Hi,
i came accross a serious problem with Apache server when it is NOT able to
read/write from/to network immediatelly.
We are using network throttling patches in our linux kernel wihch are able to
emulate low network throughput (we are using this to throttle users). When a
process reaches it's limit, read/write will result in EAGAIN which is
completely ok cos it's standard behavior. Problem is that Apache is not able to
correctly process this and is trying to read/write in a loop WITHOUT any delay
- this is, of course, resulting in 100% CPU consumption. This bug can lead to
DoS of the whole server.
Here is the strace output when Apache's child is throttled:
writev(6409, [{"n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n"..., 2900}], 1) = -1 EAGAIN
(Resource temporarily unavailable)
poll([{fd=6409, events=POLLOUT}], 1, 100000) = 1 ([{fd=6409, revents=POLLOUT}])
writev(6409, [{"n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n"..., 2900}], 1) = -1 EAGAIN
(Resource temporarily unavailable)
poll([{fd=6409, events=POLLOUT}], 1, 100000) = 1 ([{fd=6409, revents=POLLOUT}])
writev(6409, [{"n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n"..., 2900}], 1) = -1 EAGAIN
(Resource temporarily unavailable)
poll([{fd=6409, events=POLLOUT}], 1, 100000) = 1 ([{fd=6409, revents=POLLOUT}])
writev(6409, [{"n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n"..., 2900}], 1) = -1 EAGAIN
(Resource temporarily unavailable)
poll([{fd=6409, events=POLLOUT}], 1, 100000) = 1 ([{fd=6409, revents=POLLOUT}])
writev(6409, [{"n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n1n"..., 2900}], 1) = -1 EAGAIN
(Resource temporarily unavailable)
poll([{fd=6409, events=POLLOUT}], 1, 100000) = 1 ([{fd=6409, revents=POLLOUT}])
..and so on until it is able to send data again.
I suggest to insert a little delay everytime an EAGAIN is returned.
azur
