Re: [musl] [PATCH] fix MINSIGSTKSZ and SIGSTKSZ for powerpc64

[Rich Felker <dalias@libc.org>]

On Thu, Aug 29, 2024 at 06:00:52PM +0200, alice wrote:
> On Thu Aug 29, 2024 at 2:57 PM CEST, Rich Felker wrote:
> > On Thu, Aug 29, 2024 at 05:38:42AM +0200, psykose wrote:
> > > since kernel commit 2f82ec19757f58549467db568c56e7dfff8af283
> > > (https://github.com/torvalds/linux/commit/2f82ec19757f58549467db568c56e7dfff8af283)
> > > the kernel has updated these minimum values. having these small values breaks
> > > sysconf(_SC_MINSIGSTKSZ) too; it returns 4224 in musl currently which ends up
> > > returning ENOMEM from the syscall made in sigaltstack.
> > > 
> > > raising these to match the kernel fixes sigaltstack use on powerpc64(le).
> > > caught by glib's 2.82 testsuite
> >
> > I don't follow how you're claiming sysconf(_SC_MINSIGSTKSZ) is broken.
> > It will just return the kernel-provided value on new kernels that
> > insist on having a larger stack. In particular I don't see where the
> > value 4224 is supposed to be coming from. If there's something I'm
> > missing, please explain.
> 
> sysconf(_SC_MINSIGSTKSZ) returns 4224 on ppc64le (this is as far as i know
> expected).

I don't have a real system handy to test on, so I'm executing this
mentally, and not seeing where 4224 comes from.
sysconf(_SC_MINSIGSTKSZ) should return the kernel-provided value from
__getauxval(AT_MINSIGSTKSZ) unless it's less than the fixed macro
value MINSIGSTKSZ. Since that's 4096, the only way I can see this
happening is if the kernel filled in AT_MINSIGSTKSZ as 4224, which
would be a kernel bug...?

> setting stack.ss_size = 4224 (from that call) and passing it to sigaltstack
> returns ENOMEM because it is smaller than 8192 which the kernel enforces.
> 
> ...so the normal way of using sigaltstack with the smallest size (with sysconf or
> the macros) is broken.
> making the MINSIGSTKSZ match the actual value the kernel enforces fixes it.

What the kernel enforces varies by version and possibly also hardware
capabilities.

Rich