V4L/DVB (3394): Bttv: correct bttv_risc_packed buffer size

This patch fixes the strange crashes I was seeing after using
bttv card, caused by a buffer overflow in bttv_risc_packed.
The instruction buffer size calculation contains two errors:
(a) a non-zero padding value can push the start of the next bpl
section to just before a page border, leading to more scanline
splits and thus additional instructions.
(b) the first DMA region can be smaller than one page, so there can
be a scanline split even if bpl*lines is smaller than PAGE_SIZE.
For example, consider the case where offset is 0, bpl is 2, padding
is 4094, lines is smaller than 2048, the first DMA region has size 1
and all others have size PAGE_SIZE, assumed to equal 4096.  Then
all bpl regions cross page borders and the number of instructions
written is 2*lines+2, rather than lines+2 (the current estimate).
With this patch the number of instructions for this example is
estimated to be 2*lines+3.
Also, the BUG_ON that was supposed to catch buffer overflows contained
a thinko causing it fire only if the buffer was overrun by a factor of
16 or more, so it fixes the the BUG_ON's (using sizeof rather than "4").

Signed-off-by: Duncan Sands <baldrick@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/drivers/media/video/cx88/cx88-core.c b/drivers/media/video/cx88/cx88-core.c
index eda7cd8b2d4..25be3a976d0 100644
--- a/drivers/media/video/cx88/cx88-core.c
+++ b/drivers/media/video/cx88/cx88-core.c
@@ -163,7 +163,7 @@ int cx88_risc_buffer(struct pci_dev *pci, struct btcx_riscmem *risc,
 
         /* save pointer to jmp instruction address */
         risc->jmp = rp;
-        BUG_ON((risc->jmp - risc->cpu + 2) / 4 > risc->size);
+        BUG_ON((risc->jmp - risc->cpu + 2) * sizeof (*risc->cpu) > risc->size);
         return 0;
 }
 
@@ -188,7 +188,7 @@ int cx88_risc_databuffer(struct pci_dev *pci, struct btcx_riscmem *risc,
 
         /* save pointer to jmp instruction address */
         risc->jmp = rp;
-        BUG_ON((risc->jmp - risc->cpu + 2) / 4 > risc->size);
+        BUG_ON((risc->jmp - risc->cpu + 2) * sizeof (*risc->cpu) > risc->size);
         return 0;
 }