linux-kernel - Re: [PATCH] treewide: Replace zero-length arrays with flexible-array member

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Date:   Tue, 11 Feb 2020 11:38:54 -0800
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        linux-crypto@...r.kernel.org, netdev@...r.kernel.org,
        linux-usb@...r.kernel.org
Subject: Re: [PATCH] treewide: Replace zero-length arrays with flexible-array
 member

On Tue, Feb 11, 2020 at 11:32:04AM -0800, Kees Cook wrote:
> On Tue, Feb 11, 2020 at 01:20:36PM -0600, Gustavo A. R. Silva wrote:
> > 
> > 
> > On 2/11/20 12:32, Greg KH wrote:
> > > On Tue, Feb 11, 2020 at 11:41:26AM -0600, Gustavo A. R. Silva wrote:
> > >> The current codebase makes use of the zero-length array language
> > >> extension to the C90 standard, but the preferred mechanism to declare
> > >> variable-length types such as these ones is a flexible array member[1][2],
> > >> introduced in C99:
> > >>
> > >> struct foo {
> > >>         int stuff;
> > >>         struct boo array[];
> > >> };
> > >>
> > >> By making use of the mechanism above, we will get a compiler warning
> > >> in case the flexible array does not occur last in the structure, which
> > >> will help us prevent some kind of undefined behavior bugs from being
> > >> unadvertenly introduced[3] to the codebase from now on.
> > >>
> > >> All these instances of code were found with the help of the following
> > >> Coccinelle script:
> > >>
> > >> @@
> > >> identifier S, member, array;
> > >> type T1, T2;
> > >> @@
> > >>
> > >> struct S {
> > >>   ...
> > >>   T1 member;
> > >>   T2 array[
> > >> - 0
> > >>   ];
> > >> };
> > >>
> > >> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> > >> [2] https://github.com/KSPP/linux/issues/21
> > >> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> > >>
> > >> NOTE: I'll carry this in my -next tree for the v5.6 merge window.
> > > 
> > > Why not carve this up into per-subsystem patches so that we can apply
> > > them to our 5.7-rc1 trees and then you submit the "remaining" that don't
> > > somehow get merged at that timeframe for 5.7-rc2?
> > > 
> > 
> > Yep, sounds good. I'll do that.
> 
> FWIW, I'd just like to point out that since this is a mechanical change
> with no code generation differences (unlike the pre-C90 1-byte array
> conversions), it's a way better use of everyone's time to just splat
> this in all at once.
> 
> That said, it looks like Gustavo is up for it, but I'd like us to
> generally consider these kinds of mechanical changes as being easier to
> manage in a single patch. (Though getting Acks tends to be a bit
> harder...)

Hey, if this is such a mechanical patch, let's get it to Linus now,
what's preventing that from being merged now?

thanks,

greg k-h