Re: [musl] aarch64 sigsetjmp relocation truncation bug, maybe

[Rich Felker <dalias@libc.org>]

(Re-adding the OP to CC)

On Thu, Sep 07, 2023 at 08:48:28AM -0400, Rich Felker wrote:
> On Thu, Sep 07, 2023 at 05:08:13AM +0200, Markus Wichmann wrote:
> > Am Wed, Sep 06, 2023 at 08:46:32PM -0400 schrieb Peter Williams:
> > > If I'm understanding correctly, the complaint is that a branch in
> > > sigsetjmp that invokes setjmp is too far away from the definition of
> > > setjmp. My very handwavey idea is that maybe for some reason my program
> > > is causing the linker to want to locate setjmp() and sigsetjmp() really
> > > far away from each other. If that's right, perhaps it would be possible
> > > to modify the assembler code to be able to handle such a situation?
> > 
> > I'm guessing the same. Pretty much all architectures have shorter
> > conditional than unconditional branches. That is why branches to other
> > files (technically to other sections) should always be unconditional. I
> > am attaching a simple patch that should help with the situation.
> > 
> > Ciao,
> > Markus
> 
> > From dd227e22a5337d54e1cb0838410bca6672c76c43 Mon Sep 17 00:00:00 2001
> > From: Markus Wichmann <nullplan@gmx.net>
> > Date: Thu, 7 Sep 2023 05:01:23 +0200
> > Subject: [PATCH] Make branch to external symbol unconditional.
> > 
> > Conditional branches have a shorter branch length than unconditional
> > ones, and almost all ABIs require unconditional branches to external
> > symbols. Otherwise linkers may create broken binaries.
> > ---
> >  src/signal/aarch64/sigsetjmp.s | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/src/signal/aarch64/sigsetjmp.s b/src/signal/aarch64/sigsetjmp..s
> > index 75910c43..9a28e395 100644
> > --- a/src/signal/aarch64/sigsetjmp.s
> > +++ b/src/signal/aarch64/sigsetjmp.s
> > @@ -4,7 +4,7 @@
> >  .type __sigsetjmp,%function
> >  sigsetjmp:
> >  __sigsetjmp:
> > -	cbz x1,setjmp
> > +	cbz x1,1f
> > 
> >  	str x30,[x0,#176]
> >  	str x19,[x0,#176+8+8]
> > @@ -19,3 +19,5 @@ __sigsetjmp:
> > 
> >  .hidden __sigsetjmp_tail
> >  	b __sigsetjmp_tail
> > +
> > +1: b setjmp
> > --
> > 2.39.2
> 
> Are you sure this is the actual problem? I think it's that the aarch64
> (and several other archs) version of sigsetjmp is wrongly using the
> public setjmp symbol whose definition is possibly provided by a PLT
> thunk in the main program, rather than either setjmp@PLT (which would
> necessarily be the right local call point to use) or the hidden
> ___setjmp symbol that exists for this purpose (which i386, for
> example, uses).

Hmm, no, that seems to be a separate issue. The reported issue is
indeed a large static link where PLT stuff should not come into play.
So I think the cbz limited-range is indeed the issue.

Rich