* Mathieu Desnoyers:
> @deftypevar {struct rseq} __rseq_abi
> @standards{Linux, sys/rseq.h}
> @Theglibc{} implements a @code{__rseq_abi} TLS symbol to interact with the
> Restartable Sequences system call (Linux-specific). The layout of this
> structure is defined by the @file{sys/rseq.h} header. Registration of each
> thread's @code{__rseq_abi} is performed by @theglibc{} at libc library
> initialization and thread creation.
s/libc library/library/
> The main executable and shared libraries may either have an undefined
> @code{__rseq_abi} TLS symbol, or define their own, with the same
> declaration as the one present in @file{sys/rseq.h}. The dynamic linker
> will ensure that only one of those available symbols will be used at
> runtime across the process.
>
> If the main executable or shared libraries observe an uninitialized
> @code{__rseq_abi.cpu_id} field (value @code{RSEQ_CPU_ID_UNINITIALIZED}), they
> may perform rseq registration to the kernel: this means either glibc was
> prevented from doing the registration, or an older glibc version, which does
> not include rseq support, is in use. When the main executable or a library
> thus takes ownership of the registration, the memory used to hold the
> @code{__rseq_abi} TLS variable must stay allocated, and is not re-used, until
> the very end of the thread lifetime or until an explicit rseq unregistration
> for that thread is performed. It is not recommended to dlclose() libraries
> owning the @code{__rseq_abi} TLS variable.
s/dlclose()/@code{dlclose}/ (no parentheses)
Rest looks okay.
>>> + if (__rseq_abi.cpu_id == RSEQ_CPU_ID_REGISTRATION_FAILED)
>>> + return;
>>> + ret = INTERNAL_SYSCALL_CALL (rseq, &__rseq_abi, sizeof (struct rseq),
>>> + 0, RSEQ_SIG);
>>> + if (INTERNAL_SYSCALL_ERROR_P (ret) &&
>>> + INTERNAL_SYSCALL_ERRNO (ret) != EBUSY)
>>> + __rseq_abi.cpu_id = RSEQ_CPU_ID_REGISTRATION_FAILED;
>>
>> Sorry, I forgot: Please add a comment that the EBUSY error is ignored
>> because registration may have already happened in a legacy library.
>
> Considering that we now disable signals across thread creation, and that
> glibc's initialization happens before other libraries' constructors
> (as far as I remember even before LD_PRELOADed library constructors),
> in which scenario can we expect to have EBUSY here ?
That's a good point.
> Not setting __rseq_abi.cpu_id to RSEQ_CPU_ID_REGISTRATION_FAILED in case
> of EBUSY is more a way to handle "unforeseen" scenarios where somehow the
> registration would already be done. But I cannot find an "expected"
> scenario which would lead to this now.
>
> So if EBUSY really is unexpected, how should we treat that ? I don't think
> setting REGISTRATION_FAILED would be appropriate, because then it would
> break assumption of the prior successful registration that have already
> been done by this thread.
You could call __libc_fatal with an error message. ENOSYS is definitely
an expected error code here, and EPERM (and perhaps EACCES) can happen
with seccomp filters.
Thanks,
Florian
