Re: [musl] More thoughts on wrapping signal handling

[Alexey Izbyshev <izbyshev@ispras.ru>]

On 2020-10-29 09:34, Rich Felker wrote:
> In "Re: [musl] Re: [PATCH] Make abort() AS-safe (Bug 26275)."
> (20201010002612.GC17637@brightrain.aerifal.cx,
> https://www.openwall.com/lists/musl/2020/10/10/1) I raised the
> longstanding thought of having libc wrap signal handling. This is a
> little bit of a big hammer for what it was proposed for -- fixing an
> extremely-rare race between abort and execve -- but today I had a
> thought about another use of it that's really compelling.
> 
> What I noted before was that, by wrapping signal handlers, libc could
> implement a sort of "rollback" to restart a critical section that was
> interrupted. However this really only has any use when the critical
> section has no side effects aside from its final completion, and
> except for execve where replacement of the process gives the atomic
> cutoff for rollback, it requires __cp_end-like asm label of the end of
> the critical section. So it's of limited utility.
> 
> However, what's more interesting than restarting the critical section
> when a signal is received is *allowing it to complete* before handling
> the signal. This can be implemented by having the wrapper, upon seeing
> that it interrupted a critical section, save the siginfo_t in TLS and
> immediately return, leaving signals blocked, without executing the
> application-installed signal handler. Then, when leaving the critical
> section, the unlock function can see the saved siginfo_t and call the
> application's signal handler. Effectively, it's as if the signal were
> just blocked until the end of the critical section.
> 
As described, that would call the application's signal handler on the 
wrong stack in case SA_ONSTACK was used.

And what happens if the application wants to modify ucontext via the 
third argument of the signal handler?