* clang: objtool: native_machine_emergency_restart() falls through to next function
@ 2018-06-15 21:14 Matthias Kaehlcke
2018-06-18 22:17 ` Josh Poimboeuf
0 siblings, 1 reply; 3+ messages in thread
From: Matthias Kaehlcke @ 2018-06-15 21:14 UTC (permalink / raw)
To: Josh Poimboeuf; +Cc: linux-kernel, Manoj Gupta, Nick Desaulniers
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
Thanks
Matthias
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: clang: objtool: native_machine_emergency_restart() falls through to next function
2018-06-15 21:14 clang: objtool: native_machine_emergency_restart() falls through to next function Matthias Kaehlcke
@ 2018-06-18 22:17 ` Josh Poimboeuf
2018-06-18 22:52 ` Matthias Kaehlcke
0 siblings, 1 reply; 3+ messages in thread
From: Josh Poimboeuf @ 2018-06-18 22:17 UTC (permalink / raw)
To: Matthias Kaehlcke; +Cc: linux-kernel, Manoj Gupta, Nick Desaulniers
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)
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: clang: objtool: native_machine_emergency_restart() falls through to next function
2018-06-18 22:17 ` Josh Poimboeuf
@ 2018-06-18 22:52 ` Matthias Kaehlcke
0 siblings, 0 replies; 3+ messages in thread
From: Matthias Kaehlcke @ 2018-06-18 22:52 UTC (permalink / raw)
To: Josh Poimboeuf; +Cc: linux-kernel, Manoj Gupta, Nick Desaulniers
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
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2018-06-18 22:52 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2018-06-15 21:14 clang: objtool: native_machine_emergency_restart() falls through to next function Matthias Kaehlcke
2018-06-18 22:17 ` Josh Poimboeuf
2018-06-18 22:52 ` Matthias Kaehlcke
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).