[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 <mel...@google.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1250
Signed-off-by: Fangrui Song <mas...@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;

ret = PTR_ERR(ksym) ?: -ENOENT;
--
2.30.0.284.gd98b1dd5eaa7-goog

[Jessica Yu]

+++ Fangrui Song [13/01/21 21:48 -0800]:

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

[Fāng-ruì Sòng]

Hi Jessica,

I guess __this_module in scripts/mod/modpost.c:ignore_undef_symbol is
not a problem.
For PPC64 _restgpr0_* and _savegpr0_*, I am not sure ignoring the
undefined functions in kernel/module.c is right.
(I know they can be produced by gcc -Os in some cases
(https://reviews.llvm.org/D79977), but I want to learn whether that is
a real issue before adding them.)

If we ever need to ignore more symbols, the code should be refactored

to not warn for unreferenced undefined symbols as my description says.

[Jessica Yu]

+++ Fāng-ruì Sòng [14/01/21 08:57 -0800]:

Hi Fangrui,

Sorry for the confusion, I did not mean to exactly mirror
ignore_undef_symbol() in modpost. The conditions are different there
and not all of them would not apply to kernel/module.c. Like
__this_module, as you say, is not a problem as this will be resolved
once all the module *.o are linked in the final .ko. So when it
reaches the module loader it would no longer be UNDEF. I assume that
it is a similar situation for the PPC64 symbols.

What I meant that we could probably make this patch look nicer by
moving the hardcoded check for "_GLOBAL_OFFSET_TABLE_" to a helper
function ignore_undef_symbol(), similar to how modpost does it,
without adding any additional conditions for now. And yes, if we ever
need to extend this and refactor to not warn for unreferenced
undefined symbols, we should refactor to generalize this case, but for
the scope of this patch I think the suggested change is sufficient for
now.

Thank you,

Jessica

[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 <el...@google.com>

Link: https://github.com/ClangBuiltLinux/linux/issues/1250
Signed-off-by: Fangrui Song <mas...@google.com>
---

kernel/module.c | 20 ++++++++++++++++++--
1 file changed, 18 insertions(+), 2 deletions(-)
---
Changes in v2:
* Fix Marco's email address
* Add a function ignore_undef_symbol similar to scripts/mod/modpost.c:ignore_undef_symbol

diff --git a/kernel/module.c b/kernel/module.c
index 4bf30e4b3eaa..278f5129bde2 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2348,6 +2348,20 @@ static int verify_exported_symbols(struct module *mod)
return 0;
}

+static int ignore_undef_symbol(Elf_Half emachine, const char *name)
+{
+ /* On x86, PIC code and Clang non-PIC code may have call foo@PLT. GNU as
+ * before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_ on x86-64.
+ * i386 has a similar problem but may not deserve a fix.
+ *
+ * If we ever have to ignore many symbols, consider refactoring the code to
+ * only warn if referenced by a relocation.
+ */
+ if (emachine == EM_386 || emachine == EM_X86_64)
+ return !strcmp(name, "_GLOBAL_OFFSET_TABLE_");
+ return 0;
+}
+
/* Change all symbols so that st_value encodes the pointer directly. */

static int simplify_symbols(struct module *mod, const struct load_info *info)

{
@@ -2395,8 +2409,10 @@ 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 or ignored. */
+ if (!ksym &&
+ (ELF_ST_BIND(sym[i].st_info) == STB_WEAK ||
+ ignore_undef_symbol(info->hdr->e_machine, name)))

break;

ret = PTR_ERR(ksym) ?: -ENOENT;
--

2.30.0.296.g2bfb1c46d8-goog

[Nick Desaulniers]

On Thu, Jan 14, 2021 at 1:54 PM 'Fangrui Song' via Clang Built Linux
<clang-bu...@googlegroups.com> wrote:
>
> 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 <el...@google.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> Signed-off-by: Fangrui Song <mas...@google.com>

Thanks for the patch.

Reviewed-by: Nick Desaulniers <ndesau...@google.com>

Jessica, would you mind adding when applying:

Cc: <sta...@vger.kernel.org>

as I suspect we might want this fixed in stable tree's branches, too.
It might of interest to add:

Link: https://sourceware.org/bugzilla/show_bug.cgi?id=27178

too.

