Patchwork [v2,3/3] qemu-config: Add new -add-fd command line option

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Submitter Corey Bryant
Date Oct. 10, 2012, 2:20 p.m.
Message ID <1349878805-16352-4-git-send-email-coreyb@linux.vnet.ibm.com>
Download mbox | patch
Permalink /patch/190652/
State New
Headers show

Comments

Corey Bryant - Oct. 10, 2012, 2:20 p.m.
This option can be used for passing file descriptors on the
command line.  It mirrors the existing add-fd QMP command which
allows an fd to be passed to QEMU via SCM_RIGHTS and added to an
fd set.

This can be combined with commands such as -drive to link file
descriptors in an fd set to a drive:

    qemu-kvm -add-fd fd=4,set=2,opaque="rdwr:/path/to/file"
             -add-fd fd=5,set=2,opaque="rdonly:/path/to/file"
             -drive file=/dev/fdset/2,index=0,media=disk

This example adds fds 4 and 5, and the accompanying opaque
strings to the fd set with ID=2.  qemu_open() already knows how
to handle a filename of this format.  qemu_open() searches the
corresponding fd set for an fd and when it finds a match, QEMU
goes on to use a dup of that fd just like it would have used an
fd that it opened itself.

Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
---
v2:
 - The -add-fd option is new in v2 (eblake@redhat.com)

 qemu-config.c   | 22 ++++++++++++++++++++++
 qemu-options.hx | 35 +++++++++++++++++++++++++++++++++++
 vl.c            | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 93 insertions(+)
Eric Blake - Oct. 10, 2012, 10:31 p.m.
On 10/10/2012 08:20 AM, Corey Bryant wrote:
> This option can be used for passing file descriptors on the
> command line.  It mirrors the existing add-fd QMP command which
> allows an fd to be passed to QEMU via SCM_RIGHTS and added to an
> fd set.
> 
> This can be combined with commands such as -drive to link file
> descriptors in an fd set to a drive:
> 
>     qemu-kvm -add-fd fd=4,set=2,opaque="rdwr:/path/to/file"
>              -add-fd fd=5,set=2,opaque="rdonly:/path/to/file"
>              -drive file=/dev/fdset/2,index=0,media=disk
> 
> This example adds fds 4 and 5, and the accompanying opaque
> strings to the fd set with ID=2.  qemu_open() already knows how
> to handle a filename of this format.  qemu_open() searches the
> corresponding fd set for an fd and when it finds a match, QEMU
> goes on to use a dup of that fd just like it would have used an
> fd that it opened itself.

Missing some argument validation.  Earlier, I complained about set
validation, now I'm going to complain about fd validation.

> +static int parse_add_fd(QemuOpts *opts, void *opaque)
> +{
> +    int fd;
> +    int64_t fdset_id;
> +    const char *fd_opaque = NULL;
> +
> +    fd = qemu_opt_get_number(opts, "fd", -1);
> +    fdset_id = qemu_opt_get_number(opts, "set", -1);
> +    fd_opaque = qemu_opt_get(opts, "opaque");

If I call 'qemu -add-fd fd=-2,set=1', that had better fail because fd -2
does not exist.  Likewise if I call 'qemu -add-fd fd=10000000,set=1'
(here, picking an fd that I know can't be opened).  More subtly, 'qemu
-add-fd fd=4,set=1 4<&-' should fail, since fd 4 was not inherited down
to the qemu process.  Fuzzier is whether 'qemu -add-fd fd=0,set=1' makes
sense, as that would then make stdin be competing for multiple uses;
this would be a situation that the monitor command can't duplicate, so
you have introduced new ways to possibly break things from the command
line.  I'm thinking it's safest to reject fd of 0, 1, or 2 for now, and
only relax it later IF we can prove that it would be both useful and safe.

So it sounds like you need something like fcntl(fd,F_GETFL) to see that
an the fd was actually inherited, as well as validating that fd >
STDERR_FILENO.

Another missing validation check is for duplicate use.  With the monitor
command, you ALWAYS have a unique fd (thanks to SCM_RIGHTS).  But with
the command line, I can type 'qemu -add-fd fd=4,set=1 -add-fd
fd=4,set=2'.  Oops - I've now corrupted your set layout, unless you
validate that every fd requested in -add-fd does not already reside in
any existing set.



