This commit fixes tcp_shift_skb_data() so that it does not shift
SACKed data below snd_una.
This fixes an issue whose symptoms exactly match reports showing
tp->sacked_out going negative since 3.3.0-rc4 (see "WARNING: at
net/ipv4/tcp_input.c:3418" thread on netdev).
Since 2008 (832d11c5cd)
tcp_shift_skb_data() had been shifting SACKed ranges that were below
snd_una. It checked that the *end* of the skb it was about to shift
from was above snd_una, but did not check that the end of the actual
shifted range was above snd_una; this commit adds that check.
Shifting SACKed ranges below snd_una is problematic because for such
ranges tcp_sacktag_one() short-circuits: it does not declare anything
as SACKed and does not increase sacked_out.
Before the fixes in commits cc9a672ee5
and daef52bab1, shifting SACKed ranges
below snd_una happened to work because tcp_shifted_skb() was always
(incorrectly) passing in to tcp_sacktag_one() an skb whose end_seq
tcp_shift_skb_data() had already guaranteed was beyond snd_una. Hence
tcp_sacktag_one() never short-circuited and always increased
tp->sacked_out in this case.
After those two fixes, my testing has verified that shifting SACKed
ranges below snd_una could cause tp->sacked_out to go negative with
the following sequence of events:
(1) tcp_shift_skb_data() sees an skb whose end_seq is beyond snd_una,
then shifts a prefix of that skb that is below snd_una
(2) tcp_shifted_skb() increments the packet count of the
already-SACKed prev sk_buff
(3) tcp_sacktag_one() sees the end of the new SACKed range is below
snd_una, so it short-circuits and doesn't increase tp->sacked_out
(5) tcp_clean_rtx_queue() sees the SACKed skb has been ACKed,
decrements tp->sacked_out by this "inflated" pcount that was
missing a matching increase in tp->sacked_out, and hence
tp->sacked_out underflows to a u32 like 0xFFFFFFFF, which casted
to s32 is negative.
(6) this leads to the warnings seen in the recent "WARNING: at
net/ipv4/tcp_input.c:3418" thread on the netdev list; e.g.:
tcp_input.c:3418 WARN_ON((int)tp->sacked_out < 0);
More generally, I think this bug can be tickled in some cases where
two or more ACKs from the receiver are lost and then a DSACK arrives
that is immediately above an existing SACKed skb in the write queue.
This fix changes tcp_shift_skb_data() to abort this sequence at step
(1) in the scenario above by noticing that the bytes are below snd_una
and not shifting them.
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
4648dc97af