[PATCH] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for undefined symbols

[Fangrui Song]

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.

Reported-by: Marco Elver <melver@xxxxxxxxxx>
Link: https://github.com/ClangBuiltLinux/linux/issues/1250
Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
---
 kernel/module.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 4bf30e4b3eaa..2e2deea99289 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2395,8 +2395,14 @@ static int simplify_symbols(struct module *mod, const struct load_info *info)
 				break;
 			}
 
-			/* Ok if weak.  */
-			if (!ksym && ELF_ST_BIND(sym[i].st_info) == STB_WEAK)
+			/* Ok if weak. Also allow _GLOBAL_OFFSET_TABLE_:
+			 * GNU as before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_
+			 * for call foo@PLT on x86-64.  If the code ever needs to ignore
+			 * more symbols, refactor the code to only warn if referenced by
+			 * a relocation.
+			 */
+			if (!ksym && (ELF_ST_BIND(sym[i].st_info) == STB_WEAK ||
+				      !strcmp(name, "_GLOBAL_OFFSET_TABLE_")))
 				break;
 
 			ret = PTR_ERR(ksym) ?: -ENOENT;
-- 
2.30.0.284.gd98b1dd5eaa7-goog