Re: [PATCH v3] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for undefined symbols

[Jessica Yu]

+++ Fangrui Song [15/01/21 11:52 -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 <elver@xxxxxxxxxx>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> Tested-by: Marco Elver <elver@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
>
> ---
> Changes in v2:
> * Fix Marco's email address
> * Add a function ignore_undef_symbol similar to scripts/mod/modpost.c:ignore_undef_symbol
> ---
> Changes in v3:
> * Fix the style of a multi-line comment.
> * Use static bool ignore_undef_symbol.

Patch has been queued up on modules-next:

https://git.kernel.org/pub/scm/linux/kernel/git/jeyu/linux.git/commit/?h=modules-next&id=ebfac7b778fac8b0e8e92ec91d0b055f046b4604

Thanks!

Jessica