Re: objtool "no non-local symbols" error with tip of tree LLVM

[Peter Zijlstra <peterz@infradead.org>]

On Mon, May 16, 2022 at 01:47:15PM -0700, Nathan Chancellor wrote:
> Hi Josh and Peter,
> 
> After a recent change in LLVM [1], I see warnings (errors?) from objtool
> when building x86_64 allmodconfig on 5.15 and 5.17:
> 
>   $ make -skj"$(nproc)" KCONFIG_ALLCONFIG=<(echo CONFIG_WERROR) LLVM=1 allmodconfig all
>   ...
>   mm/highmem.o: warning: objtool: no non-local symbols !?
>   mm/highmem.o: warning: objtool: gelf_update_symshndx: invalid section index
>   make[2]: *** [scripts/Makefile.build:288: mm/highmem.o] Error 255
>   ...
>   security/tomoyo/load_policy.o: warning: objtool: no non-local symbols !?
>   security/tomoyo/load_policy.o: warning: objtool: gelf_update_symshndx: invalid section index
>   make[3]: *** [scripts/Makefile.build:288: security/tomoyo/load_policy.o] Error 255
>   ...
> 
> I don't see the same errors on x86_64 allmodconfig on mainline so I
> bisected the 5.17 branch and came upon commit 4abff6d48dbc ("objtool:
> Fix code relocs vs weak symbols"). I wanted to see what 5.17 might be
> missing and came to commit ed53a0d97192 ("x86/alternative: Use
> .ibt_endbr_seal to seal indirect calls") in mainline, which I think just
> hides the issue for allmodconfig. I can reproduce this problem with a
> more selective set of config values on mainline:
> 
>   $ make -skj"$(nproc)" LLVM=1 defconfig
> 
>   $ scripts/config -e KASAN -e SECURITY_TOMOYO -e SECURITY_TOMOYO_OMIT_USERSPACE_LOADER
> 
>   $ make -skj"$(nproc)" LLVM=1 olddefconfig security/tomoyo/load_policy.o
>   security/tomoyo/load_policy.o: warning: objtool: no non-local symbols !?
>   security/tomoyo/load_policy.o: warning: objtool: gelf_update_symshndx: invalid section index
>   make[3]: *** [scripts/Makefile.build:288: security/tomoyo/load_policy.o] Error 255
>   ...
> 
> Looking at the object file, the '.text.asan.module_ctor' section has
> disappeared.
> 
> Before:
> 
>   $ llvm-nm -S security/tomoyo/load_policy.o
>   0000000000000000 0000000000000001 t asan.module_ctor
> 
>   $ llvm-readelf -s security/tomoyo/load_policy.o
> 
>   Symbol table '.symtab' contains 4 entries:
>      Num:    Value          Size Type    Bind   Vis       Ndx Name
>        0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT   UND
>        1: 0000000000000000     0 FILE    LOCAL  DEFAULT   ABS load_policy.c
>        2: 0000000000000000     0 SECTION LOCAL  DEFAULT     3 .text.asan.module_ctor
>        3: 0000000000000000     1 FUNC    LOCAL  DEFAULT     3 asan.module_ctor
> 
> After:
> 
>   $ llvm-nm -S security/tomoyo/load_policy.o
>   0000000000000000 0000000000000001 t asan.module_ctor
> 
>   $ llvm-readelf -s security/tomoyo/load_policy.o
> 
>   Symbol table '.symtab' contains 3 entries:
>      Num:    Value          Size Type    Bind   Vis       Ndx Name
>        0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT   UND
>        1: 0000000000000000     0 FILE    LOCAL  DEFAULT   ABS load_policy.c
>        2: 0000000000000000     1 FUNC    LOCAL  DEFAULT     3 asan.module_ctor
> 

The problem seems to be that we need to add a local symbols because LLVM
helpfully stripped all unused section symbols.

The way we do that, is by moving a the first non-local symbol to the
end, thereby creating a hole where we can insert a new local symbol.
Because ELF very helpfully mandates that local symbols must come before
non-local symbols and keeps the symbols index of the first non-local in
sh_info.

Thing is, the above object files don't appear to have a non-local symbol
so the swizzle thing isn't needed, and apparently the value in sh_info
isn't valid either.

Does something simple like this work? If not, I'll try and reproduce
tomorrow, it shouldn't be too hard to fix.

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 583a3ec987b5..baabf38a2a11 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -618,8 +618,7 @@ static int elf_move_global_symbol(struct elf *elf, struct section *symtab,
 
 	sym = find_symbol_by_index(elf, first_non_local);
 	if (!sym) {
-		WARN("no non-local symbols !?");
-		return first_non_local;
+		return symtab->sh.sh_size / sizeof(sym->sym);
 	}
 
 	s = elf_getscn(elf->elf, symtab->idx);