linux-kernel - Re: [PATCH][next] pwm: cros-ec: Avoid -Wflex-array-member-not-at-end warnings

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Message-ID: <mlvple4tcnvu35a2liva5wxhafshc634fu46kwyrepn75wvuec@udshn3ja6lvc>
Date: Thu, 14 Aug 2025 16:08:57 +0200
From: Uwe Kleine-König <ukleinek@...nel.org>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
	Benson Leung <bleung@...omium.org>, Guenter Roeck <groeck@...omium.org>, linux-pwm@...r.kernel.org, 
	chrome-platform@...ts.linux.dev, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH][next] pwm: cros-ec: Avoid -Wflex-array-member-not-at-end
 warnings

On Thu, Aug 14, 2025 at 08:48:23PM +0900, Gustavo A. R. Silva wrote:
> 
> > diff --git a/include/linux/stddef.h b/include/linux/stddef.h
> > index dab49e2ec8c0..8ca9df87a523 100644
> > --- a/include/linux/stddef.h
> > +++ b/include/linux/stddef.h
> > @@ -108,7 +108,7 @@ enum {
> >   	union {									\
> >   		TYPE NAME;							\
> >   		struct {							\
> > -			unsigned char __offset_to_##FAM[offsetof(TYPE, FAM)];	\
> > +			unsigned char __offset_to_##FAM[sizeof(TYPE)];		\
> >   			MEMBERS							\
> >   		};								\
> >   	}
> > 
> > which only leaves one usage of FAM in the name of the padding struct
> > member. I'm sure someone is able to come up with something nice here to
> > get rid of FAM completely or point out what I'm missing.
> 
> Flexible structures (structs that contain a FAM) may have trailing padding.
> Under that scenario sizeof(TYPE) causes the overlay between FAM and MEMBERS
> to be misaligned.

That sounds wrong to me; are you sure? In that case allocating space for
such a struct using

	struct mystruct {
		unsigned short len;
		unsigned int array[];
	};

	s = malloc(sizeof(struct mystruct) + n * sizeof(unsigned int));

wouldn't do the right thing. 

I found in the net (e.g.
https://rgambord.github.io/c99-doc/sections/6/7/2/1/index.html):

	In most situations, the flexible array member is ignored. In
	particular, the size of the structure is as if the flexible
	array member were omitted except that it may have more trailing
	padding than the omission would imply.

So I'd claim that sizeof does work here as intended.

gcc here also behaves fine:

	uwe@...rus:~$ cat test.c
	#include <stdio.h>

	struct mystruct {
		unsigned short len;
		unsigned int array[];
	};

	struct mystruct2 {
		unsigned short len;
	};

	int main()
	{
		printf("sizeof(struct mystruct) = %zu\n", sizeof(struct mystruct));
		printf("sizeof(struct mystruct2) = %zu\n", sizeof(struct mystruct2));
		return 0;
	}

	uwe@...rus:~$ make test
	cc    -c -o test.o test.c
	cc   test.o   -o test

	uwe@...rus:~$ ./test
	sizeof(struct mystruct) = 4
	sizeof(struct mystruct2) = 2

Best regards
Uwe

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