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 <elver@xxxxxxxxxx> > Link: https://github.com/ClangBuiltLinux/linux/issues/1250 > Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx> > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > Tested-by: Marco Elver <elver@xxxxxxxxxx> > Cc: <stable@xxxxxxxxxxxxxxx> Reviewed-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > > --- > 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 && > + (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 > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/20210115195222.3453262-1-maskray%40google.com.
