_UNWIND@'
>  EGREP='@EGREP@'
>  PYTHON3='@PYTHON3@'
>  CFLAGS='@CFLAGS@'
> diff --git a/tests/daemon.at b/tests/daemon.at
> index d7981f9d2..1e03bf878 100644
> --- a/tests/daemon.at
> +++ b/tests/daemon.at
> @@ -234,3 +234,53 @@ OVS_WAIT_UNTIL([sc query ovsdb-server | grep STATE | grep STOPPED > /dev/null 2>
>  AT_CHECK([sc delete ovsdb-server], [0], [[[SC]] DeleteService SUCCESS
>  ])
>  AT_CLEANUP
> +
> +AT_SETUP([backtrace without monitor])
> +AT_SKIP_IF([test "$HAVE_BACKTRACE" = "no" && test "$HAVE_UNWIND" = "no"])
> +AT_SKIP_IF([test "$IS_WIN32" = "yes"])
> +
> +# This test intentionally causes SIGSEGV, so make Address Sanitizer ignore it.
> +ASAN_OPTIONS=$ASAN_OPTIONS:handle_segv=0; export ASAN_OPTIONS
> +
> +# Skip it if UB Sanitizer is being used.  There's no way to disable the
> +# SEGV check at runtime.
> +AT_SKIP_IF([test $TESTS_WITH_UBSAN = yes])
> +
> +AT_CHECK([ovsdb-server --detach --no-chdir --pidfile --no-db --log-file --verbose=DBG], [0], [ignore], [ignore])
> +
> +OVS_WAIT_UNTIL([test -s ovsdb-server.pid])
> +child=$(cat ovsdb-server.pid)
> +
> +AT_CAPTURE_FILE([ovsdb-server.log])
> +
> +AT_CHECK([kill -SEGV $child])
> +
> +OVS_WAIT_UNTIL([grep -q "^SIGSEGV detected, backtrace:" ovsdb-server.log])
> +
> +AT_CLEANUP
> +
> +AT_SETUP([backtrace with monitor])
> +AT_SKIP_IF([test "$HAVE_BACKTRACE" = "no" && test "$HAVE_UNWIND" = "no"])
> +AT_SKIP_IF([test "$IS_WIN32" = "yes"])
> +
> +# This test intentionally causes SIGSEGV, so make Address Sanitizer ignore it.
> +ASAN_OPTIONS=$ASAN_OPTIONS:handle_segv=0; export ASAN_OPTIONS
> +
> +# Skip it if UB Sanitizer is being used.  There's no way to disable the
> +# SEGV check at runtime.
> +AT_SKIP_IF([test $TESTS_WITH_UBSAN = yes])
> +
> +on_exit 'kill $(cat *.pid)'
> +
> +AT_CHECK([ovsdb-server --detach --monitor --no-chdir --pidfile --no-db --log-file --verbose=DBG], [0], [ignore], [ignore])
> +OVS_WAIT_UNTIL([test -s ovsdb-server.pid])
> +child=$(cat ovsdb-server.pid)
> +
> +AT_CAPTURE_FILE([ovsdb-server.log])
> +
> +AT_CHECK([kill -SEGV $child])
> +
> +OVS_WAIT_UNTIL([grep -q "backtrace(monitor)|WARN|SIGSEGV detected, backtrace:" ovsdb-server.log])
> +OVS_WAIT_UNTIL([grep -q "daemon_unix(monitor)|ERR|1 crashes: pid .* died, killed (Segmentation fault)" ovsdb-server.log])
> +
> +AT_CLEANUP

On Thu, Jun 8, 2023 at 3:20 PM Ilya Maximets <i.maximets@ovn.org> wrote:

> On 5/30/23 09:34, Ales Musil wrote:
> > Use the backtrace functions that is provided by libc,
> > this allows us to get backtrace that is independent of
> > the current memory map of the process. Which in turn can
> > be used for debugging/tracing purpose. The backtrace
> > is not 100% accurate due to various optimizations, most
> > notably "-fomit-frame-pointer" and LTO. This might result
> > that the line in source file doesn't correspond to the
> > real line. However, it should be able to pinpoint at least
> > the function where the backtrace was called.
> >
> > The usage for SIGSEGV is determined during compilation
> > based on available libraries.