--
Thanks,
~Nick Desaulniers

[Marco Elver]

On Thu, 14 Jan 2021 at 22:54, Fangrui Song <mas...@google.com> wrote:
> 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 <el...@google.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> Signed-off-by: Fangrui Song <mas...@google.com>

Tested-by: Marco Elver <el...@google.com>

Thank you for the patch!

> ---
> kernel/module.c | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
> ---
> Changes in v2:
> * Fix Marco's email address
> * Add a function ignore_undef_symbol similar to scripts/mod/modpost.c:ignore_undef_symbol
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 4bf30e4b3eaa..278f5129bde2 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2348,6 +2348,20 @@ static int verify_exported_symbols(struct module *mod)
> return 0;
> }
>
> +static int ignore_undef_symbol(Elf_Half emachine, const char *name)

Why not 'bool' return-type?

> +{
> + /* On x86, PIC code and Clang non-PIC code may have call foo@PLT. GNU as

Not sure if checkpatch.pl warns about this, but this multi-line
comment does not follow the normal kernel-style (see elsewhere in
file):

/*
* ...
*/

[Jessica Yu]

+++ Nick Desaulniers [14/01/21 14:01 -0800]:

Sure, will do!

Thanks,

Jessica

[Jessica Yu]

+++ Marco Elver [15/01/21 08:03 +0100]:

+1 to Marco's comments. Otherwise, patch looks good to me.

Thanks Fangrui!

Jessica

[Fāng-ruì Sòng]

Will use bool and false in v3.

> > +{
> > + /* On x86, PIC code and Clang non-PIC code may have call foo@PLT. GNU as
>
> Not sure if checkpatch.pl warns about this, but this multi-line
> comment does not follow the normal kernel-style (see elsewhere in
> file):

It doesn't warn about this. (The commit description warning cannot be
fixed, even if I place the closing paren on the next line.)

% ./scripts/checkpatch.pl
v2-0001-module-Ignore-_GLOBAL_OFFSET_TABLE_-when-warning-.patch
WARNING: Possible unwrapped commit description (prefer a maximum 75
chars per line)
#8:
https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083333232da3f1d6)

total: 0 errors, 1 warnings, 32 lines checked

NOTE: For some of the reported defects, checkpatch may be able to
mechanically convert to the typical style using --fix or --fix-inplace.

v2-0001-module-Ignore-_GLOBAL_OFFSET_TABLE_-when-warning-.patch has
style problems, please review.

NOTE: If any of the errors are false positives, please report
them to the maintainer, see CHECKPATCH in MAINTAINERS.

--
宋方睿

[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 <el...@google.com>

Link: https://github.com/ClangBuiltLinux/linux/issues/1250
Signed-off-by: Fangrui Song <mas...@google.com>

Reviewed-by: Nick Desaulniers <ndesau...@google.com>
Tested-by: Marco Elver <el...@google.com>
Cc: <sta...@vger.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.
---
kernel/module.c | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 4bf30e4b3eaa..805c49d1b86d 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2348,6 +2348,21 @@ static int verify_exported_symbols(struct module *mod)
return 0;
}

+static bool ignore_undef_symbol(Elf_Half emachine, const char *name)
+{
+ /*
+ * On x86, PIC code and Clang non-PIC code may have call foo@PLT. GNU as

+ * before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_ on x86-64.
+ * i386 has a similar problem but may not deserve a fix.
+ *
+ * If we ever have to ignore many symbols, consider refactoring the code to

+ * only warn if referenced by a relocation.
+ */

+ if (emachine == EM_386 || emachine == EM_X86_64)
+ return !strcmp(name, "_GLOBAL_OFFSET_TABLE_");

+ return false;

+}
+
/* Change all symbols so that st_value encodes the pointer directly. */

static int simplify_symbols(struct module *mod, const struct load_info *info)

{
@@ -2395,8 +2410,10 @@ 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 or ignored. */
+ if (!ksym &&

[Nathan Chancellor]

On Fri, Jan 15, 2021 at 11:52:22AM -0800, 'Fangrui Song' via Clang Built Linux wrote:
> 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 <el...@google.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> Signed-off-by: Fangrui Song <mas...@google.com>
> Reviewed-by: Nick Desaulniers <ndesau...@google.com>
> Tested-by: Marco Elver <el...@google.com>
> Cc: <sta...@vger.kernel.org>

Reviewed-by: Nathan Chancellor <natecha...@gmail.com>

> --
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[Jessica Yu]

+++ Fangrui Song [15/01/21 11:52 -0800]:

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