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Date: Sun, 18 Feb 2024 09:33:13 -0500
From: Rich Felker <dalias@...c.org>
To: Valery Ushakov <uwe@...err.spb.ru>
Cc: musl@...ts.openwall.com, toybox <toybox@...ts.landley.net>
Subject: Re: Re: Not sure how to debug this one.
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
View attachment "sigsetjmp_of.c" of type "text/plain" (261 bytes)
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