Messages in this thread | | | From | David Howells <> | Subject | Re: [syzbot] [fs?] INFO: task hung in pipe_release (4) | Date | Sat, 29 Jul 2023 00:52:20 +0100 |
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Jakub Kicinski <kuba@kernel.org> wrote:
> Hi David, any ideas about this one? Looks like it triggers on fairly > recent upstream?
I've managed to reproduce it finally. Instrumenting the pipe_lock/unlock functions, splice_to_socket() and pipe_release() seems to show that pipe_release() is being called whilst splice_to_socket() is still running.
I *think* syzbot is arranging things such that splice_to_socket() takes a significant amount of time so that another thread can close the socket as it exits.
In this sample logging, the pipe is created by pid 7101:
[ 66.205719] --pipe 7101 [ 66.209942] lock [ 66.212526] locked [ 66.215344] unlock [ 66.218103] unlocked
splice begins in 7101 also and locks the pipe:
[ 66.221057] ==>splice_to_socket() 7101 [ 66.225596] lock [ 66.228177] locked
but for some reason, pid 7100 then tries to release it:
[ 66.377781] release 7100
and hangs on the __pipe_lock() call in pipe_release():
[ 66.381059] lock
The syz reproducer does weird things with threading - and I'm wondering if there's a file struct refcount bug here. Note that splice_to_socket() can't access the pipe file structs to alter the refcount, and the involved pipe isn't communicated to udp_sendmsg() in any way - so if there is a refcount bug, it must be somewhere in the VFS, the pipe driver or the splice infrastructure:-/.
I'm also not sure what's going on inside udp_sendmsg() as yet. It doesn't show a stack in /proc/7101/stacks, which means it doesn't hit a schedule().
David
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