LLD 18 leaves leaves behind undef symbols from discarded sections

[nikic]

After 1981b1b, LLD converts symbols in discarded sections into undef symbols. This happens even if there are no references to the symbol from non-discarded sections.

This change breaks building the Linux kernel with LLD in some configurations (CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y) and does not match the behavior of previous LLD versions or the behavior of the BFD linker.

To reproduce the issue:

test.c:

__attribute__((__section__(".discard"))) __attribute__((unused)) char __pcpu_unique_foo;

module.lds:

SECTIONS {
 /DISCARD/ : {
  *(.discard)
 }
}

Then run:

clang test.c -c
ld.lld -r -T module.lds test.o -o test
objdump -t test

This will produce:

0000000000000000       O *UND*	0000000000000000 __pcpu_unique_foo

While previously it did not.

[llvmbot]

@llvm/issue-subscribers-lld-elf

Author: Nikita Popov (nikic)

After https://github.com/llvm/llvm-project/commit/1981b1b6b92f7579a30c9ed32dbdf3bc749c1b40, LLD converts symbols in discarded sections into undef symbols. This happens even if there are no references to the symbol from non-discarded sections.

This change breaks building the Linux kernel with LLD in some configurations (CONFIG_DEBUG_FORCE_WEAK_PER_CPU=y) and does not match the behavior of previous LLD versions or the behavior of the BFD linker.

To reproduce the issue:

test.c:

__attribute__((__section__(".discard"))) __attribute__((unused)) char __pcpu_unique_foo;

module.lds:

SECTIONS {
 /DISCARD/ : {
  *(.discard)
 }
}

Then run:

clang test.c -c
ld.lld -r -T module.lds test.o -o test
objdump -t test

This will produce:

0000000000000000       O *UND*	0000000000000000 __pcpu_unique_foo

While previously it did not.

[rnk]

At Google, we noticed a related issue (internal 307561801) in our Linux kernel builds, but we believed it was specific to Google-internal build configurations, and that the LLD behavior was correct.

[MaskRay]

Thanks for the detailed reproduce. A few changes have depended on this representation, so dropping demoteDefined would break many tests. #85167 should fix this forward.

[nikic]

/cherry-pick 8fe3e70

[llvmbot]

/pull-request #85266