Re: [PATCH] Fix the use of sigaltstack to return to the saved main stack.

[Szabolcs Nagy <nsz@port70.net>]

* James Y Knight <jyknight@google.com> [2019-07-10 16:11:23 -0400]:
>  int sigaltstack(const stack_t *restrict ss, stack_t *restrict old)
>  {
> +	// We must check requirements which Linux fails to verify in the syscall
> +	// itself.
>  	if (ss) {
> -		if (ss->ss_size < MINSIGSTKSZ) {
> +		// The syscall does already check against MINSIGSTKSZ, however,
> +		// the kernel's value is smaller than musl's value on some
> +		// architectures. Thus, although this check may appear
> +		// redundant, it is not.

the comment does not make sense to me, the check is obviously
not redundant.

MINSIGSTKSZ is a libc api, has nothing to do with the kernel

the kernel also defines a MINSIGSZTKSZ but musl is an
abstraction layer higher, the linux limit should not be
observable to users, only the limit defined by musl,
which ensures not only that the kernel can deliver a
signal but also reserves space of any current or future
hackery the c runtime may need to do around signal handling,
so that trivial c language signal handler is guaranteed
to work.

this is the only reasonable way to make such limit useful.
if it were only a kernel limit, then application code would
have to guess the libc signal handling overhead and add that
to the MINSIGSZTKSZ when allocating signal stacks.