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Date: Mon, 21 Oct 2019 01:04:11 -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 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.

Rich

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