>
> The usage for SIGSEGV?  This sentence looks strange.
>

Right, updated in v7.


>
> > Libunwind has higher priority
> > if both methods are available to keep the compatibility with
> > current behavior.
> >
> > The backtrace is not marked as signal safe however the backtrace
> > manual page gives more detailed explanation why it might be the
> > case [0]. Load the "libgcc" or equivalent in advance within the
> > "fatal_signal_init" which should ensure that subsequent calls
> > to backtrace* do not call malloc and are signal safe.
> >
> > The typical backtrace will look similar to the one below:
> > /lib64/libopenvswitch-3.1.so.0(backtrace_capture+0x1e) [0x7fc5db298dfe]
> > /lib64/libopenvswitch-3.1.so.0(log_backtrace_at+0x57) [0x7fc5db2999e7]
> > /lib64/libovsdb-3.1.so.0(ovsdb_txn_complete+0x7b) [0x7fc5db56247b]
> > /lib64/libovsdb-3.1.so.0(ovsdb_txn_propose_commit_block+0x8d)
> [0x7fc5db563a8d]
> > ovsdb-server(+0xa661) [0x562cfce2e661]
> > ovsdb-server(+0x7e39) [0x562cfce2be39]
> > /lib64/libc.so.6(+0x27b4a) [0x7fc5db048b4a]
> > /lib64/libc.so.6(__libc_start_main+0x8b) [0x7fc5db048c0b]
> > ovsdb-server(+0x8c35) [0x562cfce2cc35]
> >
> > backtrace.h elaborates on how to effectively get the line
> > information associated with the addressed presented in the
> > backtrace.
> >
> > [0]
> > backtrace() and backtrace_symbols_fd() don't call malloc()
> > explicitly, but they are part of libgcc, which gets loaded
> > dynamically when first used.  Dynamic loading usually triggers
> > a call to malloc(3).  If you need certain calls to these two
> > functions to not allocate memory (in signal handlers, for
> > example), you need to make sure libgcc is loaded beforehand
> >
> > Reported-at: https://bugzilla.redhat.com/2177760
> > Signed-off-by: Ales Musil <amusil@redhat.com>
> > ---
> > v2: Extend the current use of libunwind rather than replacing it.
> > v3: Allow vlog_fd to be also 0.
> >     Return the backtrace log from monitor in updated form.
> >     Return use of the "vlog_direct_write_to_log_file_unsafe".
> > v4: Rebase on top of current master.
> >     Address comments from Ilya:
> >     Address the coding style issues.
> >     Make sure that the backrace received by monitor doesn't
> >     have more than maximum allowed frames.
> >     Change the backtrace_format to accept delimiter.
> >     Remove unnecessary checks in the tests.
> > v5: Rebase on top of current master.
> >     Address missed comment from v3:
> >     Reaname the vlog_fd to vlog_get_log_file_fd_unsafe to reflect that
> >     this is really unsafe.
> >     Return the ovsdb backtrace to correct position in log and
> >     keep the previous format.
> > v6: Rebase on top of current master.
> >     Adrress comments from Ilya:
> >     Some missed style comments.
> >     Fix the compilation failure on Windows.
> >     Fix BSD test failure.
