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

[Rich Felker <dalias@libc.org>]

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).

Rich