| Message ID | Y5mQTOqFOGbJMerV@mail.google.com (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | [v2,next] pcmcia: synclink_cs: replace 1-element array with flex-array member | expand |
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.
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.
diff --git a/drivers/char/pcmcia/synclink_cs.c b/drivers/char/pcmcia/synclink_cs.c index b2735be81ab2..0b03c6d13d59 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 */
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 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> --- 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. - v1: https://lore.kernel.org/lkml/Y5mMWEtHWKOiPVU+@mail.google.com/ --- drivers/char/pcmcia/synclink_cs.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)