[5/5] nptl: Add public rseq symbols and <sys/rseq.h>

The relationship between the thread pointer and the rseq area
is made explicit.  The constant offset can be used by JIT compilers
to optimize rseq access (e.g., for really fast sched_getcpu).

Extensibility is provided through __rseq_size and __rseq_flags.
(In the future, the kernel could request a different rseq size
via the auxiliary vector.)

Co-Authored-By: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Tested on aarch64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
s390x-linux-gnu, x86_64-linux-gnu.  Built with build-many-glibcs.py
(except for a late fix to actually install the public headers).

---
 NEWS                                               | 11 +++
 manual/threads.texi                                | 81 ++++++++++++++++++++++
 sysdeps/nptl/dl-tls_init_tp.c                      | 18 ++++-
 sysdeps/unix/sysv/linux/Makefile                   |  3 +-
 sysdeps/unix/sysv/linux/Versions                   |  5 ++
 sysdeps/unix/sysv/linux/aarch64/ld.abilist         |  3 +
 sysdeps/unix/sysv/linux/alpha/ld.abilist           |  3 +
 sysdeps/unix/sysv/linux/arc/ld.abilist             |  3 +
 sysdeps/unix/sysv/linux/arm/be/ld.abilist          |  3 +
 sysdeps/unix/sysv/linux/arm/le/ld.abilist          |  3 +
 sysdeps/unix/sysv/linux/csky/ld.abilist            |  3 +
 sysdeps/unix/sysv/linux/hppa/ld.abilist            |  3 +
 sysdeps/unix/sysv/linux/i386/ld.abilist            |  3 +
 sysdeps/unix/sysv/linux/ia64/ld.abilist            |  3 +
 sysdeps/unix/sysv/linux/m68k/coldfire/ld.abilist   |  3 +
 sysdeps/unix/sysv/linux/m68k/m680x0/ld.abilist     |  3 +
 sysdeps/unix/sysv/linux/microblaze/ld.abilist      |  3 +
 sysdeps/unix/sysv/linux/mips/mips32/ld.abilist     |  3 +
 sysdeps/unix/sysv/linux/mips/mips64/n32/ld.abilist |  3 +
 sysdeps/unix/sysv/linux/mips/mips64/n64/ld.abilist |  3 +
 sysdeps/unix/sysv/linux/nios2/ld.abilist           |  3 +
 .../unix/sysv/linux/powerpc/powerpc32/ld.abilist   |  3 +
 .../sysv/linux/powerpc/powerpc64/be/ld.abilist     |  3 +
 .../sysv/linux/powerpc/powerpc64/le/ld.abilist     |  3 +
 sysdeps/unix/sysv/linux/riscv/rv32/ld.abilist      |  3 +
 sysdeps/unix/sysv/linux/riscv/rv64/ld.abilist      |  3 +
 sysdeps/unix/sysv/linux/rseq-internal.h            |  8 ++-
 sysdeps/unix/sysv/linux/s390/s390-32/ld.abilist    |  3 +
 sysdeps/unix/sysv/linux/s390/s390-64/ld.abilist    |  3 +
 sysdeps/unix/sysv/linux/sh/be/ld.abilist           |  3 +
 sysdeps/unix/sysv/linux/sh/le/ld.abilist           |  3 +
 sysdeps/unix/sysv/linux/sparc/sparc32/ld.abilist   |  3 +
 sysdeps/unix/sysv/linux/sparc/sparc64/ld.abilist   |  3 +
 sysdeps/unix/sysv/linux/sys/rseq.h                 | 10 +++
 sysdeps/unix/sysv/linux/tst-rseq-disable.c         |  6 ++
 sysdeps/unix/sysv/linux/tst-rseq.c                 |  8 +++
 sysdeps/unix/sysv/linux/x86_64/64/ld.abilist       |  3 +
 sysdeps/unix/sysv/linux/x86_64/x32/ld.abilist      |  3 +
 38 files changed, 232 insertions(+), 5 deletions(-)

