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

[Jessica Yu <jeyu@kernel.org>]

+++ Fangrui Song [13/01/21 21:48 -0800]:
>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@google.com>
>Link: https://github.com/ClangBuiltLinux/linux/issues/1250
>Signed-off-by: Fangrui Song <maskray@google.com>
>---
> 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;

Hi Fangrui,

Thanks for the patch. I am puzzled why we don't already mirror modpost
here, that particular line of code in modpost to ignore _GLOBAL_OFFSET_TABLE_
has been there long before my time. Let's properly mirror modpost
then, and create a similar helper function ignore_undef_symbol() (and
stick the _GLOBAL_OFFSET_TABLE_ check in there) to account for future
cases like this.

Thanks,

Jessica