Non-zero RELA section contents

[Alex Xu (Hello71)]

Hi,

In ELF RELA relocations, the addend is stored in the relocation instead 
of the section contents. This was the premise of commit 17c6c3b99156 
("x86-64/ELF: clear src_mask for all reloc types"); the value in the 
section contents is not used.

I recently encountered a difficult-to-trace bug in 7-Zip, eventually 
determined to be caused by JWasm-family assemblers incorrectly putting 
the addend in the section contents, and setting the RELA addend to -4. 
Previous versions of binutils added the existing value to the relocation 
result, producing a working program; new versions of binutils overwrite 
it, producing a non-working program.

Now, I want to be clear: while the relevant specifications are not clear 
about what the linker should do in this case, they are reasonably clear 
that the assembler should not generate this. The assembler is absolutely 
wrong.

With that being said, I think it would be a good idea for ld to either 
revert to the previous behavior or issue a warning or error when 
detecting such malformed object files. I think a warning at the least is 
appropriate, because binaries are either silently incorrect on the old 
version, or silently incorrect on the new version, and silently 
incorrect relocations are extremely hard to diagnose. One argument in 
favor of reverting to the previous behavior is it is better to preserve 
backwards compatibility in BFD linker when the new behavior is not 
clearly superior (e.g. faster or closer to spec). One argument in favor 
of the new behavior is that it is consistent with LLD and probably gold.

The following patch adds a warning:

diff --git a/bfd/reloc.c b/bfd/reloc.c
index 5098e0ab09f..aecdb21ec59 100644
--- a/bfd/reloc.c
+++ b/bfd/reloc.c
@@ -1509,6 +1509,11 @@ _bfd_relocate_contents (reloc_howto_type *howto,
   relocation >>= (bfd_vma) rightshift;
   relocation <<= (bfd_vma) bitpos;
 
+  if (!howto->src_mask && (x & howto->dst_mask))
+    _bfd_error_handler
+      (_("warning: %pB: existing value for %s relocation is %lx, expected 0"),
+       input_bfd, howto->name, x);
+
   /* Add RELOCATION to the right bits of X.  */
   x = ((x & ~howto->dst_mask)
        | (((x & howto->src_mask) + relocation) & howto->dst_mask));

I have tested this patch on a single test file which printed the desired 
warning. I believe %lx is probably wrong, but I don't know how to fix 
it. I suspect this patch may cause a large number of warnings in certain 
cases; however, I think adding a warning here is valid, because the 
result is either broken with old binutils or broken with new binutils 
(or both), and furthermore, proper assemblers should never generate this 
case in the first place.

I think the primary counterargument is that the point of "either broken 
with old binutils or broken with new binutils" primarily applies to 
x86-64/ELF, and may not apply to other targets. I am insufficiently 
familiar with other targets to say whether this is the case. If so, it 
may be better to add this warning only for x86-64/ELF.

Thanks,
Alex.