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Date: Mon, 21 Oct 2019 01:20:05 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: another armv7-m exception handling problem

On Mon, Oct 21, 2019 at 01:04:11AM -0400, Rich Felker wrote:
> On Mon, Oct 21, 2019 at 03:43:40PM +1100, Patrick Oppenlander wrote:
> > Not sure if this is a musl, gcc or ld bug.
> > 
> > Running gcc-8.3.0 musl 1.1.24 static pie.
> > 
> > Simple test case:
> > 
> > int main()
> > {
> >         try {
> >                 throw 1;
> >         } catch (int d) {
> >                 return d;
> >         }
> >         return 0;
> > }
> > 
> > Expected:
> > % ./a.out
> > % echo $?
> > 1
> > 
> > Actual:
> > % ./a.out
> > terminate called after throwing an instance of 'int'
> > terminate called recursively
> > Aborted (core dumped)
> > 
> > The problem is that get_eit_entry is returning _URC_FAILURE here:
> > 
> >   if (__gnu_Unwind_Find_exidx)
> >     {
> >       eitp = (const __EIT_entry *) __gnu_Unwind_Find_exidx (return_address,
> >                                                             &nrec);
> >       if (!eitp)
> >         {
> >           UCB_PR_ADDR (ucbp) = 0;
> >           return _URC_FAILURE;        <----- !!!!
> >         }
> >     }
> > 
> > Looks like a linker or program load problem to me -- the GOT entry for
> > __gnu_Unwind_Find_exidx is correctly set to 0, but after program load
> > it's been offset to some non-zero value. There's an R_ARM_RELATIVE
> > relocation on the GOT entry.
> > 
> > Not sure where this is going wrong.
> 
> OK, this is almost surely a bug in the tooling -- a weak reference has
> to use a GOT slot in PIC since it might be undefined and need to
> evaluate to 0, and the GOT slot should not have a relocation on it if
> it ends up evaluating to 0. Pretty sure it's ld's fault.

I can reproduce it with the attached trivial GNU C program.

Rich

View attachment "weakref2.c" of type "text/plain" (112 bytes)

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