• Miklos Szeredi's avatar
    proc/mounts: add cursor · 9f6c61f9
    Miklos Szeredi authored
    If mounts are deleted after a read(2) call on /proc/self/mounts (or its
    kin), the subsequent read(2) could miss a mount that comes after the
    deleted one in the list.  This is because the file position is interpreted
    as the number mount entries from the start of the list.
    
    E.g. first read gets entries #0 to #9; the seq file index will be 10.  Then
    entry #5 is deleted, resulting in #10 becoming #9 and #11 becoming #10,
    etc...  The next read will continue from entry #10, and #9 is missed.
    
    Solve this by adding a cursor entry for each open instance.  Taking the
    global namespace_sem for write seems excessive, since we are only dealing
    with a per-namespace list.  Instead add a per-namespace spinlock and use
    that together with namespace_sem taken for read to protect against
    concurrent modification of the mount list.  This may reduce parallelism of
    is_local_mountpoint(), but it's hardly a big contention point.  We could
    also use RCU freeing of cursors to make traversal not need additional
    locks, if that turns out to be neceesary.
    
    Only move the cursor once for each read (cursor is not added on open) to
    minimize cacheline invalidation.  When EOF is reached, the cursor is taken
    off the list, in order to prevent an excessive number of cursors due to
    inactive open file descriptors.
    Reported-by: default avatarKarel Zak <kzak@redhat.com>
    Signed-off-by: default avatarMiklos Szeredi <mszeredi@redhat.com>
    9f6c61f9
proc_namespace.c 7.89 KB