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From:   =?UTF-8?B?RsSBbmctcnXDrCBTw7JuZw==?= <maskray@google.com>
Date:   Fri, 15 Jan 2021 11:50:02 -0800
Message-ID: <CAFP8O3Jv23Vztn0puyXVDtpqAN9sHKd6PdHGUdzA5q7v-dmp4w@mail.gmail.com>
Subject: Re: [PATCH v2] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for
 undefined symbols
To:     Marco Elver <elver@google.com>
Cc:     LKML <linux-kernel@vger.kernel.org>, Jessica Yu <jeyu@kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Sam Ravnborg <sam@ravnborg.org>
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On Thu, Jan 14, 2021 at 11:04 PM Marco Elver <elver@google.com> wrote:
>
> On Thu, 14 Jan 2021 at 22:54, Fangrui Song <maskray@google.com> wrote:
> > clang-12 -fno-pic (since
> > https://github.com/llvm/llvm-project/commit/a084c0388e2a59b9556f2de0083=
333232da3f1d6)
> > 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 th=
e
> > 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` fo=
r
> > 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@google.com>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> > Signed-off-by: Fangrui Song <maskray@google.com>
>
> Tested-by: Marco Elver <elver@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:i=
gnore_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?

Will use bool and false in v3.

> > +{
> > +       /* On x86, PIC code and Clang non-PIC code may have call foo@PL=
T. 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/a084c0388e2a59b9556f2de00833332=
32da3f1d6)

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-inplac=
e.

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.

> /*
>  * ...
>  */
>
> > +        * before 2.37 produces an unreferenced _GLOBAL_OFFSET_TABLE_ o=
n 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 =3D=3D EM_386 || emachine =3D=3D 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) =3D=3D=
 STB_WEAK)
> > +                       /* Ok if weak or ignored.  */
> > +                       if (!ksym &&
> > +                           (ELF_ST_BIND(sym[i].st_info) =3D=3D STB_WEA=
K ||
> > +                            ignore_undef_symbol(info->hdr->e_machine, =
name)))
> >                                 break;
> >
> >                         ret =3D PTR_ERR(ksym) ?: -ENOENT;
> > --
> > 2.30.0.296.g2bfb1c46d8-goog
> >


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