linux-hardening - Re: [PATCH] tty: mxser: Remove __counted_by from mxser

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Message-ID: <2024062800-doodle-pelvis-4798@gregkh>
Date: Fri, 28 Jun 2024 16:31:46 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Kees Cook <kees@...nel.org>
Cc: Jiri Slaby <jirislaby@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	"Gustavo A. R. Silva" <gustavoars@...nel.org>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, linux-serial@...r.kernel.org,
	imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-hardening@...r.kernel.org, llvm@...ts.linux.dev,
	patches@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH] tty: mxser: Remove __counted_by from mxser_board.ports[]

On Thu, Jun 27, 2024 at 10:14:05AM -0700, Kees Cook wrote:
> On Wed, May 29, 2024 at 02:29:42PM -0700, Nathan Chancellor wrote:
> > Work for __counted_by on generic pointers in structures (not just
> > flexible array members) has started landing in Clang 19 (current tip of
> > tree). During the development of this feature, a restriction was added
> > to __counted_by to prevent the flexible array member's element type from
> > including a flexible array member itself such as:
> > 
> >   struct foo {
> >     int count;
> >     char buf[];
> >   };
> > 
> >   struct bar {
> >     int count;
> >     struct foo data[] __counted_by(count);
> >   };
> > 
> > because the size of data cannot be calculated with the standard array
> > size formula:
> > 
> >   sizeof(struct foo) * count
> > 
> > This restriction was downgraded to a warning but due to CONFIG_WERROR,
> > it can still break the build. The application of __counted_by on the
> > ports member of 'struct mxser_board' triggers this restriction,
> > resulting in:
> > 
> >   drivers/tty/mxser.c:291:2: error: 'counted_by' should not be applied to an array with element of unknown size because 'struct mxser_port' is a struct type with a flexible array member. This will be an error in a future compiler version [-Werror,-Wbounds-safety-counted-by-elt-type-unknown-size]
> >     291 |         struct mxser_port ports[] __counted_by(nports);
> >         |         ^~~~~~~~~~~~~~~~~~~~~~~~~
> >   1 error generated.
> > 
> > Remove this use of __counted_by to fix the warning/error. However,
> > rather than remove it altogether, leave it commented, as it may be
> > possible to support this in future compiler releases.
> > 
> > Cc: stable@...r.kernel.org
> > Closes: https://github.com/ClangBuiltLinux/linux/issues/2026
> > Fixes: f34907ecca71 ("mxser: Annotate struct mxser_board with __counted_by")
> > Signed-off-by: Nathan Chancellor <nathan@...nel.org>
> 
> Since this fixes a build issue under Clang, can we please land this so
> v6.7 and later will build again? Gustavo is still working on the more
> complete fix (which was already on his radar, so it won't be lost).
> 
> If it's easier/helpful, I can land this via the hardening tree? I was
> the one who sent the bad patch originally. :)
> 

I don't see this in my queue anywhere, sorry, can you resend it?  Or
feel free to take it through your trees, no objection from me there.

thanks,

greg k-h