Messages in this thread | | | Date | Mon, 16 May 2022 13:47:15 -0700 | From | Nathan Chancellor <> | Subject | objtool "no non-local symbols" error with tip of tree LLVM |
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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
As far as I understand it, the kernel uses constructors for at least KASAN and KCSAN, hence why that change impacts the kernel. Beyond that, I am not really sure whether the LLVM change is problematic or objtool just is not accounting for something that it should. I am happy to provide any additional information that might help understand what is going wrong here.
[1]: https://github.com/llvm/llvm-project/commit/badd088c57d7d18acd337b7868fe8c7974c88c5b
Cheers, Nathan
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