The 12/06/2021 14:53, Florian Weimer via Libc-alpha wrote:
> The relationship between the thread pointer and the rseq area
> is made explicit.  The constant offset can be used by JIT compilers
> to optimize rseq access (e.g., for really fast sched_getcpu).
> 
> Extensibility is provided through __rseq_size and __rseq_flags.
> (In the future, the kernel could request a different rseq size
> via the auxiliary vector.)
> 
> Co-Authored-By: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> 
> Tested on aarch64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
> s390x-linux-gnu, x86_64-linux-gnu.  Built with build-many-glibcs.py
> (except for a late fix to actually install the public headers).

minor comments below.

> 
> ---
>  NEWS                                               | 11 +++
>  manual/threads.texi                                | 81 ++++++++++++++++++++++
>  sysdeps/nptl/dl-tls_init_tp.c                      | 18 ++++-
>  sysdeps/unix/sysv/linux/Makefile                   |  3 +-
>  sysdeps/unix/sysv/linux/Versions                   |  5 ++
>  sysdeps/unix/sysv/linux/aarch64/ld.abilist         |  3 +
>  sysdeps/unix/sysv/linux/alpha/ld.abilist           |  3 +
>  sysdeps/unix/sysv/linux/arc/ld.abilist             |  3 +
>  sysdeps/unix/sysv/linux/arm/be/ld.abilist          |  3 +
>  sysdeps/unix/sysv/linux/arm/le/ld.abilist          |  3 +
>  sysdeps/unix/sysv/linux/csky/ld.abilist            |  3 +
>  sysdeps/unix/sysv/linux/hppa/ld.abilist            |  3 +
>  sysdeps/unix/sysv/linux/i386/ld.abilist            |  3 +
>  sysdeps/unix/sysv/linux/ia64/ld.abilist            |  3 +
>  sysdeps/unix/sysv/linux/m68k/coldfire/ld.abilist   |  3 +
>  sysdeps/unix/sysv/linux/m68k/m680x0/ld.abilist     |  3 +
>  sysdeps/unix/sysv/linux/microblaze/ld.abilist      |  3 +
>  sysdeps/unix/sysv/linux/mips/mips32/ld.abilist     |  3 +
>  sysdeps/unix/sysv/linux/mips/mips64/n32/ld.abilist |  3 +
>  sysdeps/unix/sysv/linux/mips/mips64/n64/ld.abilist |  3 +
>  sysdeps/unix/sysv/linux/nios2/ld.abilist           |  3 +
>  .../unix/sysv/linux/powerpc/powerpc32/ld.abilist   |  3 +
>  .../sysv/linux/powerpc/powerpc64/be/ld.abilist     |  3 +
>  .../sysv/linux/powerpc/powerpc64/le/ld.abilist     |  3 +
>  sysdeps/unix/sysv/linux/riscv/rv32/ld.abilist      |  3 +
>  sysdeps/unix/sysv/linux/riscv/rv64/ld.abilist      |  3 +
>  sysdeps/unix/sysv/linux/rseq-internal.h            |  8 ++-
>  sysdeps/unix/sysv/linux/s390/s390-32/ld.abilist    |  3 +
>  sysdeps/unix/sysv/linux/s390/s390-64/ld.abilist    |  3 +
>  sysdeps/unix/sysv/linux/sh/be/ld.abilist           |  3 +
>  sysdeps/unix/sysv/linux/sh/le/ld.abilist           |  3 +
>  sysdeps/unix/sysv/linux/sparc/sparc32/ld.abilist   |  3 +
>  sysdeps/unix/sysv/linux/sparc/sparc64/ld.abilist   |  3 +
>  sysdeps/unix/sysv/linux/sys/rseq.h                 | 10 +++
>  sysdeps/unix/sysv/linux/tst-rseq-disable.c         |  6 ++
>  sysdeps/unix/sysv/linux/tst-rseq.c                 |  8 +++
>  sysdeps/unix/sysv/linux/x86_64/64/ld.abilist       |  3 +
>  sysdeps/unix/sysv/linux/x86_64/x32/ld.abilist      |  3 +
>  38 files changed, 232 insertions(+), 5 deletions(-)
> 
> diff --git a/NEWS b/NEWS
> index 1398cf2e87..8744a92532 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -68,6 +68,17 @@ Major new features:
>    to be used by compilers for optimizing usage of 'memcmp' when its
>    return value is only used for its boolean status.
>  
> +* Support for automatically registering threads with the Linux rseq
> +  system call has been added.  This system call is implemented starting
> +  from Linux 4.18.  The Restartable Sequences ABI accelerates user-space
> +  operations on per-cpu data.  It allows user-space to perform updates
> +  on per-cpu data without requiring heavy-weight atomic operations.
> +  Automatically registering threads allows all libraries, including
> +  libc, to make immediate use of the rseq support by using the
> +  documented ABI, via the __rseq_flags, __rseq_offset, and __rseq_size
> +  variables.  The GNU C Library manual has details on integration of
> +  Restartable Sequences.
> +
>  Deprecated and removed features, and other changes affecting compatibility:
>  
>  * The r_version update in the debugger interface makes the glibc binary
> diff --git a/manual/threads.texi b/manual/threads.texi
> index 06b6b277a1..7b6170cbd0 100644
> --- a/manual/threads.texi
> +++ b/manual/threads.texi
> @@ -629,6 +629,8 @@ the standard.
>  * Waiting with Explicit Clocks::          Functions for waiting with an
>                                            explicit clock specification.
>  * Single-Threaded::                       Detecting single-threaded execution.
> +* Restartable Sequences::                 Linux-specific restartable sequences
> +                                          integration.
>  @end menu
>  
>  @node Default Thread Attributes
> @@ -958,6 +960,85 @@ application-created thread because future versions of @theglibc{} may
>  create background threads after the first thread has been created, and
>  the application has no way of knowning that these threads are present.
>  
> +@node Restartable Sequences
> +@subsubsection Restartable Sequences
> +
> +This section describes restartable sequences integration for
> +@theglibc{}.  This functionality is only available on Linux.
> +
> +@deftp {Data Type} {struct rseq}
> +@standards{Linux, sys/rseq.h}
> +The type of the restartable sequences area.  Future versions
> +of Linux may add additional fields to the end of this structure.
> +
> +
> +Users need to obtain the address of the restartable sequences area using
> +the thread pointer and the @code{__rseq_offset} variable, described
> +below.
> +
> +One use of the restartable sequences area is to read the current CPU
> +number from its @code{cpu_id} field, as an inline version of
> +@code{sched_getcpu}.  @Theglibc{} sets the @code{cpu_id} field to
> +@code{RSEQ_CPU_ID_REGISTRATION_FAILED} if registration failed or was
> +explicitly disabled.

