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clang: objtool: native_machine_emergency_restart() falls through to next function

DISCUSSION Matthias Kaehlcke 2 participants 3 messages Jun 15, 2018 – Jun 18, 2018 objtool

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Matthias Kaehlcke

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#0 lore raw

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

#1 lore raw
Quoted 25 lines · Matthias Kaehlcke

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?

Quoted 6 lines

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?

tools/objtool/check.c +1 -0
index 5409f6f6c48d..ace3f85701c8 100644
@@ -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)
author
#2 lore raw
Quoted 43 lines · Josh Poimboeuf
Quoted 25 lines · Matthias Kaehlcke

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?

Quoted 6 lines

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?

tools/objtool/check.c +0 -0
index 5409f6f6c48d..ace3f85701c8 100644

tools/objtool/check.c static int __dead_end_function(struct objtool_file *file, struct symbol *func, · line 145

Quoted 7 lines
tools/objtool/check.c +1 -0
@@ -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