Thinking aloud:
On the other hand, being able to pass in one fd to multiple sets MIGHT
be useful; in the SCM_RIGHTS monitor command case, I can pass the same
fd from the management perspective into multiple sets, even though in
qemu's perspective, there will be multiple fds created (one per call).
Perhaps instead of directly adding the inherited fd to a set, and having
to then sweep all sets to check for duplicates, it might make sense to
add dup(fd) to a set, so that if I call:

qemu -add-fd fd=4,set=1 -add-fd fd=4,set=2 -add-fd fd=5,set=2

what REALLY happens is that qemu adds dup(4)==6 to set 1, dup(4)==7 to
set 2, and dup(5)==8 to set 3.  Then, after all ALL -add-fd have been
processed, qemu then does another pass through them calling close(4) and
close(5) (to avoid holding the original fds open indefinitely if the
corresponding sets are discarded).  Hmm, notice that if you dup() before
adding to a set, then that the dup() resolves my first complaint of
validating that the inherited fd exists; it also changes the problem of
dealing with fd 0 (it would now be valid to add fd 0 to any number of
sets; but a final cleanup loop had better be careful to not call
close(0) unintentionally).

Personally, though, I'm not sure the complexity of using dup() buys us
anything, so I'm happy with the simpler solution of using fd as-is,
coupled with a check for no reuse of an fd already in a set.
Corey Bryant - Oct. 11, 2012, 2:45 p.m.
On 10/10/2012 06:31 PM, Eric Blake wrote:
> On 10/10/2012 08:20 AM, Corey Bryant wrote:
>> This option can be used for passing file descriptors on the
>> command line.  It mirrors the existing add-fd QMP command which
>> allows an fd to be passed to QEMU via SCM_RIGHTS and added to an
>> fd set.
>>
>> This can be combined with commands such as -drive to link file
>> descriptors in an fd set to a drive:
>>
>>      qemu-kvm -add-fd fd=4,set=2,opaque="rdwr:/path/to/file"
>>               -add-fd fd=5,set=2,opaque="rdonly:/path/to/file"
>>               -drive file=/dev/fdset/2,index=0,media=disk
>>
>> This example adds fds 4 and 5, and the accompanying opaque
>> strings to the fd set with ID=2.  qemu_open() already knows how
>> to handle a filename of this format.  qemu_open() searches the
>> corresponding fd set for an fd and when it finds a match, QEMU
>> goes on to use a dup of that fd just like it would have used an
>> fd that it opened itself.
>
> Missing some argument validation.  Earlier, I complained about set
> validation, now I'm going to complain about fd validation.
>
>> +static int parse_add_fd(QemuOpts *opts, void *opaque)
>> +{
>> +    int fd;
>> +    int64_t fdset_id;
>> +    const char *fd_opaque = NULL;
>> +
>> +    fd = qemu_opt_get_number(opts, "fd", -1);
>> +    fdset_id = qemu_opt_get_number(opts, "set", -1);
>> +    fd_opaque = qemu_opt_get(opts, "opaque");
>
> If I call 'qemu -add-fd fd=-2,set=1', that had better fail because fd -2
> does not exist.  Likewise if I call 'qemu -add-fd fd=10000000,set=1'
> (here, picking an fd that I know can't be opened).  More subtly, 'qemu
> -add-fd fd=4,set=1 4<&-' should fail, since fd 4 was not inherited down
> to the qemu process.  Fuzzier is whether 'qemu -add-fd fd=0,set=1' makes
> sense, as that would then make stdin be competing for multiple uses;
> this would be a situation that the monitor command can't duplicate, so
> you have introduced new ways to possibly break things from the command
> line.  I'm thinking it's safest to reject fd of 0, 1, or 2 for now, and
> only relax it later IF we can prove that it would be both useful and safe.
>
> So it sounds like you need something like fcntl(fd,F_GETFL) to see that
> an the fd was actually inherited, as well as validating that fd >
> STDERR_FILENO.

I agree.  I'll try this approach.