is it better to use RSEQ_CPU_ID_UNINITIALIZED for the disabled case?

so code that uses rseq itself can distinguish enabled but failed rseq
and disabled rseq (in the latter case it would try to register, in the
former it would assume rseq is not supported).

> +
> +Furthermore, users can store the address of a @code{struct rseq_cs}
> +object into the @code{rseq_cs} field of @code{struct rseq}, thus
> +informing the kernel that the thread enters a restartable sequence
> +critical section.  This pointer and the code areas it itself points to
> +must not be left pointing to memory areas which are freed or re-used.
> +Several approaches can guarantee this.  If the application or library
> +can guarantee that the memory used to hold the @code{struct rseq_cs} and
> +the code areas it refers to are never freed or re-used, no special
> +action must be taken.  Else, before that memory is re-used of freed, the
> +application is responsible for setting the @code{rseq_cs} field to
> +@code{NULL} in each thread's restartable sequence area to guarantee that
> +it does not leak dangling references.  Because the application does not
> +typically have knowledge of libraries' use of restartable sequences, it
> +is recommended that libraries using restartable sequences which may end
> +up freeing or re-using their memory set the @code{rseq_cs} field to
> +@code{NULL} before returning from library functions which use
> +restartable sequences.
> +
> +The manual for the @code{rseq} system call can be found
> +at @uref{https://git.kernel.org/pub/scm/libs/librseq/librseq.git/tree/doc/man/rseq.2}.
> +@end deftp
> +
> +@deftypevar {int} __rseq_offset
> +@standards{Linux, sys/rseq.h}
> +This variable contains the offset between the thread pointer (as defined
> +by @code{__builtin_thread_pointer} or the thread pointer register for
> +the architecture) and the restartable sequences area.  This value is the
> +same for all threads in the process.  If the restartable sequences area
> +is located at a lower address than the location to whic the  thread

s/whic/which/

> +pointer points, the value is negative.
> +@end deftypevar
> +
> +@deftypevar {int} __rseq_size

declaration below uses unsigned int.

> +@standards{Linux, sys/rseq.h}
> +This variable is either zero (if restartable sequence registration
> +failed or has been disabled) or the size of the restartable sequence
> +registration.  This can be less can be different from the size of

s/can be less//

> +@code{struct rseq} if the kernel has extended the size of the
> +registration.  If registration is successful, @code{__rseq_size} is at
> +least 32 (the initial size of @code{struct rseq}.

missing )

> +@end deftypevar
> +
> +@deftypevar {unsigned int} __rseq_flags
> +@standards{Linux, sys/rseq.h}
> +The flags used during restartable sequence registration with the kernel.

the syscall argument is documented to be int and our header
declares it as enum rseq_flags, not sure if this matters.

