linux-kernel - Re: [PATCH] objtool: ignore .L prefixed local symbols

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Date:   Thu, 13 Feb 2020 14:37:34 -0800
From:   Fangrui Song <maskray@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>, peterz@...radead.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: ignore .L prefixed local symbols

On 2020-02-13, Josh Poimboeuf wrote:
>On Thu, Feb 13, 2020 at 10:47:08AM -0800, Nick Desaulniers wrote:
>> 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.

R_X86_64_PLT32 was fixed (just now) by
https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=292676c15a615b5a95bede9ee91004d3f7ee7dfd
It will be included in binutils 2.35 and probably a bug fix release of 2.34.x

>> commit 644592d32837 ("objtool: Fail the kernel build on fatal errors")
>> exposed this issue.
>
>Since I'm going to be dropping 644592d32837 ("objtool: Fail the kernel
>build on fatal errors") anyway, I wonder if this patch is still needed?
>
>At least the error will be downgraded to a warning.  And while the
>warning could be more user friendly, it still has value because it
>reveals a toolchain bug.

I still consider such a check (tools/objtool/check.c:679) unneeded.

st_type doesn't have to be STT_FUNC. Either STT_NOTYPE or STT_FUNC is
ok. If STT_GNU_IFUNC is used, it can be ok as well.
(My clang patch skips STT_GNU_IFUNC just because rtld typically doesn't
  cache R_*_IRELATIVE results. Having two STT_GNU_IFUNC symbols with same st_shndx and
  st_value can create two R_*_IRELATIVE, which need to be resolved twice
  at runtime.)

	} else if (rela->sym->type == STT_SECTION) {
		insn->call_dest = find_symbol_by_offset(rela->sym->sec,
							rela->addend+4);
		if (!insn->call_dest ||
		    insn->call_dest->type != STT_FUNC) {
			WARN_FUNC("can't find call dest symbol at %s+0x%x",
				  insn->sec, insn->offset,
				  rela->sym->sec->name,
				  rela->addend + 4);
			return -1;
		}


	.section	.init.text,"ax",@progbits
	call	printk
	call	.Lprintk$local
	.text
	.globl	printk
	.type	printk,@function
printk:
.Lprintk$local:
  ret

% llvm-mc -filetype=obj -triple=riscv64 a.s -mattr=+relax -o a.o
% readelf -Wr a.o

Relocation section '.rela.init.text' at offset 0xa0 contains 4 entries:
     Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
0000000000000000  0000000200000012 R_RISCV_CALL           0000000000000000 printk + 0
0000000000000000  0000000000000033 R_RISCV_RELAX                             0
0000000000000008  0000000100000012 R_RISCV_CALL           0000000000000000 .Lprintk$local + 0
0000000000000008  0000000000000033 R_RISCV_RELAX                             0


On RISC-V, when relaxation is enabled, .L cannot be resolved at assembly
time because sections can shrink.

https://sourceware.org/binutils/docs/as/Symbol-Names.html

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

I consider the GNU as issue a missed optimization, instead of a bug.
There is no rigid rule that .L symbols cannot be saved in object files.