From 2e456ccf0c34a056e3ccafac4a0c7effef14d918 Mon Sep 17 00:00:00 2001 From: Florian Weimer Date: Mon, 8 Jul 2024 21:14:00 +0200 Subject: [PATCH] Linux: Make __rseq_size useful for feature detection (bug 31965) 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 Reviewed-by: Mathieu Desnoyers --- NEWS | 3 +++ manual/threads.texi | 8 ++++++-- sysdeps/nptl/dl-tls_init_tp.c | 8 +------- sysdeps/unix/sysv/linux/rseq-internal.h | 23 +++++++++++++++++++++-- sysdeps/unix/sysv/linux/tst-rseq.c | 10 +++++++++- 5 files changed, 40 insertions(+), 12 deletions(-) diff --git a/NEWS b/NEWS index f626896aa0..3824815917 100644 --- a/NEWS +++ b/NEWS @@ -49,6 +49,9 @@ Deprecated and removed features, and other changes affecting compatibility: (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] diff --git a/manual/threads.texi b/manual/threads.texi index e5544ff3da..25e99c9606 100644 --- a/manual/threads.texi +++ b/manual/threads.texi @@ -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 diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c index 7eb35fb133..7803e19fd1 100644 --- a/sysdeps/nptl/dl-tls_init_tp.c +++ b/sysdeps/nptl/dl-tls_init_tp.c @@ -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 diff --git a/sysdeps/unix/sysv/linux/rseq-internal.h b/sysdeps/unix/sysv/linux/rseq-internal.h index 48eebc1e16..7ea935b4ad 100644 --- a/sysdeps/unix/sysv/linux/rseq-internal.h +++ b/sysdeps/unix/sysv/linux/rseq-internal.h @@ -25,15 +25,34 @@ #include #include +/* 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; } diff --git a/sysdeps/unix/sysv/linux/tst-rseq.c b/sysdeps/unix/sysv/linux/tst-rseq.c index 2c90409ba0..08a9533130 100644 --- a/sysdeps/unix/sysv/linux/tst-rseq.c +++ b/sysdeps/unix/sysv/linux/tst-rseq.c @@ -29,6 +29,7 @@ # include # include # include +# include # include # include # 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 */