PCI: Replace zero-length array with flexible-array

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@kernel.org>
---
 drivers/pci/pci.c   |    2 +-
 include/linux/pci.h |    4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

On Thu, May 07, 2020 at 02:05:44PM -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@kernel.org>

Applied to pci/misc for v5.8, thanks!

I assume this takes care of everything in drivers/pci/, right?  I'd
like to do them all at once, so if there are others, send another
patch and I'll squash them.  I took a quick look but didn't see any.

> ---
>  drivers/pci/pci.c   |    2 +-
>  include/linux/pci.h |    4 ++--
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 595fcf59843f..bb78f580814e 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -1578,7 +1578,7 @@ 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[];
>  };
>  
>  /**
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 83ce1cdf5676..0453ee458ab1 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -279,7 +279,7 @@ struct pci_cap_saved_data {
>  	u16		cap_nr;
>  	bool		cap_extended;
>  	unsigned int	size;
> -	u32		data[0];
> +	u32		data[];
>  };
>  
>  struct pci_cap_saved_state {
> @@ -532,7 +532,7 @@ struct pci_host_bridge {
>  			resource_size_t start,
>  			resource_size_t size,
>  			resource_size_t align);
> -	unsigned long	private[0] ____cacheline_aligned;
> +	unsigned long	private[] ____cacheline_aligned;
>  };
>  
>  #define	to_pci_host_bridge(n) container_of(n, struct pci_host_bridge, dev)
>

On Thu, May 07, 2020 at 03:15:44PM -0500, Bjorn Helgaas wrote:
> On Thu, May 07, 2020 at 02:05:44PM -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[];
> > };
> > 
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
> 
> Applied to pci/misc for v5.8, thanks!
> 
> I assume this takes care of everything in drivers/pci/, right?  I'd
> like to do them all at once, so if there are others, send another
> patch and I'll squash them.  I took a quick look but didn't see any.
> 

Yep. I can confirm that these are the last zero-length arrays in
drivers/pci/ :)

Thanks
--
Gustavo