> +Currently zero.
> +@end deftypevar
> +
> +@deftypevr Macro int RSEQ_SIG
> +@standards{Linux, sys/rseq.h}
> +Each supported architecture provides a @code{RSEQ_SIG} macro in
> +@file{sys/rseq.h} which contains a signature.  That signature is
> +expected to be present in the code before each restartable sequences
> +abort handler.  Failure to provide the expected signature may terminate
> +the process with a segmentation fault.
> +@end deftypevr
> +
>  @c FIXME these are undocumented:
>  @c pthread_atfork
>  @c pthread_attr_destroy
> diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c
> index b39dfbff2c..82aa7083c1 100644
> --- a/sysdeps/nptl/dl-tls_init_tp.c
> +++ b/sysdeps/nptl/dl-tls_init_tp.c
> @@ -22,6 +22,7 @@
>  #include <pthreadP.h>
>  #include <tls.h>
>  #include <rseq-internal.h>
> +#include <thread_pointer.h>
>  
>  #define TUNABLE_NAMESPACE pthread
>  #include <dl-tunables.h>
> @@ -43,6 +44,10 @@ rtld_mutex_dummy (pthread_mutex_t *lock)
>  }
>  #endif
>  
> +const unsigned int __rseq_flags;
> +const unsigned int __rseq_size attribute_relro;
> +const int __rseq_offset attribute_relro;
> +
>  void
>  __tls_pre_init_tp (void)
>  {
> @@ -100,7 +105,18 @@ __tls_init_tp (void)
>  #if HAVE_TUNABLES
>      do_rseq = TUNABLE_GET (rseq, int, NULL);
>  #endif
> -    rseq_register_current_thread (pd, do_rseq);
> +    if (rseq_register_current_thread (pd, do_rseq))
> +      {
> +        /* We need a writable view of the variables.  They are in
> +           .data.relro and are not yet write-protected.  */
> +        extern unsigned int size __asm__ ("__rseq_size");
> +        size = sizeof (pd->rseq_area);
> +      }
> +
> +    /* This should be a compile-time constant, but the current
> +       infrastructure makes it difficult to determine its value.  */
> +    extern int offset __asm__ ("__rseq_offset");
> +    offset = (char *) &pd->rseq_area - (char *) __thread_pointer ();
>    }
>  
>    /* Set initial thread's stack block from 0 up to __libc_stack_end.
> diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
> index 62a796f214..61acc1987d 100644
> --- a/sysdeps/unix/sysv/linux/Makefile
> +++ b/sysdeps/unix/sysv/linux/Makefile
> @@ -112,7 +112,8 @@ sysdep_headers += sys/mount.h sys/acct.h \
>  		  bits/types/struct_semid64_ds_helper.h \
>  		  bits/types/struct_shmid64_ds.h \
>  		  bits/types/struct_shmid64_ds_helper.h \
> -		  bits/pthread_stack_min.h bits/pthread_stack_min-dynamic.h
> +		  bits/pthread_stack_min.h bits/pthread_stack_min-dynamic.h \
> +		  sys/rseq.h bits/rseq.h
>  
>  tests += tst-clone tst-clone2 tst-clone3 tst-fanotify tst-personality \
>  	 tst-quota tst-sync_file_range tst-sysconf-iov_max tst-ttyname \
> diff --git a/sysdeps/unix/sysv/linux/Versions b/sysdeps/unix/sysv/linux/Versions
> index 26452f3f17..3f8809a158 100644
> --- a/sysdeps/unix/sysv/linux/Versions
> +++ b/sysdeps/unix/sysv/linux/Versions
> @@ -316,6 +316,11 @@ librt {
>  }
>  
>  ld {
> +  GLIBC_2.35 {
> +    __rseq_flags;
> +    __rseq_offset;
> +    __rseq_size;
> +  }
>    GLIBC_PRIVATE {
>      __nptl_change_stack_perm;
>    }
> diff --git a/sysdeps/unix/sysv/linux/aarch64/ld.abilist b/sysdeps/unix/sysv/linux/aarch64/ld.abilist
