PING: V7 [PATCH] sysconf: Add _SC_MINSIGSTKSZ/_SC_SIGSTKSZ [BZ #20305]

[Zack Weinberg]

On Thu, Nov 19, 2020 at 10:00 AM Szabolcs Nagy via Libc-alpha
<libc-alpha@sourceware.org> wrote:
...
> but to let the critical section with all its side
> effects complete and delay the signal handler
> until then. (the slow and easy way to do this is
> masking signals using syscalls around critical
> sections, a fast solution needs signal wrapping
> and saving the sigcontext.)
>
> for example the entire malloc call can be a critical
> section and an incomming signal delayed until malloc
> completes.

I read this and I'm wondering how impractical it would be to invent a
way to perform pthread_sigmask() operations without a system call.
Abstractly, the signal mask for each thread _could_ be placed in user
space at an ABI-specified location where the kernel can find it
(bottom of the thread stack, maybe, or an offset from the TLS base);
this doesn't make anything new possible (the kernel will just continue
to ignore the bits for SIGKILL, SIGSEGV, etc.) so it wouldn't be a
privilege violation.

Seems like that would be much easier than copying sigcontexts around,
and less likely to break code that thinks it knows stuff about signal
frames.

zw