On Thu, 2020-09-17 at 17:44 +0100, Al Viro wrote: > On Thu, Sep 17, 2020 at 10:10:27AM -0400, Qian Cai wrote: > > > [ 81.942909] generic_file_read_iter+0x23b/0x4b0 > > [ 81.942918] fuse_file_read_iter+0x280/0x4e0 [fuse] > > [ 81.942931] ? fuse_direct_IO+0xd30/0xd30 [fuse] > > [ 81.942949] ? _raw_spin_lock_irqsave+0x80/0xe0 > > [ 81.942957] ? timerqueue_add+0x15e/0x280 > > [ 81.942960] ? _raw_spin_lock_irqsave+0x80/0xe0 > > [ 81.942966] new_sync_read+0x3b7/0x620 > > [ 81.942968] ? __ia32_sys_llseek+0x2e0/0x2e0 > > Interesting... Basic logics in there: > ->direct_IO() might consume more (on iov_iter_get_pages() > and friends) than it actually reads. We want to revert the > excess. Suppose by the time we call ->direct_IO() we had > N bytes already consumed and C bytes left. We expect that > after ->direct_IO() returns K, we have C' bytes left, N + (C - C') > consumed and N + K out of those actually read. So we revert by > C - K - C'. You end up trying to revert beyond the beginning. > > Use of iov_iter_truncate() is problematic here, since it > changes the amount of data left without having consumed anything. > Basically, it changes the position of end, and the logics in the > caller expects that to remain unchanged. iov_iter_reexpand() use > should restore the position of end. > > How much IO does it take to trigger that on your reproducer? I can even reproduce this with a single child of the trinity: https://people.redhat.com/qcai/iov_iter_revert/single/ [ 77.841021] BUG: KASAN: stack-out-of-bounds in iov_iter_revert+0x693/0x8c0 [ 77.842055] Read of size 8 at addr ffff8886efe47d98 by task trinity-c0/1449
