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

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Message-ID: <20200305112923.GA5576@e121166-lin.cambridge.arm.com>
Date:   Thu, 5 Mar 2020 11:29:31 +0000
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] PCI: Replace zero-length array with flexible-array member

On Wed, Feb 12, 2020 at 06:52:21PM -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
> 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]
> 
> 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/pci/pci.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Applied to pci/hv for v5.7, thanks.

Lorenzo

> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index d828ca835a98..3217b8cdb1e0 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1521,7 +1521,7 @@ EXPORT_SYMBOL(pci_restore_state);
>  
>  struct pci_saved_state {
>  	u32 config_space[16];
> -	struct pci_cap_saved_data cap[0];
> +	struct pci_cap_saved_data cap[];
>  };
>  
>  /**
> -- 
> 2.23.0
>