[PATCH] bttv: correct bttv_risc_packed buffer size

[Duncan Sands]

This patch fixes the strange crashes I was seeing after using
my bttv card to watch television.  They were 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.

I didn't check whether similar mistakes exist elsewhere in the bttv
code.

Signed-off-by: Duncan Sands <baldrick@xxxxxxx>

PS: I'm sending the patch as an attachment because for some reason my
mailer crashes if I try to insert it into the email.
Index: Linux/drivers/media/video/bttv-risc.c
===================================================================
--- Linux.orig/drivers/media/video/bttv-risc.c	2006-01-24 10:09:21.000000000 +0100
+++ Linux/drivers/media/video/bttv-risc.c	2006-01-24 10:16:06.000000000 +0100
@@ -51,8 +51,10 @@
 	int rc;
 
 	/* estimate risc mem: worst case is one write per page border +
-	   one write per scan line + sync + jump (all 2 dwords) */
-	instructions  = (bpl * lines) / PAGE_SIZE + lines;
+	   one write per scan line + sync + jump (all 2 dwords).  padding
+	   can cause next bpl to start close to a page border.  First DMA
+	   region may be smaller than PAGE_SIZE */
+	instructions  = 1 + ((bpl + padding) * lines) / PAGE_SIZE + lines;
 	instructions += 2;
 	if ((rc = btcx_riscmem_alloc(btv->c.pci,risc,instructions*8)) < 0)
 		return rc;
@@ -104,7 +106,7 @@
 
 	/* save pointer to jmp instruction address */
 	risc->jmp = rp;
-	BUG_ON((risc->jmp - risc->cpu + 2) / 4 > risc->size);
+	BUG_ON(4 * (risc->jmp - risc->cpu + 2) > risc->size);
 	return 0;
 }