Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation

Szabolcs Nagy
2018-11-22T16:25:42Z

On 22/11/18 15:33, Mathieu Desnoyers wrote:
> ----- On Nov 22, 2018, at 10:21 AM, Florian Weimer fweimer@redhat.com wrote:
>> Right, but in case of user-supplied stacks, we actually free TLS memory
>> at this point, so signals need to be blocked because the TCB is
>> (partially) gone after that.
>
> Unfortuntately, disabling signals is not enough.
>
> With rseq registered, the kernel accesses the rseq TLS area when returning to
> user-space after _preemption_ of user-space, which can be triggered at any
> point by an interrupt or a fault, even if signals are blocked.
>
> So if there are cases where the TLS memory is freed while the thread is still
> running, we _need_ to explicitly unregister rseq beforehand.

i think the man page should point this out.

the memory of a registered rseq object must not be freed
before thread exit. (either unregister it or free later)

and ideally also point out that c language thread storage
duration does not provide this guarantee: it may be freed
by the implementation before thread exit (which is currently
not observable, but with the rseq syscall it is).