On Thu, Jan 14, 2021 at 11:04 PM Marco Elver <el...@google.com> wrote:
>
> On Thu, 14 Jan 2021 at 22:54, Fangrui Song <mask...@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 <mask...@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?
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.
> /*
> * ...
> */
>
> > + * 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
> >
--
宋方睿