>
> Another missing validation check is for duplicate use.  With the monitor
> command, you ALWAYS have a unique fd (thanks to SCM_RIGHTS).  But with
> the command line, I can type 'qemu -add-fd fd=4,set=1 -add-fd
> fd=4,set=2'.  Oops - I've now corrupted your set layout, unless you
> validate that every fd requested in -add-fd does not already reside in
> any existing set.
>

I don't see this validation check for duplicate use of fd's being 
necessary.  Like you say below, in the QMP add-fd case we can add the 
same fd multiple times.  So we should be able to add the same fd 
multiple times via the command line.  The only difference between QMP 
and command line in this case is that the QMP fd is a dup and therefore 
a different number and the command line fd will be the same fd.  I'd 
prefer to leave this alone unless there's a compelling reason to block 
adding of the same fd.

>
>
> Thinking aloud:
> On the other hand, being able to pass in one fd to multiple sets MIGHT
> be useful; in the SCM_RIGHTS monitor command case, I can pass the same
> fd from the management perspective into multiple sets, even though in
> qemu's perspective, there will be multiple fds created (one per call).
> Perhaps instead of directly adding the inherited fd to a set, and having
> to then sweep all sets to check for duplicates, it might make sense to
> add dup(fd) to a set, so that if I call:
>
> qemu -add-fd fd=4,set=1 -add-fd fd=4,set=2 -add-fd fd=5,set=2
>
> what REALLY happens is that qemu adds dup(4)==6 to set 1, dup(4)==7 to
> set 2, and dup(5)==8 to set 3.  Then, after all ALL -add-fd have been
> processed, qemu then does another pass through them calling close(4) and
> close(5) (to avoid holding the original fds open indefinitely if the
> corresponding sets are discarded).  Hmm, notice that if you dup() before
> adding to a set, then that the dup() resolves my first complaint of
> validating that the inherited fd exists; it also changes the problem of
> dealing with fd 0 (it would now be valid to add fd 0 to any number of
> sets; but a final cleanup loop had better be careful to not call
> close(0) unintentionally).
>
> Personally, though, I'm not sure the complexity of using dup() buys us
> anything, so I'm happy with the simpler solution of using fd as-is,
> coupled with a check for no reuse of an fd already in a set.
>
Eric Blake - Oct. 11, 2012, 3:55 p.m.
On 10/11/2012 08:45 AM, Corey Bryant wrote:

>> Another missing validation check is for duplicate use.  With the monitor
>> command, you ALWAYS have a unique fd (thanks to SCM_RIGHTS).  But with
>> the command line, I can type 'qemu -add-fd fd=4,set=1 -add-fd
>> fd=4,set=2'.  Oops - I've now corrupted your set layout, unless you
>> validate that every fd requested in -add-fd does not already reside in
>> any existing set.
>>
> 
> I don't see this validation check for duplicate use of fd's being
> necessary.  Like you say below, in the QMP add-fd case we can add the
> same fd multiple times.  So we should be able to add the same fd
> multiple times via the command line.  The only difference between QMP
> and command line in this case is that the QMP fd is a dup and therefore
> a different number and the command line fd will be the same fd.  I'd
> prefer to leave this alone unless there's a compelling reason to block
> adding of the same fd.

There is a compelling reason to prevent duplicates among your sets:
qemu_close().

Suppose I add fd 4 into set 1 and 2, and then discard set 2 via monitor
commands.  Then, when qemu_close() drops the last reference to set 2, it
steps through and calls close() on all fds in that set, including fd 4.
 Oops - now set 1 is invalid, because it is tracking a closed fd.  And
worse, if qemu then does something else to open a new fd, it will get fd
4 again, and now set 1 will be tracking the WRONG fd.
Eric Blake - Oct. 11, 2012, 4:04 p.m.
On 10/10/2012 04:31 PM, Eric Blake wrote:

