I've traced down an off-by-one TCP checksum calculation error under
the following conditions:
1) The TCP code needs to split a full-sized packet due to a reduced
MSS (typically due to the addition of TCP options mid-stream like
SACK).
_AND_
2) The checksum of the 2nd fragment is larger than the checksum of the
original packet. After subtraction this results in a checksum for
the 1st fragment with bits 16..31 set to 1. (this is ok)
_AND_
3) The checksum of the 1st fragment's TCP header plus the previously
32bit checksum of the 1st fragment DOES NOT cause a 32bit overflow
when added together. This results in a checksum of the TCP header
plus TCP data that still has the upper 16 bits as 1's.
_THEN_
4) The TCP+data checksum is added to the checksum of the pseudo IP
header with csum_tcpudp_nofold() incorrectly (the bug).
The problem is the checksum of the TCP+data is passed to
csum_tcpudp_nofold() as an 32bit unsigned value, however the assembly
code acts on it as if it is a 64bit unsigned value.
This causes an incorrect 32->64bit extension if the sum has bit 31
set. The resulting checksum is off by one.
This problems is data and TCP header dependent due to #2 and #3
above so it doesn't occur on every TCP packet split.
Signed-off-by: Dave Johnson <djohnson+linux-mips@sw.starentnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Dave Johnson