Hi, Ben, Commit d05c5f7ba164 "vfs,mm: fix return value of read() at s_maxbytes" need be backported with this patch, otherwise we'll get an error when reading at the end of the file. Thanks, Wei On 2017/2/16 6:41, Ben Hutchings wrote: > 3.16.40-rc1 review patch. If anyone has any objections, please let me know. > > ------------------ > > From: Wei Fang <fangwei1@xxxxxxxxxx> > > commit c2a9737f45e27d8263ff9643f994bda9bac0b944 upstream. > > We triggered a deadloop in truncate_inode_pages_range() on 32 bits > architecture with the test case bellow: > > ... > fd = open(); > write(fd, buf, 4096); > preadv64(fd, &iovec, 1, 0xffffffff000); > ftruncate(fd, 0); > ... > > Then ftruncate() will not return forever. > > The filesystem used in this case is ubifs, but it can be triggered on > many other filesystems. > > When preadv64() is called with offset=0xffffffff000, a page with > index=0xffffffff will be added to the radix tree of ->mapping. Then > this page can be found in ->mapping with pagevec_lookup(). After that, > truncate_inode_pages_range(), which is called in ftruncate(), will fall > into an infinite loop: > > - find a page with index=0xffffffff, since index>=end, this page won't > be truncated > > - index++, and index become 0 > > - the page with index=0xffffffff will be found again > > The data type of index is unsigned long, so index won't overflow to 0 on > 64 bits architecture in this case, and the dead loop won't happen. > > Since truncate_inode_pages_range() is executed with holding lock of > inode->i_rwsem, any operation related with this lock will be blocked, > and a hung task will happen, e.g.: > > INFO: task truncate_test:3364 blocked for more than 120 seconds. > ... > call_rwsem_down_write_failed+0x17/0x30 > generic_file_write_iter+0x32/0x1c0 > ubifs_write_iter+0xcc/0x170 > __vfs_write+0xc4/0x120 > vfs_write+0xb2/0x1b0 > SyS_write+0x46/0xa0 > > The page with index=0xffffffff added to ->mapping is useless. Fix this > by checking the read position before allocating pages. > > Link: http://lkml.kernel.org/r/1475151010-40166-1-git-send-email-fangwei1@xxxxxxxxxx > Signed-off-by: Wei Fang <fangwei1@xxxxxxxxxx> > Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx> > Cc: Dave Chinner <david@xxxxxxxxxxxxx> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > [bwh: Backported to 3.16: adjust context] > Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> > --- > mm/filemap.c | 4 ++++ > 1 file changed, 4 insertions(+) > > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -1464,6 +1464,10 @@ static ssize_t do_generic_file_read(stru > unsigned int prev_offset; > int error = 0; > > + if (unlikely(*ppos >= inode->i_sb->s_maxbytes)) > + return -EINVAL; > + iov_iter_truncate(iter, inode->i_sb->s_maxbytes); > + > index = *ppos >> PAGE_CACHE_SHIFT; > prev_index = ra->prev_pos >> PAGE_CACHE_SHIFT; > prev_offset = ra->prev_pos & (PAGE_CACHE_SIZE-1); >