Re: [musl] Re: Not sure how to debug this one.

[Rich Felker <dalias@libc.org>]

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On Sun, Feb 18, 2024 at 03:55:36PM +0300, Valery Ushakov wrote:
> On Sat, Feb 17, 2024 at 20:40:50 -0500, Rich Felker wrote:
> 
> > due to incorrect base address register when attempting to reload the
> > saved value of r8, the caller's value of r8 was not preserved.
> > ---
> >  src/signal/sh/sigsetjmp.s | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/src/signal/sh/sigsetjmp.s b/src/signal/sh/sigsetjmp.s
> > index 1e2270be..f0f604e2 100644
> > --- a/src/signal/sh/sigsetjmp.s
> > +++ b/src/signal/sh/sigsetjmp.s
> > @@ -27,7 +27,7 @@ __sigsetjmp:
> >  
> >  	mov.l 3f, r0
> >  4:	braf r0
> > -	 mov.l @(4+8,r4), r8
> > +	 mov.l @(4+8,r6), r8
> >  
> >  9:	mov.l 5f, r0
> >  6:	braf r0
> 
> That takes care of restoring caller's r8 for the first return from
> sigsetjmp, but isn't there still the problem that the jump buffer
> contains the wrong one, so on the second return from sigsetjmp the
> caller will have clobbered r8?
> 
> Sorry for a drive-by reply.  I'll try to take a closer look in the
> evening.

No, that's the return path for both returns.

The whole reason a call-saved register like r8 is used here is so that
we can return twice into the body of sigsetjmp, in order to tailcall
__sigsetjmp_tail at both the first return and subsequent return. This
is what makes it possible to restore the signal mask from the
returned-to frame rather than the returning-from frame (which is why
the attached doesn't crash with stack overflow on musl like it does on
glibc).

Rich

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#include <signal.h>
#include <setjmp.h>

static volatile long cnt = 1000000;
sigjmp_buf jb;

void handle(int s)
{
	if (!cnt--) return;
	raise(s);
	siglongjmp(jb, 1);
}

int main()
{
	if (sigsetjmp(jb, 1)) return 0;
	signal(SIGALRM, handle);
	raise(SIGALRM);
}