> index 80b2fe6725..717a35f242 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/aarch64/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.17 __stack_chk_guard D 0x8
>  GLIBC_2.17 __tls_get_addr F
>  GLIBC_2.17 _dl_mcount F
>  GLIBC_2.17 _r_debug D 0x28
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/alpha/ld.abilist b/sysdeps/unix/sysv/linux/alpha/ld.abilist
> index 98a03f611f..76911bd7f8 100644
> --- a/sysdeps/unix/sysv/linux/alpha/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/alpha/ld.abilist
> @@ -2,4 +2,7 @@ GLIBC_2.0 _r_debug D 0x28
>  GLIBC_2.1 __libc_stack_end D 0x8
>  GLIBC_2.1 _dl_mcount F
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x8
> diff --git a/sysdeps/unix/sysv/linux/arc/ld.abilist b/sysdeps/unix/sysv/linux/arc/ld.abilist
> index 048f17c848..71c67f9803 100644
> --- a/sysdeps/unix/sysv/linux/arc/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/arc/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.32 __stack_chk_guard D 0x4
>  GLIBC_2.32 __tls_get_addr F
>  GLIBC_2.32 _dl_mcount F
>  GLIBC_2.32 _r_debug D 0x14
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/arm/be/ld.abilist b/sysdeps/unix/sysv/linux/arm/be/ld.abilist
> index cc8825c3bc..3859433b21 100644
> --- a/sysdeps/unix/sysv/linux/arm/be/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/arm/be/ld.abilist
> @@ -1,3 +1,6 @@
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __libc_stack_end D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x4
>  GLIBC_2.4 __tls_get_addr F
> diff --git a/sysdeps/unix/sysv/linux/arm/le/ld.abilist b/sysdeps/unix/sysv/linux/arm/le/ld.abilist
> index cc8825c3bc..3859433b21 100644
> --- a/sysdeps/unix/sysv/linux/arm/le/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/arm/le/ld.abilist
> @@ -1,3 +1,6 @@
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __libc_stack_end D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x4
>  GLIBC_2.4 __tls_get_addr F
> diff --git a/sysdeps/unix/sysv/linux/csky/ld.abilist b/sysdeps/unix/sysv/linux/csky/ld.abilist
> index 564ac09737..6bfc582b73 100644
> --- a/sysdeps/unix/sysv/linux/csky/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/csky/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.29 __stack_chk_guard D 0x4
>  GLIBC_2.29 __tls_get_addr F
>  GLIBC_2.29 _dl_mcount F
>  GLIBC_2.29 _r_debug D 0x14
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/hppa/ld.abilist b/sysdeps/unix/sysv/linux/hppa/ld.abilist
> index d155a59843..efccd6a023 100644
> --- a/sysdeps/unix/sysv/linux/hppa/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/hppa/ld.abilist
> @@ -2,4 +2,7 @@ GLIBC_2.2 __libc_stack_end D 0x4
>  GLIBC_2.2 _dl_mcount F
>  GLIBC_2.2 _r_debug D 0x14
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x4
> diff --git a/sysdeps/unix/sysv/linux/i386/ld.abilist b/sysdeps/unix/sysv/linux/i386/ld.abilist
> index 0478e22071..1eb94ae75f 100644
> --- a/sysdeps/unix/sysv/linux/i386/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/i386/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.1 __libc_stack_end D 0x4
>  GLIBC_2.1 _dl_mcount F
>  GLIBC_2.3 ___tls_get_addr F
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/ia64/ld.abilist b/sysdeps/unix/sysv/linux/ia64/ld.abilist
> index 33f91199bf..2cc68bcf7b 100644
