diff options
author | Jamie Liu <jamieliu@google.com> | 2020-09-15 13:23:14 -0700 |
---|---|---|
committer | gVisor bot <gvisor-bot@google.com> | 2020-09-15 13:25:58 -0700 |
commit | 0d790cbaea5cd40ceabdf8f59978efff1a0626f1 (patch) | |
tree | 14a0d03184804079a69814bebbda703376c5a24c /pkg/sentry/syscalls/linux/vfs2 | |
parent | 86b31a80777dd906b275fb4b21888e93836f8a9b (diff) |
Read vfs2 epoll events atomically.
Discovered by ayushranjan@:
VFS2 was employing the following algorithm for fetching ready events from an
epoll instance:
- Create a statically sized EpollEvent slice on the stack of size 16.
- Pass that to EpollInstance.ReadEvents() to populate.
- EpollInstance.ReadEvents() requeues level-triggered events that it returns
back into the ready queue.
- Write the results to usermem.
- If the number of results were = 16 then recall EpollInstance.ReadEvents() in
the hopes of getting more. But this will cause duplication of the "requeued"
ready level-triggered events.
So if the ready queue has >= 16 ready events, the EpollWait for loop will spin
until it fills the usermem with `maxEvents` events.
Fixes #3521
PiperOrigin-RevId: 331840527
Diffstat (limited to 'pkg/sentry/syscalls/linux/vfs2')
-rw-r--r-- | pkg/sentry/syscalls/linux/vfs2/epoll.go | 49 |
1 files changed, 11 insertions, 38 deletions
diff --git a/pkg/sentry/syscalls/linux/vfs2/epoll.go b/pkg/sentry/syscalls/linux/vfs2/epoll.go index c62f03509..d0cbb77eb 100644 --- a/pkg/sentry/syscalls/linux/vfs2/epoll.go +++ b/pkg/sentry/syscalls/linux/vfs2/epoll.go @@ -24,7 +24,6 @@ import ( ktime "gvisor.dev/gvisor/pkg/sentry/kernel/time" "gvisor.dev/gvisor/pkg/sentry/vfs" "gvisor.dev/gvisor/pkg/syserror" - "gvisor.dev/gvisor/pkg/usermem" "gvisor.dev/gvisor/pkg/waiter" ) @@ -141,50 +140,26 @@ func EpollWait(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sys return 0, nil, syserror.EINVAL } - // Use a fixed-size buffer in a loop, instead of make([]linux.EpollEvent, - // maxEvents), so that the buffer can be allocated on the stack. + // Allocate space for a few events on the stack for the common case in + // which we don't have too many events. var ( - events [16]linux.EpollEvent - total int + eventsArr [16]linux.EpollEvent ch chan struct{} haveDeadline bool deadline ktime.Time ) for { - batchEvents := len(events) - if batchEvents > maxEvents { - batchEvents = maxEvents - } - n := ep.ReadEvents(events[:batchEvents]) - maxEvents -= n - if n != 0 { - // Copy what we read out. - copiedBytes, err := linux.CopyEpollEventSliceOut(t, eventsAddr, events[:n]) + events := ep.ReadEvents(eventsArr[:0], maxEvents) + if len(events) != 0 { + copiedBytes, err := linux.CopyEpollEventSliceOut(t, eventsAddr, events) copiedEvents := copiedBytes / sizeofEpollEvent // rounded down - eventsAddr += usermem.Addr(copiedEvents * sizeofEpollEvent) - total += copiedEvents - if err != nil { - if total != 0 { - return uintptr(total), nil, nil - } - return 0, nil, err - } - // If we've filled the application's event buffer, we're done. - if maxEvents == 0 { - return uintptr(total), nil, nil - } - // Loop if we read a full batch, under the expectation that there - // may be more events to read. - if n == batchEvents { - continue + if copiedEvents != 0 { + return uintptr(copiedEvents), nil, nil } + return 0, nil, err } - // We get here if n != batchEvents. If we read any number of events - // (just now, or in a previous iteration of this loop), or if timeout - // is 0 (such that epoll_wait should be non-blocking), return the - // events we've read so far to the application. - if total != 0 || timeout == 0 { - return uintptr(total), nil, nil + if timeout == 0 { + return 0, nil, nil } // In the first iteration of this loop, register with the epoll // instance for readability events, but then immediately continue the @@ -207,8 +182,6 @@ func EpollWait(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sys if err == syserror.ETIMEDOUT { err = nil } - // total must be 0 since otherwise we would have returned - // above. return 0, nil, err } } |