Messages in this thread | | | Date | Thu, 15 Dec 2022 09:19:57 +1300 | From | Paulo Miguel Almeida <> | Subject | Re: [PATCH v2] [next] pcmcia: synclink_cs: replace 1-element array with flex-array member |
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On Wed, Dec 14, 2022 at 12:43:48PM +0200, Andy Shevchenko wrote: > On Wed, Dec 14, 2022 at 10:58 AM Paulo Miguel Almeida > <paulo.miguel.almeida.rodenas@gmail.com> 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. No changes were required > > within the source code because of the existing padding in RXBUF struct > > You shouldn't rely on padding. Make you change robust independently on > the padding. See also below. > > > It's worth mentioning that doing a build before/after this patch > > results in no binary output differences. > > This is interesting... > > > 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] > > > > > The blank lines are not allowed in the tag block (in case you want to > have Link: to be recognized as a tag). > > > Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com> > > --- > > Changelog: > > > > - v2: removed changes to how the size of RXBUF was calculated. I > > changed my mind after thinking about the existing padding in the > > struct. Happy to discuss it if anyone sees it differently. > > I feel worried about in particular this code: > > /* each buffer has header and data */ > info->rx_buf_size = sizeof(RXBUF) + info->max_frame_size; > > which means that entire rx_alloc_buffers() should be revisited. Also > take into account the use of one or more macros from overflow.h for > memory allocation. > > -- > With Best Regards, > Andy Shevchenko
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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