Messages in this thread | | | Date | Mon, 18 Jun 2018 15:52:30 -0700 | From | Matthias Kaehlcke <> | Subject | Re: clang: objtool: native_machine_emergency_restart() falls through to next function |
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On Mon, Jun 18, 2018 at 05:17:16PM -0500, Josh Poimboeuf wrote: > On Fri, Jun 15, 2018 at 02:14:41PM -0700, Matthias Kaehlcke wrote: > > Hi Josh, > > > > with your work on objtool and the upcoming implementation of > > -fno-delete-null-pointer-checks (https://reviews.llvm.org/D47894, > > https://reviews.llvm.org/D47895) in clang most objtool warnings > > for clang builds will be fixed. > > > > However even with -fno-delete-null-pointer-checks we currently still > > get a warning about native_machine_emergency_restart(): > > > > arch/x86/kernel/reboot.o: warning: objtool: > > native_machine_emergency_restart() > > falls through to next function machine_power_off() > > > > This only occurs when building with -Oz. > > > > One of our compiler engineers looked into this, LLVM optimizes the > > return statement away since the function has an endless loop, and > > actually never returns. > > > > Is there a way to tell objtool that the function is not expected to > > return (I tried the attribute __noreturn, but it doesn't make a > > difference), or do we have to tell clang to restrain from optimizing > > returns out? > > > > To repro: > > > > git checkout v4.16 > > make CC=clang defconfig > > echo "CONFIG_CC_OPTIMIZE_FOR_SIZE=y" >> .config > > make CC=clang arch/x86/kernel/reboot.o > > Hi Matthias, > > Objtool can't detect 'noreturn' functions, so unfortunately we have to > keep a manual list. Does this fix it? > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c > index 5409f6f6c48d..ace3f85701c8 100644 > --- a/tools/objtool/check.c > +++ b/tools/objtool/check.c > @@ -139,6 +139,7 @@ static int __dead_end_function(struct objtool_file *file, struct symbol *func, > "lbug_with_loc", > "fortify_panic", > "usercopy_abort", > + "machine_real_restart", > }; > > if (func->bind == STB_WEAK)
Yes this helps, thanks for having a look!
Could you send this as an official patch or should I do it (crediting you)?
Thanks
Matthias
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