Re: [PATCH] ext4: fix ext4_tune_sb_params padding

[David Laight <david.laight.linux@gmail.com>]

On Thu, 04 Dec 2025 14:42:06 +0100
"Arnd Bergmann" <arnd@arndb.de> wrote:

> On Thu, Dec 4, 2025, at 13:35, David Laight wrote:
> > On Thu,  4 Dec 2025 11:19:10 +0100
> > Arnd Bergmann <arnd@kernel.org> wrote:
> >  
> >> From: Arnd Bergmann <arnd@arndb.de>
> >> 
> >> The padding at the end of struct ext4_tune_sb_params is architecture
> >> specific and in particular is different between x86-32 and x86-64,
> >> since the __u64 member only enforces struct alignment on the latter.  
> >
> > Is it worth adding a compile-time check for the size somewhere?
> > Since the intention seems to be that any extensions will use the padding.  
> 
> There is already ABI checking with abigail that ensures that struct
> members and sizes don't change in the future, which I think covers
> that. I would also like to push my series to enable -Werror=padded
> in the header checks, but I'm not sure yet what others think of the
> idea.

Putting it in the command line is going to be griefsome (at least in the
short term) even for uapi headers - where you really don't want padding.
(Tell that to some of the standards bodies...)
It is a shame there isn't an attribute, but you can wrap definitions:

#define check_padding(...) _Pragma("GCC diagnostic push"); \
    _Pragma("GCC diagnostic error \"-Wpadded\""); \
    __VA_ARGS__ \
    _Pragma("GCC diagnostic pop");
    
check_padding(

typedef struct fubar {
    int a;
    char b;
} fred;

) /* check_padding */

I've thought about doing something similar to avoid the 'type-limits' check
inside statically_true() and the like for W=1 builds.

	David