> Another missing validation check is for duplicate use.  With the monitor
> command, you ALWAYS have a unique fd (thanks to SCM_RIGHTS).  But with
> the command line, I can type 'qemu -add-fd fd=4,set=1 -add-fd
> fd=4,set=2'.  Oops - I've now corrupted your set layout, unless you
> validate that every fd requested in -add-fd does not already reside in
> any existing set.
> 
> Thinking aloud:
> On the other hand, being able to pass in one fd to multiple sets MIGHT
> be useful; in the SCM_RIGHTS monitor command case, I can pass the same
> fd from the management perspective into multiple sets, even though in
> qemu's perspective, there will be multiple fds created (one per call).
> Perhaps instead of directly adding the inherited fd to a set, and having
> to then sweep all sets to check for duplicates, it might make sense to
> add dup(fd) to a set, so that if I call:
> 
> qemu -add-fd fd=4,set=1 -add-fd fd=4,set=2 -add-fd fd=5,set=2
> 
> what REALLY happens is that qemu adds dup(4)==6 to set 1, dup(4)==7 to
> set 2, and dup(5)==8 to set 3.  Then, after all ALL -add-fd have been
> processed, qemu then does another pass through them calling close(4) and
> close(5) (to avoid holding the original fds open indefinitely if the
> corresponding sets are discarded).

Another idea: a hybrid approach - the _first_ -add-fd 4 directly adds 4
to the set, all other -add-fd 4 end up adding dup(4) instead (well,
fcntl(F_DUPFD_CLOEXEC), but you get the picture).  That is, do the
duplicate scanning, and if there is no duplicate, use the fd directly;
if there IS a duplicate, then put a unique fd number as a copy into the
remaining sets.  That way, you don't have to do a final close() sweep
across the -add-fd arguments passed on the command line, and you still
don't have to worry about duplicated fds across multiple sets causing
mayhem in qemu_close().
Eric Blake - Oct. 11, 2012, 4:11 p.m.
On 10/11/2012 10:04 AM, Eric Blake wrote:
> Another idea: a hybrid approach - the _first_ -add-fd 4 directly adds 4
> to the set, all other -add-fd 4 end up adding dup(4) instead (well,
> fcntl(F_DUPFD_CLOEXEC), but you get the picture).  That is, do the
> duplicate scanning, and if there is no duplicate, use the fd directly;
> if there IS a duplicate, then put a unique fd number as a copy into the
> remaining sets.  That way, you don't have to do a final close() sweep
> across the -add-fd arguments passed on the command line, and you still
> don't have to worry about duplicated fds across multiple sets causing
> mayhem in qemu_close().

Hmm, you may also need to be careful of corner cases.  If I do:

qemu -add-fd fd=4,set=1 -add-fd fd=4,set=2 -add-fd fd=5,set=3 5<&-

with fd 5 not inherited, then a dup(4) would give 5; you don't want to
accidentally add a copy of fd 4 into set 3, but rather fail because fd 5
was not inherited.
Corey Bryant - Oct. 11, 2012, 5:36 p.m.
On 10/11/2012 11:55 AM, Eric Blake wrote:
> On 10/11/2012 08:45 AM, Corey Bryant wrote:
>
>>> Another missing validation check is for duplicate use.  With the monitor
>>> command, you ALWAYS have a unique fd (thanks to SCM_RIGHTS).  But with
>>> the command line, I can type 'qemu -add-fd fd=4,set=1 -add-fd
>>> fd=4,set=2'.  Oops - I've now corrupted your set layout, unless you
>>> validate that every fd requested in -add-fd does not already reside in
>>> any existing set.
>>>
>>
>> I don't see this validation check for duplicate use of fd's being
>> necessary.  Like you say below, in the QMP add-fd case we can add the
>> same fd multiple times.  So we should be able to add the same fd
>> multiple times via the command line.  The only difference between QMP
>> and command line in this case is that the QMP fd is a dup and therefore
>> a different number and the command line fd will be the same fd.  I'd
>> prefer to leave this alone unless there's a compelling reason to block
>> adding of the same fd.
>
> There is a compelling reason to prevent duplicates among your sets:
> qemu_close().
>
> Suppose I add fd 4 into set 1 and 2, and then discard set 2 via monitor
> commands.  Then, when qemu_close() drops the last reference to set 2, it
> steps through and calls close() on all fds in that set, including fd 4.
>   Oops - now set 1 is invalid, because it is tracking a closed fd.  And
> worse, if qemu then does something else to open a new fd, it will get fd
> 4 again, and now set 1 will be tracking the WRONG fd.
>