> --- a/sysdeps/unix/sysv/linux/ia64/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/ia64/ld.abilist
> @@ -2,3 +2,6 @@ GLIBC_2.2 __libc_stack_end D 0x8
>  GLIBC_2.2 _dl_mcount F
>  GLIBC_2.2 _r_debug D 0x28
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/m68k/coldfire/ld.abilist b/sysdeps/unix/sysv/linux/m68k/coldfire/ld.abilist
> index cc8825c3bc..3859433b21 100644
> --- a/sysdeps/unix/sysv/linux/m68k/coldfire/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/m68k/coldfire/ld.abilist
> @@ -1,3 +1,6 @@
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __libc_stack_end D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x4
>  GLIBC_2.4 __tls_get_addr F
> diff --git a/sysdeps/unix/sysv/linux/m68k/m680x0/ld.abilist b/sysdeps/unix/sysv/linux/m68k/m680x0/ld.abilist
> index 3ba474c27f..e62b2742af 100644
> --- a/sysdeps/unix/sysv/linux/m68k/m680x0/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/m68k/m680x0/ld.abilist
> @@ -2,4 +2,7 @@ GLIBC_2.0 _r_debug D 0x14
>  GLIBC_2.1 __libc_stack_end D 0x4
>  GLIBC_2.1 _dl_mcount F
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x4
> diff --git a/sysdeps/unix/sysv/linux/microblaze/ld.abilist b/sysdeps/unix/sysv/linux/microblaze/ld.abilist
> index a4933c3541..5d63d74e8f 100644
> --- a/sysdeps/unix/sysv/linux/microblaze/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/microblaze/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.18 __stack_chk_guard D 0x4
>  GLIBC_2.18 __tls_get_addr F
>  GLIBC_2.18 _dl_mcount F
>  GLIBC_2.18 _r_debug D 0x14
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/mips/mips32/ld.abilist b/sysdeps/unix/sysv/linux/mips/mips32/ld.abilist
> index be09641a48..53ca22de2f 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips32/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/mips/mips32/ld.abilist
> @@ -2,4 +2,7 @@ GLIBC_2.0 _r_debug D 0x14
>  GLIBC_2.2 __libc_stack_end D 0x4
>  GLIBC_2.2 _dl_mcount F
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x4
> diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n32/ld.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n32/ld.abilist
> index be09641a48..53ca22de2f 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips64/n32/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/mips/mips64/n32/ld.abilist
> @@ -2,4 +2,7 @@ GLIBC_2.0 _r_debug D 0x14
>  GLIBC_2.2 __libc_stack_end D 0x4
>  GLIBC_2.2 _dl_mcount F
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x4
> diff --git a/sysdeps/unix/sysv/linux/mips/mips64/n64/ld.abilist b/sysdeps/unix/sysv/linux/mips/mips64/n64/ld.abilist
> index 1ea36e13f2..d1cdd68333 100644
> --- a/sysdeps/unix/sysv/linux/mips/mips64/n64/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/mips/mips64/n64/ld.abilist
> @@ -2,4 +2,7 @@ GLIBC_2.0 _r_debug D 0x28
>  GLIBC_2.2 __libc_stack_end D 0x8
>  GLIBC_2.2 _dl_mcount F
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
>  GLIBC_2.4 __stack_chk_guard D 0x8
> diff --git a/sysdeps/unix/sysv/linux/nios2/ld.abilist b/sysdeps/unix/sysv/linux/nios2/ld.abilist
> index 52178802dd..bcbba1823e 100644
> --- a/sysdeps/unix/sysv/linux/nios2/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/nios2/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.21 __stack_chk_guard D 0x4
