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module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for undefined symbols

clang-12 -fno-pic (since
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)
can emit `call __stack_chk_fail@PLT` instead of `call __stack_chk_fail`
on x86. The two forms should have identical behaviors on x86-64 but the
former causes GNU as<2.37 to produce an unreferenced undefined symbol
_GLOBAL_OFFSET_TABLE_.

(On x86-32, there is an R_386_PC32 vs R_386_PLT32 difference but the
linker behavior is identical as far as Linux kernel is concerned.)

Simply ignore _GLOBAL_OFFSET_TABLE_ for now, like what
scripts/mod/modpost.c:ignore_undef_symbol does. This also fixes the
problem for gcc/clang -fpie and -fpic, which may emit `call foo@PLT` for
external function calls on x86.

Note: ld -z defs and dynamic loaders do not error for unreferenced
undefined symbols so the module loader is reading too much. If we ever
need to ignore more symbols, the code should be refactored to ignore
unreferenced symbols.

Cc: <stable@vger.kernel.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/1250
Link: https://sourceware.org/bugzilla/show_bug.cgi?id=27178
Reported-by: Marco Elver <elver@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Marco Elver <elver@google.com>
Signed-off-by: Fangrui Song <maskray@google.com>
Signed-off-by: Jessica Yu <jeyu@kernel.org>

authored by

Fangrui Song and committed by
Jessica Yu ebfac7b7 19c329f6

+19 -2
+19 -2
kernel/module.c
··· 2348 2348 return 0; 2349 2349 } 2350 2350 2351 + static bool ignore_undef_symbol(Elf_Half emachine, const char *name) 2352 + { 2353 + /* 2354 + * On x86, PIC code and Clang non-PIC code may have call foo@PLT. GNU as 2355 + * before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_ on x86-64. 2356 + * i386 has a similar problem but may not deserve a fix. 2357 + * 2358 + * If we ever have to ignore many symbols, consider refactoring the code to 2359 + * only warn if referenced by a relocation. 2360 + */ 2361 + if (emachine == EM_386 || emachine == EM_X86_64) 2362 + return !strcmp(name, "_GLOBAL_OFFSET_TABLE_"); 2363 + return false; 2364 + } 2365 + 2351 2366 /* Change all symbols so that st_value encodes the pointer directly. */ 2352 2367 static int simplify_symbols(struct module *mod, const struct load_info *info) 2353 2368 { ··· 2410 2395 break; 2411 2396 } 2412 2397 2413 - /* Ok if weak. */ 2414 - if (!ksym && ELF_ST_BIND(sym[i].st_info) == STB_WEAK) 2398 + /* Ok if weak or ignored. */ 2399 + if (!ksym && 2400 + (ELF_ST_BIND(sym[i].st_info) == STB_WEAK || 2401 + ignore_undef_symbol(info->hdr->e_machine, name))) 2415 2402 break; 2416 2403 2417 2404 ret = PTR_ERR(ksym) ?: -ENOENT;