linux-kernel - Re: [PATCH v3] module: Ignore _GLOBAL_OFFSET_TABLE

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Date:   Mon, 18 Jan 2021 11:14:55 +0100
From:   Jessica Yu <jeyu@...nel.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        Sam Ravnborg <sam@...nborg.org>,
        Marco Elver <elver@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH v3] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for
 undefined symbols

+++ 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@...` 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@...` 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@...gle.com>
>Link: https://github.com/ClangBuiltLinux/linux/issues/1250
>Signed-off-by: Fangrui Song <maskray@...gle.com>
>Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>Tested-by: Marco Elver <elver@...gle.com>
>Cc: <stable@...r.kernel.org>
>
>---
>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