What can a signal handler do with SIGSTKSZ?

[Florian Weimer]

* Carlos O'Donell:

> In the continuing series of "What can X do with Y?" [1]
>
> I would like to get consensus on what a signal handler can do with
> SIGSTKSZ amount of space.
>
> I propose the following:
> ~~~
> The implementation only guarantees that a signal handler can
> manipulate a reasonable amount of local variables (no more than
> 2 KiB worth), and can read and write to memory, carry out atomic
> operations, and call simple C library functions that do similar
> memory and simple string operations e.g. memcpy, memset, strcmp,
> strcpy. The amount of signal stack allocated for SIGSTKSZ is not
> sufficient to call complex signal-safe functions e.g. fork, _exit,
> abort, nor any that can be canceled (requires enough stack for
> cancellation). Any other operations or function calls in the
> signal handler should be evaluated for runtime stack usage and
> additional stack beyond SIGSTKSZ should be allocated.
> ~~~
>
> Thoughts?

I have posted some numbers here:

[PATCH] nptl: New test nptl/tst-stack-usage
<https://sourceware.org/ml/libc-alpha/2018-12/msg00271.html>

x86_64: MINSIGSTKSZ too small for AVX-512F support
<https://sourceware.org/bugzilla/show_bug.cgi?id=20305>

I did not measure dynamic linker overhead.  On x86-64, due to the use of
XSAVE, I expect it to be comparable to signal delivery, perhaps slightly
worse.  That means that the default SIGSTKSZ (8192 bytes I think) is
really tight with your 2 KiB rule and lazy binding.

It's also non-obvious that vfprintf needs more than 8192 bytes of stack
with some streams (which includes the fake stream in dprintf, I think).

Furthermore, you'd also have to use sigaction with a proper sa_mask
value for correctness, which I believe is rare.

Thanks,
Florian