Re: [PATCH] [next] pcmcia: synclink_cs: replace 1-element array with flex-array member

[Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>]

On Wed, Dec 14, 2022 at 11:29:37AM -0800, Kees Cook wrote:
> On Wed, Dec 14, 2022 at 09:42:00PM +1300, Paulo Miguel Almeida wrote:
> > One-element arrays are deprecated, and we are replacing them with
> > flexible array members instead. So, replace one-element array with
> > flexible-array member in struct RXBUF and refactor the rest of the code
> > accordingly.
> > 
> > It's worth mentioning that doing a build before/after this patch
> > results in no binary output differences.
> > 
> > This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
> > routines on memcpy() and help us make progress towards globally
> > enabling -fstrict-flex-arrays=3 [1].
> > 
> > Link: https://github.com/KSPP/linux/issues/79
> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]
> > 
> > Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
> > ---
> >  drivers/char/pcmcia/synclink_cs.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c
> > index b2735be81ab2..1ab2d552f498 100644
> > --- a/drivers/char/pcmcia/synclink_cs.c
> > +++ b/drivers/char/pcmcia/synclink_cs.c
> > @@ -105,7 +105,7 @@ static MGSL_PARAMS default_params = {
> >  typedef struct {
> >  	int count;
> >  	unsigned char status;
> > -	char data[1];
> > +	char data[];
> >  } RXBUF;
> >  
> >  /* The queue of BH actions to be performed */
> > @@ -2611,7 +2611,8 @@ static int mgslpc_proc_show(struct seq_file *m, void *v)
> >  static int rx_alloc_buffers(MGSLPC_INFO *info)
> >  {
> >  	/* each buffer has header and data */
> > -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> > +	info->rx_buf_size = max(offsetof(typeof(RXBUF), data) + 1, sizeof(RXBUF))
> > +		+ info->max_frame_size;
> 
> It seems like there is an existing size bug here, and likely should be
> fixed separately?
> 
> i.e. this was already allocating 1 byte "too much". I'd expect this
> first:
> 
> -	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> +	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> 
> and then the next patch:
> 
> -	char data[1];
> +	char data[];
> ...
> -	info->rx_buf_size = sizeof(RXBUF) - 1 + info->max_frame_size;
> +	info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
> 
> The above would induce a binary output change, and the second would not.
> 
> Though this results in what you had for the v2 patch (but I can't
> believe it had no binary changes...)
> 
> -- 
> Kees Cook

Hi Kees, Hi Andy, Thanks for taking the time to review this patch.

As both of you had similar points, I will reply them here.

The reasons why it had no binary changes was because of the combination
of this 2 things:

1) Existing padding - so sizeof(RXBUF) returned 8 bytes in both cases.

pahole -C RXBUF gcc/before/drivers/char/pcmcia/synclink_cs.ko
typedef struct {
	int                        count;                /*     0     4 */
	unsigned char              status;               /*     4     1 */
	char                       data[1];              /*     5     1 */

	/* size: 8, cachelines: 1, members: 3 */
	/* padding: 2 */
	/* last cacheline: 8 bytes */
} RXBUF;

pahole -C RXBUF gcc/after/drivers/char/pcmcia/synclink_cs.ko
typedef struct {
	int                        count;                /*     0     4 */
	unsigned char              status;               /*     4     1 */
	char                       data[];               /*     5     0 */

	/* size: 8, cachelines: 1, members: 3 */
	/* padding: 3 */
	/* last cacheline: 8 bytes */
} RXBUF;

2) RXBUF (as implemented now) is just  like a pair of lenses from which a
developer can have access to one of the circular buffers in MGSLPC_INFO
struct called 'rx_buf'. 

2611 static int rx_alloc_buffers(MGSLPC_INFO *info)
2612 {
2613         /* each buffer has header and data */
2614         info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size;
2615
2616         /* calculate total allocation size for 8 buffers */
2617         info->rx_buf_total_size = info->rx_buf_size * 8;
2618
2619         /* limit total allocated memory */
2620         if (info->rx_buf_total_size > 0x10000)
2621                 info->rx_buf_total_size = 0x10000;
2622
2623         /* calculate number of buffers */
2624         info->rx_buf_count = info->rx_buf_total_size / info->rx_buf_size;
2625
2626         info->rx_buf = kmalloc(info->rx_buf_total_size, GFP_KERNEL);

To be honest, char data[_1_] in RXBUF was never required to be there.
The code base seems to make sure that it doesn't run past its limits by
keeping track of size buffer on MGSLPC_INFO->rx_buf_size (and sometimes
RXBUF->count)

(Addressing one point made by Andy about using of of the macros in
overflow.h)
	struct_size(buf, data, 1) would return 9 bytes which could
	potentially break the existing driver as it produces binary
	changes.

Let me know your thoughts

thanks!

- Paulo A.