Re: [PATCH v2] module: Ignore _GLOBAL_OFFSET_TABLE_ when warning for undefined symbols

[Jessica Yu]

+++ Marco Elver [15/01/21 08:03 +0100]:
> On Thu, 14 Jan 2021 at 22:54, Fangrui Song <maskray@xxxxxxxxxx> 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 <elver@xxxxxxxxxx>
> > Link: https://github.com/ClangBuiltLinux/linux/issues/1250
> > Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
>
> Tested-by: Marco Elver <elver@xxxxxxxxxx>
>
> 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):
>
> /*
> * ...
> */

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

Thanks Fangrui!

Jessica