> > ---
> >  include/openvswitch/vlog.h |   3 +
> >  lib/backtrace.c            | 128 ++++++++++++++++++++++++++-----------
> >  lib/backtrace.h            |  58 ++++++++++-------
> >  lib/fatal-signal.c         |  53 +++++++++++++--
> >  lib/ovsdb-error.c          |  12 +---
> >  lib/vlog.c                 |   7 ++
> >  m4/openvswitch.m4          |   9 ++-
> >  tests/atlocal.in           |   2 +
> >  tests/daemon.at            |  50 +++++++++++++++
> >  9 files changed, 245 insertions(+), 77 deletions(-)
> >
> > diff --git a/include/openvswitch/vlog.h b/include/openvswitch/vlog.h
> > index e53ce6d81..481e1c0f0 100644
> > --- a/include/openvswitch/vlog.h
> > +++ b/include/openvswitch/vlog.h
> > @@ -148,6 +148,9 @@ void vlog_set_syslog_target(const char *target);
> >  /* Write directly to log file. */
> >  void vlog_direct_write_to_log_file_unsafe(const char *s);
> >
> > +/* Return the current log file descriptor. */
> > +int vlog_get_log_file_fd_unsafe(void);
> > +
> >  /* Initialization. */
> >  void vlog_init(void);
> >  void vlog_enable_async(void);
> > diff --git a/lib/backtrace.c b/lib/backtrace.c
> > index 2853d5ff1..3a61e5591 100644
> > --- a/lib/backtrace.c
> > +++ b/lib/backtrace.c
> > @@ -32,12 +32,27 @@ VLOG_DEFINE_THIS_MODULE(backtrace);
> >  void
> >  backtrace_capture(struct backtrace *b)
> >  {
> > -    void *frames[BACKTRACE_MAX_FRAMES];
> > -    int i;
> > +    b->n_frames = backtrace(b->frames, BACKTRACE_MAX_FRAMES);
> > +}
> > +
> > +void
> > +backtrace_format(struct ds *ds, const struct backtrace *bt,
> > +                 const char *delimiter)
> > +{
> > +    if (bt->n_frames) {
> > +        char **symbols = backtrace_symbols(bt->frames, bt->n_frames);
> > +
> > +        if (!symbols) {
> > +            return;
> > +        }
> >
> > -    b->n_frames = backtrace(frames, BACKTRACE_MAX_FRAMES);
> > -    for (i = 0; i < b->n_frames; i++) {
> > -        b->frames[i] = (uintptr_t) frames[i];
> > +        for (int i = 0; i < bt->n_frames - 1; i++) {
> > +            ds_put_format(ds, "%s%s", symbols[i], delimiter);
> > +        }
> > +
> > +        ds_put_format(ds, "%s", symbols[bt->n_frames - 1]);
> > +
> > +        free(symbols);
> >      }
> >  }
> >
> > @@ -47,23 +62,14 @@ backtrace_capture(struct backtrace *backtrace)
> >  {
> >      backtrace->n_frames = 0;
> >  }
> > -#endif
> >
> > -static char *
> > -backtrace_format(const struct backtrace *b, struct ds *ds)
> > +void
> > +backtrace_format(struct ds *ds, const struct backtrace *bt OVS_UNUSED,
> > +                 const char *delimiter OVS_UNUSED)
> >  {
> > -    if (b->n_frames) {
> > -        int i;
> > -
> > -        ds_put_cstr(ds, " (backtrace:");
> > -        for (i = 0; i < b->n_frames; i++) {
> > -            ds_put_format(ds, " 0x%08"PRIxPTR, b->frames[i]);
> > -        }
> > -        ds_put_cstr(ds, ")");
> > -    }
> > -
> > -    return ds_cstr(ds);
> > +    ds_put_cstr(ds, "backtrace() is not supported!\n");
> >  }
> > +#endif
> >
> >  void
> >  log_backtrace_at(const char *msg, const char *where)
> > @@ -77,41 +83,85 @@ log_backtrace_at(const char *msg, const char *where)
> >      }
> >
> >      ds_put_cstr(&ds, where);
> > -    VLOG_ERR("%s", backtrace_format(&b, &ds));
> > +    ds_put_cstr(&ds, " backtrace:\n");
> > +    backtrace_format(&ds, &b, "\n");
> > +    VLOG_ERR("%s", ds_cstr_ro(&ds));
> >
> >      ds_destroy(&ds);
> >  }
> >
> > +#if defined(HAVE_UNWIND) || defined(HAVE_BACKTRACE)
> > +static bool
> > +read_received_backtrace(int fd, void *dest, size_t len)
> > +{
> > +    VLOG_WARN("%s fd %d", __func__, fd);
>
> This log is a bit strange.  It's a debug log printed at WARN level.
> It was WARN before, but maybe we should change it to DBG?
>

Updated in v7.


>
> > +    fcntl(fd, F_SETFL, O_NONBLOCK);
> > +    memset(dest, 0, len);
> > +
> > +    int byte_read = read(fd, dest, len);
> > +    if (byte_read < 0) {
> > +        VLOG_ERR("Read fd %d failed: %s", fd, ovs_strerror(errno));
> > +        return false;
> > +    }
> > +    return true;
>
> This fucntion will return 'true' if the read buffer is empty.
> For exmaple, this is causing 'SIGSEGV detected' message after
> the process dying from SIGABRT.  Which is incorrect.
