linux-kernel - RE: [PATCH] regulator: da9062: Replace zero-length array with flexible-array member

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Message-ID: <AM6PR10MB2263140BCCAEDB6BC0D258E8801B0@AM6PR10MB2263.EURPRD10.PROD.OUTLOOK.COM>
Date:   Wed, 12 Feb 2020 14:55:07 +0000
From:   Adam Thomson <Adam.Thomson.Opensource@...semi.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Adam Thomson <Adam.Thomson.Opensource@...semi.com>,
        Support Opensource <Support.Opensource@...semi.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        Mark Brown <broonie@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] regulator: da9062: Replace zero-length array with
 flexible-array member

On 12 February 2020 14:29, Gustavo A. R. Silva wrote:

> On 2/12/20 05:12, Adam Thomson wrote:
> > On 11 February 2020 23:46, 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
> >> inadvertenly introduced[3] to the codebase from now on.
> >>
> >> 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 <gustavo@...eddedor.com>
> >> ---
> >>  drivers/regulator/da9062-regulator.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/regulator/da9062-regulator.c b/drivers/regulator/da9062-
> >> regulator.c
> >> index b064d8a19d4c..c3b6ba9bafdf 100644
> >> --- a/drivers/regulator/da9062-regulator.c
> >> +++ b/drivers/regulator/da9062-regulator.c
> >> @@ -86,7 +86,7 @@ struct da9062_regulators {
> >>  	int					irq_ldo_lim;
> >>  	unsigned				n_regulators;
> >>  	/* Array size to be defined during init. Keep at end. */
> >> -	struct da9062_regulator			regulator[0];
> >> +	struct da9062_regulator			regulator[];
> >
> > I don't think is the correct change here for this driver. In the probe
> > 'struct_size()' is used to determine the actual size requested from 'malloc()'
> > when allocating memory for this structure. It's not statically initialised.
> > Your change will break that code I believe.
> >
> 
> 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]
> 
> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html

Yeah, ok misinterpreted the descriptions in there. In which case:

Acked-by: Adam Thomson <Adam.Thomson.Opensource@...semi.com>

> 
> Thanks
> --
> Gustavo