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

[Dave Chinner <david@fromorbit.com>]

On Sun, Dec 05, 2021 at 09:49:51AM -0800, 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>
> ---
>  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 <label>;
> - *   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!

This comment belongs at the top of the file before all the includes,
not at the end of it. Otherwise looks ok for a quick fix.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

But I have to wonder - why is this file even exported to userspace?
It's mostly the xfsprogs source build wrapper stuff for linux -
there's no public API in it except for xfsctl()....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com