>  GLIBC_2.21 __tls_get_addr F
>  GLIBC_2.21 _dl_mcount F
>  GLIBC_2.21 _r_debug D 0x14
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/ld.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc32/ld.abilist
> index 4bbfba7a61..0d033cb8bd 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/ld.abilist
> @@ -4,3 +4,6 @@ GLIBC_2.1 _dl_mcount F
>  GLIBC_2.22 __tls_get_addr_opt F
>  GLIBC_2.23 __parse_hwcap_and_convert_at_platform F
>  GLIBC_2.3 __tls_get_addr F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/ld.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/ld.abilist
> index 283fb4510b..9c627b1ddf 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/be/ld.abilist
> @@ -4,3 +4,6 @@ GLIBC_2.3 __libc_stack_end D 0x8
>  GLIBC_2.3 __tls_get_addr F
>  GLIBC_2.3 _dl_mcount F
>  GLIBC_2.3 _r_debug D 0x28
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/ld.abilist b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/ld.abilist
> index b1f313c7cd..3a748c2817 100644
> --- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/le/ld.abilist
> @@ -4,3 +4,6 @@ GLIBC_2.17 _dl_mcount F
>  GLIBC_2.17 _r_debug D 0x28
>  GLIBC_2.22 __tls_get_addr_opt F
>  GLIBC_2.23 __parse_hwcap_and_convert_at_platform F
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/riscv/rv32/ld.abilist b/sysdeps/unix/sysv/linux/riscv/rv32/ld.abilist
> index 94ca64c43d..4c67ea18d6 100644
> --- a/sysdeps/unix/sysv/linux/riscv/rv32/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/riscv/rv32/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.33 __stack_chk_guard D 0x4
>  GLIBC_2.33 __tls_get_addr F
>  GLIBC_2.33 _dl_mcount F
>  GLIBC_2.33 _r_debug D 0x14
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/riscv/rv64/ld.abilist b/sysdeps/unix/sysv/linux/riscv/rv64/ld.abilist
> index 845f356c3c..09596f09e2 100644
> --- a/sysdeps/unix/sysv/linux/riscv/rv64/ld.abilist
> +++ b/sysdeps/unix/sysv/linux/riscv/rv64/ld.abilist
> @@ -3,3 +3,6 @@ GLIBC_2.27 __stack_chk_guard D 0x8
>  GLIBC_2.27 __tls_get_addr F
>  GLIBC_2.27 _dl_mcount F
>  GLIBC_2.27 _r_debug D 0x28
> +GLIBC_2.35 __rseq_flags D 0x4
> +GLIBC_2.35 __rseq_offset D 0x4
> +GLIBC_2.35 __rseq_size D 0x4
> diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h
> index 15bc7ffd6e..9e8f99fd51 100644
> --- a/sysdeps/unix/sysv/linux/rseq-internal.h
> +++ b/sysdeps/unix/sysv/linux/rseq-internal.h
> @@ -26,7 +26,7 @@
>  #include <sys/rseq.h>
>  
>  #ifdef RSEQ_SIG
> -static inline void
> +static inline bool
>  rseq_register_current_thread (struct pthread *self, bool do_rseq)
>  {
>    if (do_rseq)
> @@ -35,15 +35,17 @@ rseq_register_current_thread (struct pthread *self, bool do_rseq)
>                                         sizeof (self->rseq_area),
>                                         0, RSEQ_SIG);
>        if (!INTERNAL_SYSCALL_ERROR_P (ret))
> -        return;
> +        return true;
>      }
>    THREAD_SETMEM (self, rseq_area.cpu_id, RSEQ_CPU_ID_REGISTRATION_FAILED);
> +  return false;
>  }
>  #else /* RSEQ_SIG */
> -static inline void
> +static inline bool
>  rseq_register_current_thread (struct pthread *self, bool do_rseq)
>  {
>    THREAD_SETMEM (self, rseq_area.cpu_id, RSEQ_CPU_ID_REGISTRATION_FAILED);
> +  return false;
>  }
>  #endif /* RSEQ_SIG */

