Messages in this thread Patch in this message | | | Date | Mon, 16 May 2022 23:40:05 +0200 | From | Peter Zijlstra <> | Subject | Re: objtool "no non-local symbols" error with tip of tree LLVM |
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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);
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