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    SubjectRe: [PATCH 3.16 105/306] vfs,mm: fix a dead loop in truncate_inode_pages_range()
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    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@huawei.com>
    >
    > 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@huawei.com
    > Signed-off-by: Wei Fang <fangwei1@huawei.com>
    > Cc: Christoph Hellwig <hch@infradead.org>
    > Cc: Dave Chinner <david@fromorbit.com>
    > Cc: Al Viro <viro@zeniv.linux.org.uk>
    > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    > Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    > [bwh: Backported to 3.16: adjust context]
    > Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    > ---
    > 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);
    >

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