> The old implementation handled a zero bytes read correctly.
>

Ah right, fixed in v7.


>
> > +}
> > +#else
> > +static bool
> > +read_received_backtrace(int fd OVS_UNUSED, void *dest OVS_UNUSED,
> > +                        size_t len OVS_UNUSED)
> > +{
> > +    return false;
> > +}
> > +#endif
> > +
> >  #ifdef HAVE_UNWIND
> >  void
> > -log_received_backtrace(int fd) {
> > -    int byte_read;
> > +log_received_backtrace(int fd)
> > +{
> >      struct unw_backtrace backtrace[UNW_MAX_DEPTH];
> >
> > -    VLOG_WARN("%s fd %d", __func__, fd);
> > -    fcntl(fd, F_SETFL, O_NONBLOCK);
> > -    memset(backtrace, 0, UNW_MAX_BUF);
> > +    if (read_received_backtrace(fd, backtrace, UNW_MAX_BUF)) {
> > +        struct ds ds = DS_EMPTY_INITIALIZER;
> > +
> > +        ds_put_cstr(&ds, BACKTRACE_DUMP_MSG);
> >
> > -    byte_read = read(fd, backtrace, UNW_MAX_BUF);
> > -    if (byte_read < 0) {
> > -        VLOG_ERR("Read fd %d failed: %s", fd,
> > -                 ovs_strerror(errno));
> > -    } else if (byte_read > 0) {
> > -        VLOG_WARN("SIGSEGV detected, backtrace:");
> >          for (int i = 0; i < UNW_MAX_DEPTH; i++) {
> >              if (backtrace[i].func[0] == 0) {
> >                  break;
> >              }
> > -            VLOG_WARN("0x%016"PRIxPTR" <%s+0x%"PRIxPTR">\n",
> > -                      backtrace[i].ip,
> > -                      backtrace[i].func,
> > -                      backtrace[i].offset);
> > +            ds_put_format(&ds, "0x%016"PRIxPTR" <%s+0x%"PRIxPTR">\n",
> > +                          backtrace[i].ip,
> > +                          backtrace[i].func,
> > +                          backtrace[i].offset);
> >          }
> > +
> > +        VLOG_WARN("%s", ds_cstr_ro(&ds));
> > +
> > +        ds_destroy(&ds);
> >      }
> >  }
> > -#else /* !HAVE_UNWIND */
> > +#elif HAVE_BACKTRACE
> >  void
> > -log_received_backtrace(int daemonize_fd OVS_UNUSED) {
> > -    VLOG_WARN("Backtrace using libunwind not supported.");
> > +log_received_backtrace(int fd)
> > +{
> > +    struct backtrace bt;
> > +
> > +    if (read_received_backtrace(fd, &bt, sizeof bt)) {
> > +        struct ds ds = DS_EMPTY_INITIALIZER;
> > +
> > +        bt.n_frames = MIN(bt.n_frames, BACKTRACE_MAX_FRAMES);
> > +
> > +        ds_put_cstr(&ds, BACKTRACE_DUMP_MSG);
> > +        backtrace_format(&ds, &bt, "\n");
> > +        VLOG_WARN("%s", ds_cstr_ro(&ds));
> > +
> > +        ds_destroy(&ds);
> > +    }
> >  }
> > -#endif /* HAVE_UNWIND */
> > +#else
> > +void
> > +log_received_backtrace(int daemonize_fd OVS_UNUSED)
> > +{
> > +    VLOG_WARN("Backtrace using libunwind or backtrace() is not
> supported.");
> > +}
> > +#endif
> > diff --git a/lib/backtrace.h b/lib/backtrace.h
> > index 5708bf9c6..9ccafd6d4 100644
> > --- a/lib/backtrace.h
> > +++ b/lib/backtrace.h
> > @@ -36,41 +36,53 @@
> >   *       log_backtrace_msg("your message");         <-- with a message
> >   *
> >   *
> > - * A typical log will look like the following. The hex numbers listed
> after
> > - * "backtrace" are the addresses of the backtrace.
