[Bug 207898] kernel linker behaves differently on amd64 vs. i386

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=207898

Justin Cady <desk at justincady.com> changed:

           What    |Removed                     |Added
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                 CC|                            |desk at justincady.com

--- Comment #4 from Justin Cady <desk at justincady.com> ---
> Why would the linker ignore the flag for local symbols?  That seems like it could be the source of difficult to debug problems.

I have the same question, and I can confirm that such problems are very
difficult to debug. :(

In my case, Module-A, which depends on Modules B and C, is intended to pick up
a globally exported symbol from Module-B, but picks up a _local_ symbol of the
same name from Module-C (which happens to be scanned by the linker first). I
believe that is because of this bug.

I verified this was causing my issue by editing link_elf_lookup_symbol() in
sys/kern/link_elf_obj.c to populate the sym pointer and return success only if
the symbol has global binding (STB_GLOBAL). With that change, the local symbol
from Module-C is ignored, and the global symbol from Module-B is correctly
selected.

1. Am I correct that because of this bug, symbol names on amd64 are effectively
required to be unique across all kernel module dependencies?

2. Is there any risk to actually fixing this? I tried to understand all of the
potential callers of link_elf_lookup_symbol(), but doing so is not
straightforward through all the indirect calls (function pointers, macros).
Stated differently: is there any expectation that link_elf_lookup_symbol()
should return a local symbol?

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