Ah yes, that is compelling.  So we do need something here.  I'll reply 
to your other email regarding the approach to take.
Corey Bryant - Oct. 11, 2012, 5:49 p.m.
On 10/11/2012 12:04 PM, Eric Blake wrote:
> On 10/10/2012 04:31 PM, Eric Blake wrote:
>
>> Another missing validation check is for duplicate use.  With the monitor
>> command, you ALWAYS have a unique fd (thanks to SCM_RIGHTS).  But with
>> the command line, I can type 'qemu -add-fd fd=4,set=1 -add-fd
>> fd=4,set=2'.  Oops - I've now corrupted your set layout, unless you
>> validate that every fd requested in -add-fd does not already reside in
>> any existing set.
>>
>> Thinking aloud:
>> On the other hand, being able to pass in one fd to multiple sets MIGHT
>> be useful; in the SCM_RIGHTS monitor command case, I can pass the same
>> fd from the management perspective into multiple sets, even though in
>> qemu's perspective, there will be multiple fds created (one per call).
>> Perhaps instead of directly adding the inherited fd to a set, and having
>> to then sweep all sets to check for duplicates, it might make sense to
>> add dup(fd) to a set, so that if I call:
>>
>> qemu -add-fd fd=4,set=1 -add-fd fd=4,set=2 -add-fd fd=5,set=2
>>
>> what REALLY happens is that qemu adds dup(4)==6 to set 1, dup(4)==7 to
>> set 2, and dup(5)==8 to set 3.  Then, after all ALL -add-fd have been
>> processed, qemu then does another pass through them calling close(4) and
>> close(5) (to avoid holding the original fds open indefinitely if the
>> corresponding sets are discarded).
>
> Another idea: a hybrid approach - the _first_ -add-fd 4 directly adds 4
> to the set, all other -add-fd 4 end up adding dup(4) instead (well,
> fcntl(F_DUPFD_CLOEXEC), but you get the picture).  That is, do the
> duplicate scanning, and if there is no duplicate, use the fd directly;
> if there IS a duplicate, then put a unique fd number as a copy into the
> remaining sets.  That way, you don't have to do a final close() sweep
> across the -add-fd arguments passed on the command line, and you still
> don't have to worry about duplicated fds across multiple sets causing
> mayhem in qemu_close().
>

This would simplify the code, but I wonder if it would be confusing to 
users when they call query-fdsets and only see a single fd 4.  It may 
make more sense to dup all fds that are passed with -add-fd, and then it 
basically works the same as the QMP add-fd via SCM_RIGHTS.

On a somewhat related note, one major difference between the QMP add-fd 
and  command line -add-fd, is that -add-fd doesn't return the fd that 
was added.  So opaque strings will be even more important when passing 
fds on the command line.

Patch

diff --git a/qemu-config.c b/qemu-config.c
index cd1ec21..0bd67ca 100644
--- a/qemu-config.c
+++ b/qemu-config.c
@@ -653,6 +653,27 @@  QemuOptsList qemu_boot_opts = {
     },
 };
 
+static QemuOptsList qemu_add_fd_opts = {
+    .name = "add-fd",
+    .head = QTAILQ_HEAD_INITIALIZER(qemu_add_fd_opts.head),
+    .desc = {
+        {
+            .name = "fd",
+            .type = QEMU_OPT_NUMBER,
+            .help = "file descriptor to add to fd set",
+        },{
+            .name = "set",
+            .type = QEMU_OPT_NUMBER,
+            .help = "ID of the fd set to add fd to",
+        },{
+            .name = "opaque",
+            .type = QEMU_OPT_STRING,
+            .help = "free-form string used to describe fd",
+        },
+        { /* end of list */ }
+    },
+};
+
 static QemuOptsList *vm_config_groups[32] = {
     &qemu_drive_opts,
     &qemu_chardev_opts,
@@ -669,6 +690,7 @@  static QemuOptsList *vm_config_groups[32] = {
     &qemu_boot_opts,
     &qemu_iscsi_opts,
     &qemu_sandbox_opts,
+    &qemu_add_fd_opts,
     NULL,
 };
 
diff --git a/qemu-options.hx b/qemu-options.hx
index 7d97f96..884fcb6 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -257,6 +257,14 @@  qemu-system-i386 -drive file=file,index=2,media=disk
 qemu-system-i386 -drive file=file,index=3,media=disk
 @end example
 
