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SubjectRe: [PATCH] kbuild: fix kernel/bounds.c 'W=1' warning
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On 05/10/18 10:07, Arnd Bergmann wrote:
> On Fri, Oct 5, 2018 at 10:52 AM David Laight <David.Laight@aculab.com> wrote:
>>
>> From: Arnd Bergmann
>>> Sent: 05 October 2018 09:33
>>>
>>> Building any configuration with 'make W=1' produces a warning:
>>>
>>> kernel/bounds.c:16:6: warnign: no previous prototype for 'foo' [-Wmissing-prototypes]
>>>
>>> When also passing -Werror, this prevents us from building any
>>> other files. Nobody ever calls the function, but we can't make
>>> it 'static' either since we want the compiler output.
>>>
>>> Calling it 'main' instead however avoids the warning, because gcc
>>> does not insist on having a declaration for main.
>>
>> Ugg.
>> main() might be special in other ways too.
>> It wouldn't surprise me if some linkers don't do special stuff for it.

I worried about this but didn't think it would be too much of an issue.
But perhaps we should check...

<compile bounds.s in both configurations> as bounds.s.foo and bounds.s.main:


diff -Nurp bounds.s.*
--- bounds.s.foo 2018-10-05 10:20:53.269941404 +0100
+++ bounds.s.main 2018-10-05 10:20:31.375891260 +0100
@@ -108,11 +108,12 @@

.global _mcount
#NO_APP
+ .section .text.startup,"ax",@progbits
.align 2
.p2align 3,,7
- .global foo
- .type foo, %function
-foo:
+ .global main
+ .type main, %function
+main:
stp x29, x30, [sp, -16]! //,,,
add x29, sp, 0 //,,
// /home/linuxembedded/iob/renesas/vsp1/sources/linux/kernel/bounds.c:17: {
@@ -139,10 +140,11 @@ foo:

.ascii "->SPINLOCK_SIZE 56 sizeof(spinlock_t)" //
// 0 "" 2
-// /home/linuxembedded/iob/renesas/vsp1/sources/linux/kernel/bounds.c:26: }
+// /home/linuxembedded/iob/renesas/vsp1/sources/linux/kernel/bounds.c:28: }
#NO_APP
+ mov w0, 0 //,
ldp x29, x30, [sp], 16 //,,,
ret
- .size foo, .-foo
+ .size main, .-main
.ident "GCC: (Ubuntu/Linaro 7.3.0-16ubuntu3) 7.3.0"
.section .note.GNU-stack,"",@progbits


compiled with aarch64-linux-gnu-gcc, and with no debug enabled.

Other than the entry point rename (and section name) and the return
value being added, I can't see anything problematic here.

And as far as I know - this file gets processed after to extract
definitions which should be independent. This file is not executed or
further compiled as far as I am aware.

--
Kieran



>>
>> What is wrong with just putting and extra "void foo(void);" before
>> the function?
>
> Greg objected to that on the basis that we don't want declarations
> in .c files -- they should be in a shared header:
>
> https://lkml.org/lkml/2018/9/21/735
>
> I don't see what could go wrong here with calling it main(), after
> all we are just interested in the assembler output, not even
> creating an object file.
>
> Arnd
>

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