Re: SIGSTKSZ is now a run-time variable

[Bruno Haible <bruno@clisp.org>]

Eric Blake wrote:
> I can open a defect against POSIX if we decide that is needed, but want
> some consensus first on whether it is glibc's change that went too far,
> or POSIX's requirements that are too restrictive for what glibc wants to do.

Thanks for opening the discussion, Eric.

Here are a couple of questions, to understand the motivation and the possible
alternative solutions to the problem:

1) As far as I understand, the issue occurs with certain x86 or x86_64
   processors.

   1.1) What has been the value of MINSIGSTKSZ on x86 and x86_64 so far?
   1.2) What value of MINSIGSTKSZ is needed for AVX-512F support?
   1.3) Will the trend to larger MINSIGSTKSZ values continue for Intel
        processors?

2) Regarding the change of the macro MINSIGSTKSZ:
   Would it possible to just change the value of MINSIGSTKSZ to a larger
   constant?

   If there is a fear regarding ABI compatibility between a library and a
   program: How likely is it that a library offers an interface that takes
   a char[MINSIGSTKSZ] as argument, or that defines a variable of type
   char[MINSIGSTKSZ]?

3) Regarding the change of the macro SIGSTKSZ:
   Likewise, would it be possible to just change the value of SIGSTKSZ to a
   larger constant?

4) Since SIGSTKSZ has other uses than MINSIGSTKSZ, has it been considered
   to make MINSIGSTKSZ non-constant but keep SIGSTKSZ constant?

5) POSIX:2018 [1] defines SIGSTKSZ as the stack size for "the usual case".
   So, it should be composed of MINSIGSTKSZ for the initial stack frame,
   plus a certain amount of stack, depending on CPU, ABI, and compiler,
   for doing what a "usual" signal handler would do.

   What is the reason, then, for the computation
     SIGSTKSZ >= 4 * MINSIGSTKSZ
   in [2]? Shouldn't it be something like
     SIGSTKSZ >= MINSIGSTKSZ + (64 KB on SPARC and powerpc, 8 KB on other
                                processors)
   ?

Bruno

[1] https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigaltstack.html
[2] https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/sysconf-sigstksz.h