On Wed, Jul 3, 2019 at 7:08 AM Frank de Brabander <debraban...@gmail.com> wrote: > > In commit 5cd8d46e a fix was applied for data corruption in > tpacket_snd. A selftest was added in commit 358be656 which > validates this fix. > > Unfortunately this bug still persists, although since this fix less > likely to trigger. This bug was initially observed using a PACKET_MMAP > application, but can also be seen by tweaking the kernel selftest. > > By tweaking the selftest txring_overwrite.c to run > as an infinite loop, the data corruption will still trigger. It > seems to occur faster by generating interrupts (e.g. by plugging > in USB devices). Tested with kernel version 5.2-RC7. > > Cause for this bug is still unclear.
The cause of the original bug is well understood. The issue you report I expect is due to background traffic. And more about the test than the kernel implementation. Can you reproduce the issue when running the modified test in a network namespace (./in_netns.sh ./txring_overwrite)? I observe the issue report outside that, but not inside. That implies that what we're observing is random background traffic. The modified test then drops the unexpected packet because it mismatches on length. As a result the next read (the test always sends two packets, then reads both) will report a data mismatch. Because it is reading the first test packet, but expecting the second. Output with a bit more data: count: 200 count: 300 count: 400 count: 500 read: 90B != 100B wrong pattern: 0x61 != 0x62 count: 600 count: 700 count: 800 read: 90B != 100B wrong pattern: 0x61 != 0x62 count: 900 read: 90B != 100B wrong pattern: 0x61 != 0x62 Notice the clear pattern. This does not trigger inside a network namespace, which is how kselftest invokes txring_override (from run_afpackettests).
