[PATCH] parisc: Fix csum_ipv6_magic on 64-bit systems

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hppa 64-bit systems calculates the IPv6 checksum using 64-bit add
operations. The last add folds protocol and length fields into the 64-bit
result. While unlikely, this operation can overflow. The overflow can be
triggered with a code sequence such as the following.

	/* try to trigger massive overflows */
	memset(tmp_buf, 0xff, sizeof(struct in6_addr));
	csum_result = csum_ipv6_magic((struct in6_addr *)tmp_buf,
				      (struct in6_addr *)tmp_buf,
				      0xffff, 0xff, 0xffffffff);

Fix the problem by adding any overflows from the final add operation into
the calculated checksum. Fortunately, we can do this without additional
cost by replacing the add operation used to fold the checksum into 32 bit
with "add,dc" to add in the missing carry.

Cc: Charlie Jenkins <charlie@xxxxxxxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
This patch does not completely fix the problems with csum_ipv6_magic seen
when running 64-bit parisc images with the C3700 emulation in qemu. That
is due to unaligned 64-bit load operations which (presumably as part of
unaligned trap handling) generate bad carry flags. It is unknown if that
is a problem with the qemu emulation or with the Linux kernel, so it is not
addressed here.

 arch/parisc/include/asm/checksum.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/parisc/include/asm/checksum.h b/arch/parisc/include/asm/checksum.h
index e619e67440db..c949aa20fa16 100644
--- a/arch/parisc/include/asm/checksum.h
+++ b/arch/parisc/include/asm/checksum.h
@@ -137,8 +137,8 @@ static __inline__ __sum16 csum_ipv6_magic(const struct in6_addr *saddr,
 "	add,dc		%3, %0, %0\n"  /* fold in proto+len | carry bit */
 "	extrd,u		%0, 31, 32, %4\n"/* copy upper half down */
 "	depdi		0, 31, 32, %0\n"/* clear upper half */
-"	add		%4, %0, %0\n"	/* fold into 32-bits */
-"	addc		0, %0, %0\n"	/* add carry */
+"	add,dc		%4, %0, %0\n"	/* fold into 32-bits, plus carry */
+"	addc		0, %0, %0\n"	/* add final carry */

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