this and the rest looks good.

* Szabolcs Nagy via Libc-alpha:

>> +One use of the restartable sequences area is to read the current CPU
>> +number from its @code{cpu_id} field, as an inline version of
>> +@code{sched_getcpu}.  @Theglibc{} sets the @code{cpu_id} field to
>> +@code{RSEQ_CPU_ID_REGISTRATION_FAILED} if registration failed or was
>> +explicitly disabled.
>
> is it better to use RSEQ_CPU_ID_UNINITIALIZED for the disabled case?
>
> so code that uses rseq itself can distinguish enabled but failed rseq
> and disabled rseq (in the latter case it would try to register, in the
> former it would assume rseq is not supported).

I think RSEQ_CPU_ID_UNINITIALIZED was previously intended for a scenario
that is just not observable at all in the current glibc implementation,
basically an initialization ordering issue.

My expectation is that an application that knows about glibc's rseq
implementation will not want to use rseq if it has been explicitly
disabled.  That tunable is only for getting old rseq users to work
again.  If the application goes behind glibc's back and starts rseq
using again, it defeats the tunable (because there could be another rseq
user in the process image).

>> +@deftypevar {int} __rseq_offset
>> +@standards{Linux, sys/rseq.h}
>> +This variable contains the offset between the thread pointer (as defined
>> +by @code{__builtin_thread_pointer} or the thread pointer register for
>> +the architecture) and the restartable sequences area.  This value is the
>> +same for all threads in the process.  If the restartable sequences area
>> +is located at a lower address than the location to whic the  thread
>
> s/whic/which/
>
>> +pointer points, the value is negative.
>> +@end deftypevar
>> +
>> +@deftypevar {int} __rseq_size
>
> declaration below uses unsigned int.
>
>> +@standards{Linux, sys/rseq.h}
>> +This variable is either zero (if restartable sequence registration
>> +failed or has been disabled) or the size of the restartable sequence
>> +registration.  This can be less can be different from the size of
>
> s/can be less//
>
>> +@code{struct rseq} if the kernel has extended the size of the
>> +registration.  If registration is successful, @code{__rseq_size} is at
>> +least 32 (the initial size of @code{struct rseq}.
>
> missing )

All fixed, thanks.

>> +@end deftypevar
>> +
>> +@deftypevar {unsigned int} __rseq_flags
>> +@standards{Linux, sys/rseq.h}
>> +The flags used during restartable sequence registration with the kernel.
>
> the syscall argument is documented to be int and our header
> declares it as enum rseq_flags, not sure if this matters.

It shouldn't matter, unsigned int seems more conservative.  I'm always a
bit wary about enums and bitmasks.

Thanks,
Florian