linux-kernel - Re: [PATCH] ibft: Replace zero-length array with flexible-array

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Date:   Tue, 23 Jun 2020 09:46:56 -0400
From:   Konrad Rzeszutek Wilk <konrad@...nok.org>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Peter Jones <pjones@...hat.com>,
        Konrad Rzeszutek Wilk <konrad@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ibft: Replace zero-length array with flexible-array

On Thu, May 21, 2020 at 01:32:35PM -0500, Gustavo A. R. Silva wrote:
> Hi all,
> 
> Friendly ping: who can take this?

It is in the tree. Let me send out a git pull to Linus in a week or two.

Thanks!
> 
> Thanks
> --
> Gustavo
> 
> On Thu, May 07, 2020 at 01:55:44PM -0500, 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
> > inadvertently introduced[3] to the codebase from now on.
> > 
> > Also, notice that, dynamic memory allocations won't be affected by
> > this change:
> > 
> > "Flexible array members have incomplete type, and so the sizeof operator
> > may not be applied. As a quirk of the original implementation of
> > zero-length arrays, sizeof evaluates to zero."[1]
> > 
> > sizeof(flexible-array-member) triggers a warning because flexible array
> > members have incomplete type[1]. There are some instances of code in
> > which the sizeof operator is being incorrectly/erroneously applied to
> > zero-length arrays and the result is zero. Such instances may be hiding
> > some bugs. So, this work (flexible-array member conversions) will also
> > help to get completely rid of those sorts of issues.
> > 
> > This issue was found with the help of Coccinelle.
> > 
> > [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")
> > 
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > ---
> >  drivers/firmware/iscsi_ibft.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/firmware/iscsi_ibft.c b/drivers/firmware/iscsi_ibft.c
> > index 96758b71a8db..7127a04bca19 100644
> > --- a/drivers/firmware/iscsi_ibft.c
> > +++ b/drivers/firmware/iscsi_ibft.c
> > @@ -104,7 +104,7 @@ struct ibft_control {
> >  	u16 tgt0_off;
> >  	u16 nic1_off;
> >  	u16 tgt1_off;
> > -	u16 expansion[0];
> > +	u16 expansion[];
> >  } __attribute__((__packed__));
> >  
> >  struct ibft_initiator {
> >