Bug#1000974: [PATCH xfsprogs-5.14.2 URGENT] libxfs: hide the drainbamaged fallthrough macro from xfslibs

Eric Sandeen
2021-12-06T14:36:19Z

On 12/5/21 11:49 AM, Darrick J. Wong wrote:
> From: Darrick J. Wong <djwong@kernel.org>
>
> Back in mid-2021, Kees and Gustavo rammed into the kernel a bunch of
> static checker "improvements" that redefined '/* fallthrough */'
> comments for switch statements as a macro that virtualizes either that
> same comment, a do-while loop, or a compiler __attribute__. This was
> necessary to work around the poor decision-making of the clang, gcc, and
> C language standard authors, who collectively came up with four mutually
> incompatible ways to document a lack of branching in a code flow.
>
> Having received ZERO HELP porting this to userspace, Eric and I
> foolishly dumped that crap into linux.h, which was a poor decision
> because we keep forgetting that linux.h is exported as a userspace
> header. This has now caused downstream regressions in Debian[1] and
> will probably cause more problems in the other distros.
>
> Move it to platform_defs.h since that's not shipped publicly and leave a
> warning to anyone else who dare modify linux.h.
>
> [1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1000974
>
> Fixes: df9c7d8d ("xfs: Fix fall-through warnings for Clang")
> Cc: 1000974@bugs.debian.org, gustavoars@kernel.org, keescook@chromium.org
> Signed-off-by: Darrick J. Wong <djwong@kernel.org>

Thanks Darrick, I'll get a 5.14.2 pushed out today. Not sure if it was you
or I who stuffed it into this header originally, but apologies for not
thinking that through before merging it in any case.

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

-Eric

> ---
> include/linux.h | 20 ++------------------
> include/platform_defs.h.in | 21 +++++++++++++++++++++
> 2 files changed, 23 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux.h b/include/linux.h
> index 24650228..054117aa 100644
> --- a/include/linux.h
> +++ b/include/linux.h
> @@ -360,24 +360,8 @@ fsmap_advance(
> #endif /* HAVE_MAP_SYNC */
>
> /*
> - * Add the pseudo keyword 'fallthrough' so case statement blocks
> - * must end with any of these keywords:
> - * break;
> - * fallthrough;
> - * continue;
> - * goto ;
> - * return [expression];
> - *
> - * gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
> + * Reminder: anything added to this file will be compiled into downstream
> + * userspace projects!
> */
> -#if defined __has_attribute
> -# if __has_attribute(__fallthrough__)
> -# define fallthrough __attribute__((__fallthrough__))
> -# else
> -# define fallthrough do {} while (0) /* fallthrough */
> -# endif
> -#else
> -# define fallthrough do {} while (0) /* fallthrough */
> -#endif
>
> #endif /* __XFS_LINUX_H__ */
> diff --git a/include/platform_defs.h.in b/include/platform_defs.h.in
> index 7c6b3ada..6e6f26ef 100644
> --- a/include/platform_defs.h.in
> +++ b/include/platform_defs.h.in
> @@ -113,4 +113,25 @@ static inline size_t __ab_c_size(size_t a, size_t b, size_t c)
> sizeof(*(p)->member) + __must_be_array((p)->member), \
> sizeof(*(p)))
>
> +/*
> + * Add the pseudo keyword 'fallthrough' so case statement blocks
> + * must end with any of these keywords:
> + * break;
> + * fallthrough;
> + * continue;
> + * goto ;
> + * return [expression];
> + *
> + * gcc: https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html#Statement-Attributes
> + */
> +#if defined __has_attribute
> +# if __has_attribute(__fallthrough__)
> +# define fallthrough __attribute__((__fallthrough__))
> +# else
> +# define fallthrough do {} while (0) /* fallthrough */
> +# endif
> +#else
> +# define fallthrough do {} while (0) /* fallthrough */
> +#endif
> +
> #endif /* __XFS_PLATFORM_DEFS_H__ */
>