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    Subject[PATCH 1/2] vfs: check dentry is still valid in get_link()
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    When following a trailing symlink in rcu-walk mode it's possible for
    the dentry to become invalid between the last dentry seq lock check
    and getting the link (eg. an unlink) leading to a backtrace similar
    to this:

    crash> bt
    PID: 10964 TASK: ffff951c8aa92f80 CPU: 3 COMMAND: "TaniumCX"

    #7 [ffffae44d0a6fbe0] page_fault at ffffffff8d6010fe
    [exception RIP: unknown or invalid address]
    RIP: 0000000000000000 RSP: ffffae44d0a6fc90 RFLAGS: 00010246
    RAX: ffffffff8da3cc80 RBX: ffffae44d0a6fd30 RCX: 0000000000000000
    RDX: ffffae44d0a6fd98 RSI: ffff951aa9af3008 RDI: 0000000000000000
    RBP: 0000000000000000 R8: ffffae44d0a6fb94 R9: 0000000000000000
    R10: ffff951c95d8c318 R11: 0000000000080000 R12: ffffae44d0a6fd98
    R13: ffff951aa9af3008 R14: ffff951c8c9eb840 R15: 0000000000000000
    ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
    #8 [ffffae44d0a6fc90] trailing_symlink at ffffffff8cf24e61
    #9 [ffffae44d0a6fcc8] path_lookupat at ffffffff8cf261d1
    #10 [ffffae44d0a6fd28] filename_lookup at ffffffff8cf2a700
    #11 [ffffae44d0a6fe40] vfs_statx at ffffffff8cf1dbc4
    #12 [ffffae44d0a6fe98] __do_sys_newstat at ffffffff8cf1e1f9
    #13 [ffffae44d0a6ff38] do_syscall_64 at ffffffff8cc0420b

    Most of the time this is not a problem because the inode is unchanged
    while the rcu read lock is held.

    But xfs can re-use inodes which can result in the inode ->get_link()
    method becoming invalid (or NULL).

    This case needs to be checked for in fs/namei.c:get_link() and if
    detected the walk re-started.

    Signed-off-by: Ian Kent <raven@themaw.net>
    ---
    fs/namei.c | 5 ++++-
    1 file changed, 4 insertions(+), 1 deletion(-)

    diff --git a/fs/namei.c b/fs/namei.c
    index 1946d9667790..9a48a6106516 100644
    --- a/fs/namei.c
    +++ b/fs/namei.c
    @@ -1760,8 +1760,11 @@ static const char *pick_link(struct nameidata *nd, struct path *link,
    if (!res) {
    const char * (*get)(struct dentry *, struct inode *,
    struct delayed_call *);
    - get = inode->i_op->get_link;
    + get = READ_ONCE(inode->i_op->get_link);
    if (nd->flags & LOOKUP_RCU) {
    + /* Does the inode still match the associated dentry? */
    + if (unlikely(read_seqcount_retry(&link->dentry->d_seq, last->seq)))
    + return ERR_PTR(-ECHILD);
    res = get(NULL, inode, &last->done);
    if (res == ERR_PTR(-ECHILD) && try_to_unlazy(nd))
    res = get(link->dentry, inode, &last->done);

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