Messages in this thread | | | Date | Thu, 15 Dec 2022 09:09:46 +1300 | From | Paulo Miguel Almeida <> | Subject | Re: [PATCH] [next] pcmcia: synclink_cs: replace 1-element array with flex-array member |
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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.
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