> > + * A typical backtrace will look like the following example:
> > + * /lib64/libopenvswitch-3.1.so.0(backtrace_capture+0x1e)
> [0x7fc5db298dfe]
> > + * /lib64/libopenvswitch-3.1.so.0(log_backtrace_at+0x57)
> [0x7fc5db2999e7]
> > + * /lib64/libovsdb-3.1.so.0(ovsdb_txn_complete+0x7b) [0x7fc5db56247b]
> > + * /lib64/libovsdb-3.1.so.0(ovsdb_txn_propose_commit_block+0x8d)
> > + * [0x7fc5db563a8d]
> > + * ovsdb-server(+0xa661) [0x562cfce2e661]
> > + * ovsdb-server(+0x7e39) [0x562cfce2be39]
> > + * /lib64/libc.so.6(+0x27b4a) [0x7fc5db048b4a]
> > + * /lib64/libc.so.6(__libc_start_main+0x8b) [0x7fc5db048c0b]
> > + * ovsdb-server(+0x8c35) [0x562cfce2cc35]
> >   *
> > - *
> 2014-03-13T23:18:11.979Z|00002|backtrace(revalidator_6)|ERR|lib/dpif-netdev.c:1312:
> (backtrace: 0x00521f57 0x00460365 0x00463ea4 0x0046470b 0x0043b32d
> 0x0043bac3 0x0043bae2 0x0043943b 0x004c22b3 0x2b5b3ac94e9a 0x2b5b3b4a33fd)
> > + * GDB can be used to view the exact line of the code for particular
> backtrace.
> > + * One thing to keep in mind is that the lines in source files might not
> > + * 100% correspond with the backtrace due to various optimizations as
> LTO etc.
> > + * (The effect can be seen in this example).
> >   *
> > - * The following bash command can be used to  view backtrace in
> > - * a more readable form.
> > - * addr2line -p -e vswitchd/ovs-vswitchd <cut-and-paste back traces>
> > + * Assuming that debuginfo for the library or binary is installed load
> it to
> > + * GDB:
> > + * $ gdb ovsdb-server
> > + * (gdb) list *(+0x7e39)
> > + * 0x7e39 is in main (ovsdb/ovsdb-server.c:278).
> > + * (gdb) list *(+0xa661)
> > + * 0xa661 is in commit_txn (ovsdb/ovsdb-server.c:1173)
> >   *
> > - * An typical run and output will look like:
> > - * addr2line -p -e vswitchd/ovs-vswitchd  0x00521f57 0x00460365
> 0x00463ea4
> > - * 0x0046470b 0x0043b32d 0x0043bac3 0x0043bae2 0x0043943b 0x004c22b3
> > - * 0x2b5b3ac94e9a 0x2b5b3b4a33fd
> > + * $ gdb /lib64/libovsdb-3.1.so.0
> > + * (gdb) list *(ovsdb_txn_propose_commit_block+0x8d)
> > + * 0x3aa8d is in ovsdb_txn_propose_commit_block
> (ovsdb/transaction.c:1328)
> > + * (gdb) list *(ovsdb_txn_complete+0x7b)
> > + * 0x3947b is in ovsdb_txn_complete (./include/openvswitch/list.h:321)
> >   *
> > - * openvswitch/lib/backtrace.c:33
> > - * openvswitch/lib/dpif-netdev.c:1312
> > - * openvswitch/lib/dpif.c:937
> > - * openvswitch/lib/dpif.c:1258
> > - * openvswitch/ofproto/ofproto-dpif-upcall.c:1440
> > - * openvswitch/ofproto/ofproto-dpif-upcall.c:1595
> > - * openvswitch/ofproto/ofproto-dpif-upcall.c:160
> > - * openvswitch/ofproto/ofproto-dpif-upcall.c:717
> > - * openvswitch/lib/ovs-thread.c:268
> > - * ??:0
> > - * ??:0
> > + * $ gdb /lib64/libopenvswitch-3.1.so.0
> > + * (gdb) list *(log_backtrace_at+0x57)
> > + * 0x999e7 is in log_backtrace_at (lib/backtrace.c:77)
> > + * (gdb) list *(backtrace_capture+0x1e)
> > + * 0x98dfe is in backtrace_capture (lib/backtrace.c:35)
> >   */
> >
> >  #define log_backtrace() log_backtrace_at(NULL, OVS_SOURCE_LOCATOR);
> >  #define log_backtrace_msg(msg) log_backtrace_at(msg,
> OVS_SOURCE_LOCATOR);
> >
> >  #define BACKTRACE_MAX_FRAMES 31
> > +#define BACKTRACE_DUMP_MSG "SIGSEGV detected, backtrace:\n"
