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Linux: Make __rseq_size useful for feature detection (bug 31965)

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The __rseq_size value is now the active area of struct rseq
(so 20 initially), not the full struct size including padding
at the end (32 initially).

Update misc/tst-rseq to print some additional diagnostics.

Reviewed-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
This commit is contained in:
Florian Weimer 2024-07-08 21:14:00 +02:00
parent 7e7f35278c
commit 2e456ccf0c
5 changed files with 40 additions and 12 deletions
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3
NEWS
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@ -49,6 +49,9 @@ Deprecated and removed features, and other changes affecting compatibility:
<utmpx.h> (except for login_tty) due to locking and session management
problems.
* __rseq_size now denotes the size of the active rseq area (20 bytes
initially), not the size of struct rseq (32 bytes initially).
Changes to build and runtime requirements:
[Add changes to build and runtime requirements here]

8
manual/threads.texi
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@ -1007,8 +1007,12 @@ 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 different from the size of @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}).
registration is successful, @code{__rseq_size} is at least 20 (the
initially active size of @code{struct rseq}).
Previous versions of @theglibc{} set this to 32 even if the kernel only
supported the initial area of 20 bytes because the value included unused
padding at the end of the restartable sequence area.
@end deftypevar
@deftypevar {unsigned int} __rseq_flags

8
sysdeps/nptl/dl-tls_init_tp.c
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@ -46,10 +46,6 @@ rtld_mutex_dummy (pthread_mutex_t *lock)
const unsigned int __rseq_flags;
/* The variables are in .data.relro but are not yet write-protected. */
extern unsigned int _rseq_size attribute_hidden;
extern ptrdiff_t _rseq_offset attribute_hidden;
void
__tls_pre_init_tp (void)
{
@ -106,9 +102,7 @@ __tls_init_tp (void)
bool do_rseq = true;
do_rseq = TUNABLE_GET (rseq, int, NULL);
if (rseq_register_current_thread (pd, do_rseq))
{
_rseq_size = sizeof (pd->rseq_area);
}
_rseq_size = RSEQ_AREA_SIZE_INITIAL_USED;
#ifdef RSEQ_SIG
/* This should be a compile-time constant, but the current

23
sysdeps/unix/sysv/linux/rseq-internal.h
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@ -25,15 +25,34 @@
#include <stdio.h>
#include <sys/rseq.h>
/* 32 is the initially required value for the area size. The
actually used rseq size may be less (20 bytes initially). */
#define RSEQ_AREA_SIZE_INITIAL 32
#define RSEQ_AREA_SIZE_INITIAL_USED 20
/* The variables are in .data.relro but are not yet write-protected. */
extern unsigned int _rseq_size attribute_hidden;
extern ptrdiff_t _rseq_offset attribute_hidden;
#ifdef RSEQ_SIG
static inline bool
rseq_register_current_thread (struct pthread *self, bool do_rseq)
{
if (do_rseq)
{
unsigned int size;
#if IS_IN (rtld)
/* Use the hidden symbol in ld.so. */
size = _rseq_size;
#else
size = __rseq_size;
#endif
if (size < RSEQ_AREA_SIZE_INITIAL)
/* The initial implementation used only 20 bytes out of 32,
but still expected size 32. */
size = RSEQ_AREA_SIZE_INITIAL;
int ret = INTERNAL_SYSCALL_CALL (rseq, &self->rseq_area,
sizeof (self->rseq_area),
0, RSEQ_SIG);
size, 0, RSEQ_SIG);
if (!INTERNAL_SYSCALL_ERROR_P (ret))
return true;
}

10
sysdeps/unix/sysv/linux/tst-rseq.c
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@ -29,6 +29,7 @@
# include <stdlib.h>
# include <string.h>
# include <syscall.h>
# include <sys/auxv.h>
# include <thread_pointer.h>
# include <tls.h>
# include "tst-rseq.h"
@ -42,7 +43,8 @@ do_rseq_main_test (void)
TEST_COMPARE (__rseq_flags, 0);
TEST_VERIFY ((char *) __thread_pointer () + __rseq_offset
== (char *) &pd->rseq_area);
TEST_COMPARE (__rseq_size, sizeof (pd->rseq_area));
/* The current implementation only supports the initial size. */
TEST_COMPARE (__rseq_size, 20);
}
static void
@ -52,6 +54,12 @@ do_rseq_test (void)
{
FAIL_UNSUPPORTED ("kernel does not support rseq, skipping test");
}
printf ("info: __rseq_size: %u\n", __rseq_size);
printf ("info: __rseq_offset: %td\n", __rseq_offset);
printf ("info: __rseq_flags: %u\n", __rseq_flags);
printf ("info: getauxval (AT_RSEQ_FEATURE_SIZE): %ld\n",
getauxval (AT_RSEQ_FEATURE_SIZE));
printf ("info: getauxval (AT_RSEQ_ALIGN): %ld\n", getauxval (AT_RSEQ_ALIGN));
do_rseq_main_test ();
}
#else /* RSEQ_SIG */