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    Subject[PATCH 3.2 86/94] proc: prevent stacking filesystems on top
    3.2.82-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Jann Horn <jannh@google.com>

    commit e54ad7f1ee263ffa5a2de9c609d58dfa27b21cd9 upstream.

    This prevents stacking filesystems (ecryptfs and overlayfs) from using
    procfs as lower filesystem. There is too much magic going on inside
    procfs, and there is no good reason to stack stuff on top of procfs.

    (For example, procfs does access checks in VFS open handlers, and
    ecryptfs by design calls open handlers from a kernel thread that doesn't
    drop privileges or so.)

    Signed-off-by: Jann Horn <jannh@google.com>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    [bwh: Backported to 3.2: adjust context]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    fs/proc/root.c | 7 +++++++
    1 file changed, 7 insertions(+)

    --- a/fs/proc/root.c
    +++ b/fs/proc/root.c
    @@ -53,6 +53,13 @@ static struct dentry *proc_mount(struct
    if (IS_ERR(sb))
    return ERR_CAST(sb);

    + /*
    + * procfs isn't actually a stacking filesystem; however, there is
    + * too much magic going on inside it to permit stacking things on
    + * top of it
    + */
    + sb->s_stack_depth = FILESYSTEM_MAX_STACK_DEPTH;
    +
    if (!sb->s_root) {
    sb->s_flags = flags;
    err = proc_fill_super(sb);
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