> >
> >  struct backtrace {
> >      int n_frames;
> > -    uintptr_t frames[BACKTRACE_MAX_FRAMES];
> > +    void *frames[BACKTRACE_MAX_FRAMES];
> >  };
> >
> >  #ifdef HAVE_UNWIND
> > @@ -88,6 +100,8 @@ struct unw_backtrace {
> >
> >  void backtrace_capture(struct backtrace *);
> >  void log_backtrace_at(const char *msg, const char *where);
> > +void backtrace_format(struct ds *, const struct backtrace *,
> > +                      const char *delimiter);
> >  void log_received_backtrace(int fd);
> >
> >  #endif /* backtrace.h */
> > diff --git a/lib/fatal-signal.c b/lib/fatal-signal.c
> > index bbb31ef27..f80f32182 100644
> > --- a/lib/fatal-signal.c
> > +++ b/lib/fatal-signal.c
> > @@ -35,10 +35,14 @@
> >
> >  #include "openvswitch/type-props.h"
> >
> > -#ifdef HAVE_UNWIND
> > +#if defined(HAVE_UNWIND) || defined(HAVE_BACKTRACE)
> >  #include "daemon-private.h"
> >  #endif
> >
> > +#ifdef HAVE_BACKTRACE
> > +#include <execinfo.h>
> > +#endif
> > +
> >  #ifndef SIG_ATOMIC_MAX
> >  #define SIG_ATOMIC_MAX TYPE_MAXIMUM(sig_atomic_t)
> >  #endif
> > @@ -94,6 +98,17 @@ fatal_signal_init(void)
> >          inited = true;
> >
> >          ovs_mutex_init_recursive(&mutex);
> > +
> > +        /* The dummy backtrace is needed.
> > +         * See comment for send_backtrace_to_monitor(). */
> > +        struct backtrace dummy_bt;
> > +
> > +        backtrace_capture(&dummy_bt);
> > +
> > +        if (!dummy_bt.n_frames) {
> > +            VLOG_DBG("Capturing of dummy backtrace has failed.");
> > +        }
> > +
> >  #ifndef _WIN32
> >          xpipe_nonblocking(signal_fds);
> >  #else
> > @@ -181,7 +196,8 @@ llong_to_hex_str(unsigned long long value, char *str)
> >   * library functions used here must be async-signal-safe.
> >   */
> >  static inline void
> > -send_backtrace_to_monitor(void) {
> > +send_backtrace_to_monitor(void)
> > +{
> >      /* volatile added to prevent a "clobbered" error on ppc64le with
> gcc */
> >      volatile int dep;
> >      struct unw_backtrace unw_bt[UNW_MAX_DEPTH];
> > @@ -211,11 +227,10 @@ send_backtrace_to_monitor(void) {
> >          /* Since there is no monitor daemon running, write backtrace
> >           * in current process.
> >           */
> > -        char str[] = "SIGSEGV detected, backtrace:\n";
> >          char ip_str[16], offset_str[6];
> >          char line[64], fn_name[UNW_MAX_FUNCN];
> >
> > -        vlog_direct_write_to_log_file_unsafe(str);
> > +        vlog_direct_write_to_log_file_unsafe(BACKTRACE_DUMP_MSG);
> >
> >          for (int i = 0; i < dep; i++) {
> >              memset(line, 0, sizeof line);
> > @@ -239,6 +254,36 @@ send_backtrace_to_monitor(void) {
> >          }
> >      }
> >  }
> > +#elif HAVE_BACKTRACE
> > +/* Send the backtrace to monitor thread.
> > + *
> > + * Note that this runs in the signal handling context, any system
> > + * library functions used here must be async-signal-safe.
> > + * backtrace() is only signal safe if the "libgcc" or equivalent was
> loaded
> > + * before the signal handler. In order to keep it safe the
> fatal_signal_init()
> > + * should always call backtrace_capture which will ensure that "libgcc"
> or
> > + * equivlent is loaded.
