This section describes restartable sequences integration for the GNU C Library. This functionality is only available on Linux.
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 __rseq_offset variable, described
below.
One use of the restartable sequences area is to read the current CPU
number from its cpu_id field, as an inline version of
sched_getcpu. The GNU C Library sets the cpu_id field to
RSEQ_CPU_ID_REGISTRATION_FAILED if registration failed or was
explicitly disabled.
Furthermore, users can store the address of a struct rseq_cs
object into the rseq_cs field of 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 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 rseq_cs field to
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 rseq_cs field to
NULL before returning from library functions which use
restartable sequences.
The manual for the rseq system call can be found
at https://git.kernel.org/pub/scm/libs/librseq/librseq.git/tree/doc/man/rseq.2.
ptrdiff_t __rseq_offset ¶This variable contains the offset between the thread pointer (as defined
by __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 which the thread
pointer points, the value is negative.
unsigned int __rseq_size ¶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 struct rseq
if the kernel has extended the size of the registration. If
registration is successful, __rseq_size is at least 32 (the
initial size of struct rseq).
unsigned int __rseq_flags ¶The flags used during restartable sequence registration with the kernel. Currently zero.
int RSEQ_SIG ¶Each supported architecture provides a RSEQ_SIG macro in
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.