+You can open an image using pre-opened file descriptors from an fd set:
+@example
+qemu-system-i386
+-add-fd fd=4,set=2,opaque="rdwr:/path/to/file"
+-add-fd fd=5,set=2,opaque="rdonly:/path/to/file"
+-drive file=/dev/fdset/2,index=0,media=disk
+@end example
+
 You can connect a CDROM to the slave of ide0:
 @example
 qemu-system-i386 -drive file=file,if=ide,index=1,media=cdrom
@@ -289,6 +297,33 @@  qemu-system-i386 -hda a -hdb b
 @end example
 ETEXI
 
+DEF("add-fd", HAS_ARG, QEMU_OPTION_add_fd,
+    "-add-fd fd=fd,set=set[,opaque=opaque]\n"
+    "                Add 'fd' to fd 'set'\n", QEMU_ARCH_ALL)
+STEXI
+@item -add-fd fd=@var{fd},set=@var{set}[,opaque=@var{opaque}]
+@findex -add-fd
+
+Add a file descriptor to an fd set.  Valid options are:
+
+@table @option
+@item fd=@var{fd}
+This option defines the file descriptor that is to be added to the fd set.
+@item set=@var{set}
+This option defines the ID of the fd set to add the file descriptor to.
+@item opaque=@var{opaque}
+This option defines a free-form string that can be used to describe @var{fd}.
+@end table
+
+You can open an image using pre-opened file descriptors from an fd set:
+@example
+qemu-system-i386
+-add-fd fd=4,set=2,opaque="rdwr:/path/to/file"
+-add-fd fd=5,set=2,opaque="rdonly:/path/to/file"
+-drive file=/dev/fdset/2,index=0,media=disk
+@end example
+ETEXI
+
 DEF("set", HAS_ARG, QEMU_OPTION_set,
     "-set group.id.arg=value\n"
     "                set <arg> parameter for item <id> of type <group>\n"
diff --git a/vl.c b/vl.c
index 8d305ca..0265712 100644
--- a/vl.c
+++ b/vl.c
@@ -790,6 +790,33 @@  static int parse_sandbox(QemuOpts *opts, void *opaque)
     return 0;
 }
 
+static int parse_add_fd(QemuOpts *opts, void *opaque)
+{
+    int fd;
+    int64_t fdset_id;
+    const char *fd_opaque = NULL;
+
+    fd = qemu_opt_get_number(opts, "fd", -1);
+    fdset_id = qemu_opt_get_number(opts, "set", -1);
+    fd_opaque = qemu_opt_get(opts, "opaque");
+
+    if (fd == -1) {
+        qerror_report(ERROR_CLASS_GENERIC_ERROR, "fd option is required");
+        return -1;
+    }
+
+    if (fdset_id == -1) {
+        qerror_report(ERROR_CLASS_GENERIC_ERROR, "set option is required");
+        return -1;
+    }
+
+    /* add the fd, and optionally the opaque string, to the fd set */
+    monitor_fdset_add_fd(fd, true, fdset_id, fd_opaque ? true : false,
+                         fd_opaque);
+
+    return 0;
+}
+
 /***********************************************************/
 /* QEMU Block devices */
 
@@ -3299,6 +3326,11 @@  int main(int argc, char **argv, char **envp)
                     exit(0);
                 }
                 break;
+            case QEMU_OPTION_add_fd:
+                opts = qemu_opts_parse(qemu_find_opts("add-fd"), optarg, 0);
+                if (!opts) {
+                    exit(0);
+                }
             default:
                 os_parse_cmd_args(popt->index, optarg);
             }
@@ -3310,6 +3342,10 @@  int main(int argc, char **argv, char **envp)
         exit(1);
     }
 
+    if (qemu_opts_foreach(qemu_find_opts("add-fd"), parse_add_fd, NULL, 1)) {
+        exit(1);
+    }
+
     if (machine == NULL) {
         fprintf(stderr, "No machine found.\n");
         exit(1);