> > + */
> > +static inline void
> > +send_backtrace_to_monitor(void)
> > +{
> > +    struct backtrace bt;
> > +
> > +    backtrace_capture(&bt);
> > +
> > +    if (monitor && daemonize_fd > -1) {
> > +        ignore(write(daemonize_fd, &bt, sizeof bt));
> > +    } else {
> > +        int log_fd = vlog_get_log_file_fd_unsafe();
> > +
> > +        if (log_fd < 0) {
> > +            return;
> > +        }
> > +
> > +        vlog_direct_write_to_log_file_unsafe(BACKTRACE_DUMP_MSG);
> > +        backtrace_symbols_fd(bt.frames, bt.n_frames, log_fd);
> > +    }
> > +}
> >  #else
> >  static inline void
> >  send_backtrace_to_monitor(void) {
> > diff --git a/lib/ovsdb-error.c b/lib/ovsdb-error.c
> > index a75ad36b7..3966f421b 100644
> > --- a/lib/ovsdb-error.c
> > +++ b/lib/ovsdb-error.c
> > @@ -139,16 +139,10 @@ ovsdb_internal_error(struct ovsdb_error
> *inner_error,
> >          va_end(args);
> >      }
> >
> > +    ds_put_cstr(&ds, " (backtrace:");
> >      backtrace_capture(&backtrace);
> > -    if (backtrace.n_frames) {
> > -        int i;
> > -
> > -        ds_put_cstr(&ds, " (backtrace:");
> > -        for (i = 0; i < backtrace.n_frames; i++) {
> > -            ds_put_format(&ds, " 0x%08"PRIxPTR, backtrace.frames[i]);
> > -        }
> > -        ds_put_char(&ds, ')');
> > -    }
> > +    backtrace_format(&ds, &backtrace, ", ");
> > +    ds_put_char(&ds, ')');
>
> Why printing an empty backtrace?
>

Updated in v7.


>
> >
> >      ds_put_format(&ds, " (%s %s)", program_name, VERSION);
> >
> > diff --git a/lib/vlog.c b/lib/vlog.c
> > index 9ddea48b8..b2653142f 100644
> > --- a/lib/vlog.c
> > +++ b/lib/vlog.c
> > @@ -664,6 +664,13 @@ vlog_direct_write_to_log_file_unsafe(const char *s)
> >      }
> >  }
> >
> > +int
> > +vlog_get_log_file_fd_unsafe(void)
> > +    OVS_NO_THREAD_SAFETY_ANALYSIS
> > +{
> > +    return log_fd;
> > +}
> > +
> >  /* Returns 'false' if 'facility' is not a valid string. If 'facility'
> >   * is a valid string, sets 'value' with the integer value of 'facility'
> >   * and returns 'true'. */
> > diff --git a/m4/openvswitch.m4 b/m4/openvswitch.m4
> > index 14d9249b8..f4943b140 100644
> > --- a/m4/openvswitch.m4
> > +++ b/m4/openvswitch.m4
> > @@ -359,9 +359,12 @@ AC_DEFUN([OVS_CHECK_DBDIR],
> >
> >  dnl Defines HAVE_BACKTRACE if backtrace() is found.
> >  AC_DEFUN([OVS_CHECK_BACKTRACE],
> > -  [AC_SEARCH_LIBS([backtrace], [execinfo ubacktrace],
> > -                  [AC_DEFINE([HAVE_BACKTRACE], [1],
> > -                             [Define to 1 if you have
> backtrace(3).])])])
> > +  [AC_SEARCH_LIBS([backtrace], [execinfo ubacktrace],
> [HAVE_BACKTRACE=yes], [HAVE_BACKTRACE=no])
> > +   if test "$HAVE_BACKTRACE" = "yes"; then
> > +     AC_DEFINE([HAVE_BACKTRACE], [1], [Define to 1 if you have
> backtrace(3).])
> > +   fi
> > +   AM_CONDITIONAL([HAVE_BACKTRACE], [test "$HAVE_BACKTRACE" = "yes"])
> > +   AC_SUBST([HAVE_BACKTRACE])])
> >
> >  dnl Defines HAVE_PERF_EVENT if linux/perf_event.h is found.
