[PATCH] objtool: ignore .L prefixed local symbols

[Nick Desaulniers]

Top of tree LLVM has optimizations related to
-fno-semantic-interposition to avoid emitting PLT relocations for
references to symbols located in the same translation unit, where it
will emit "local symbol" references.

Clang builds fall back on GNU as for assembling, currently. It appears a
bug in GNU as introduced around 2.31 is keeping around local labels in
the symbol table, despite the documentation saying:

"Local symbols are defined and used within the assembler, but they are
normally not saved in object files."

When objtool searches for a symbol at a given offset, it's finding the
incorrectly kept .L<symbol>$local symbol that should have been discarded
by the assembler.

A patch for GNU as has been authored.  For now, objtool should not treat
local symbols as the expected symbol for a given offset when iterating
the symbol table.

commit 644592d32837 ("objtool: Fail the kernel build on fatal errors")
exposed this issue.

Link: https://github.com/ClangBuiltLinux/linux/issues/872
Link: https://sourceware.org/binutils/docs/as/Symbol-Names.html#Symbol-Names
Link: https://sourceware.org/ml/binutils/2020-02/msg00243.html
Link: https://travis-ci.com/ClangBuiltLinux/continuous-integration/jobs/286292010
Debugged-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
Debugged-by: Fangrui Song <maskray@xxxxxxxxxx>
Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Build tested allyesconfig with ToT Clang and GCC 9.2.1.
Boot tested defconfig with ToT Clang and GCC 9.2.1.

 tools/objtool/elf.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index edba4745f25a..9c1e3cc928b0 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -63,7 +63,8 @@ struct symbol *find_symbol_by_offset(struct section *sec, unsigned long offset)
 
 	list_for_each_entry(sym, &sec->symbol_list, list)
 		if (sym->type != STT_SECTION &&
-		    sym->offset == offset)
+		    sym->offset == offset &&
+		    strstr(sym->name, ".L") != sym->name)
 			return sym;
 
 	return NULL;
-- 
2.25.0.225.g125e21ebc7-goog