Re: sigsegv, c-stack: Avoid compilation error with glibc >= 2.34

[Paul Eggert]

On 5/18/21 10:34 AM, Bruno Haible wrote:

> malloc'ed memory is not executable. But GCC's trampolines (used by nested
> functions) require an executable stack. When an object file has such nested
> functions, the toolchain arranges for the executable to ask the OS to
> allocate an executable stack. But that only has an effect on the primary
> stack of each thread. It does not have an effect on an alternate stack
> (AFAIK).

That should be OK so long as stack-overflow handlers don't used nested 
functions. Conversely, even programs that assume SIGSTKSZ is a constant 
(and allocate fixed-size arrays on the stack or in static storage) will 
have problems on such platforms, no? Stack and static storage won't be 
executable any more than heap storage is.

> I wouldn't mind, if it's done properly:
>    - The problem with non-executable malloc'ed memory would need to be
>      solved.

Or, we could just document that stack-overflow handers can't use 
trampolines. That is a reasonable restriction for most apps, I would think.

>    - The declaration in sigsegv.h needs to stay, at least for the next
>      couple of years, because it is necessary for programs that use SIGSTKSZ
>      (such as GNU m4 and GNU clisp) to compile fine.

Why is it needed for GNU m4 and GNU clisp? I just looked at the latest 
development source code for these programs, and neither mentions 
SIGSTKSZ. GNU Emacs doesn't either (except in a configure.ac test that 
does not assume SIGSTKSZ is a constant).

The only place I found that assumed that SIGSTKSZ is a constant, was in 
Gnulib test cases. I removed that assumption by installing the first 
attached patch.

And I propose the second attached patch to remove sigsegv.h's guarantee 
that SIGSTKSZ is a constant, as glibc has gone a different way and it's 
better if Gnulib tracks glibc.

0001-sigsegv-don-t-assume-SIGSTKSZ-is-a-constant.patch
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0002-sigsegv-do-not-guarantee-SIGSTKSZ-is-constant.patch
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