> >  AC_DEFUN([OVS_CHECK_PERF_EVENT],
> > diff --git a/tests/atlocal.in b/tests/atlocal.in
> > index 859668586..18d5efae0 100644
> > --- a/tests/atlocal.in
> > +++ b/tests/atlocal.in
> > @@ -2,6 +2,8 @@
> >  HAVE_OPENSSL='@HAVE_OPENSSL@'
> >  OPENSSL_SUPPORTS_SNI='@OPENSSL_SUPPORTS_SNI@'
> >  HAVE_UNBOUND='@HAVE_UNBOUND@'
> > +HAVE_BACKTRACE='@HAVE_BACKTRACE@'
> > +HAVE_UNWIND='@HAVE_UNWIND@'
> >  EGREP='@EGREP@'
> >  PYTHON3='@PYTHON3@'
> >  CFLAGS='@CFLAGS@'
> > diff --git a/tests/daemon.at b/tests/daemon.at
> > index d7981f9d2..1e03bf878 100644
> > --- a/tests/daemon.at
> > +++ b/tests/daemon.at
> > @@ -234,3 +234,53 @@ OVS_WAIT_UNTIL([sc query ovsdb-server | grep STATE
> | grep STOPPED > /dev/null 2>
> >  AT_CHECK([sc delete ovsdb-server], [0], [[[SC]] DeleteService SUCCESS
> >  ])
> >  AT_CLEANUP
> > +
> > +AT_SETUP([backtrace without monitor])
> > +AT_SKIP_IF([test "$HAVE_BACKTRACE" = "no" && test "$HAVE_UNWIND" =
> "no"])
> > +AT_SKIP_IF([test "$IS_WIN32" = "yes"])
> > +
> > +# This test intentionally causes SIGSEGV, so make Address Sanitizer
> ignore it.
> > +ASAN_OPTIONS=$ASAN_OPTIONS:handle_segv=0; export ASAN_OPTIONS
> > +
> > +# Skip it if UB Sanitizer is being used.  There's no way to disable the
> > +# SEGV check at runtime.
> > +AT_SKIP_IF([test $TESTS_WITH_UBSAN = yes])
> > +
> > +AT_CHECK([ovsdb-server --detach --no-chdir --pidfile --no-db --log-file
> --verbose=DBG], [0], [ignore], [ignore])
> > +
> > +OVS_WAIT_UNTIL([test -s ovsdb-server.pid])
> > +child=$(cat ovsdb-server.pid)
> > +
> > +AT_CAPTURE_FILE([ovsdb-server.log])
> > +
> > +AT_CHECK([kill -SEGV $child])
> > +
> > +OVS_WAIT_UNTIL([grep -q "^SIGSEGV detected, backtrace:"
> ovsdb-server.log])
> > +
> > +AT_CLEANUP
> > +
> > +AT_SETUP([backtrace with monitor])
> > +AT_SKIP_IF([test "$HAVE_BACKTRACE" = "no" && test "$HAVE_UNWIND" =
> "no"])
> > +AT_SKIP_IF([test "$IS_WIN32" = "yes"])
> > +
> > +# This test intentionally causes SIGSEGV, so make Address Sanitizer
> ignore it.
> > +ASAN_OPTIONS=$ASAN_OPTIONS:handle_segv=0; export ASAN_OPTIONS
> > +
> > +# Skip it if UB Sanitizer is being used.  There's no way to disable the
> > +# SEGV check at runtime.
> > +AT_SKIP_IF([test $TESTS_WITH_UBSAN = yes])
> > +
> > +on_exit 'kill $(cat *.pid)'
> > +
> > +AT_CHECK([ovsdb-server --detach --monitor --no-chdir --pidfile --no-db
> --log-file --verbose=DBG], [0], [ignore], [ignore])
> > +OVS_WAIT_UNTIL([test -s ovsdb-server.pid])
> > +child=$(cat ovsdb-server.pid)
> > +
> > +AT_CAPTURE_FILE([ovsdb-server.log])
> > +
> > +AT_CHECK([kill -SEGV $child])
> > +
> > +OVS_WAIT_UNTIL([grep -q "backtrace(monitor)|WARN|SIGSEGV detected,
> backtrace:" ovsdb-server.log])
> > +OVS_WAIT_UNTIL([grep -q "daemon_unix(monitor)|ERR|1 crashes: pid .*
> died, killed (Segmentation fault)" ovsdb-server.log])
> > +
> > +AT_CLEANUP
>
> Thanks,
Ales