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    Subject[ review 26/84] make timerfd return a u64 and fix the __put_user

    From: Davide Libenzi <>

    Davi fixed a missing cast in the __put_user(), that was making timerfd
    return a single byte instead of the full value.

    Talking with Michael about the timerfd man page, we think it'd be better to
    use a u64 for the returned value, to align it with the eventfd

    This is an ABI change. The timerfd code is new in 2.6.22 and if we merge this
    into 2.6.23 then we should also merge it into 2.6.22.x. That will leave a few
    early 2.6.22 kernels out in the wild which might misbehave when a future
    timerfd-enabled glibc is run on them.

    mtk says:
    The difference would be that read() will only return 4 bytes,
    while the application will expect 8. If the application is
    checking the size of returned value, as it should, then it will
    be able to detect the problem (it could even be sophisticated
    enough to know that if this is a 4-byte return, then it is
    running on an old 2.6.22 kernel). If the application is not
    checking the return from read(), then its 8-byte buffer will not
    be filled -- the contents of the last 4 bytes will be undefined,
    so the u64 value as a whole will be junk.

    When I wrote up that description above, I forgot a crucial
    detail. The above description described the difference between
    the new behavior implemented by the patch, and the current
    (i.e., 2.6.22) *intended* behavior. However, as I originally
    remarked to Davide, the 2.6.22 read() behavior is broken: it
    should return 4 bytes on a read(), but as originally
    implemented, only the least significant byte contained valid
    information. (In other words, the top 3 bytes of overrun
    information were simply being discarded.)

    So the patch both fixes a bug in the originally intended
    behavior, and changes the intended behavior (to return 8 bytes
    from a read() instead of 4).

    Signed-off-by: Davide Libenzi <>
    Cc: Michael Kerrisk <>
    Cc: Davi Arnaut <>
    Signed-off-by: Andrew Morton <>
    Signed-off-by: Greg Kroah-Hartman <>

    fs/timerfd.c | 6 +++---
    1 file changed, 3 insertions(+), 3 deletions(-)

    --- a/fs/timerfd.c
    +++ b/fs/timerfd.c
    @@ -95,7 +95,7 @@ static ssize_t timerfd_read(struct file
    struct timerfd_ctx *ctx = file->private_data;
    ssize_t res;
    - u32 ticks = 0;
    + u64 ticks = 0;
    DECLARE_WAITQUEUE(wait, current);

    if (count < sizeof(ticks))
    @@ -130,7 +130,7 @@ static ssize_t timerfd_read(struct file
    * callback to avoid DoS attacks specifying a very
    * short timer period.
    - ticks = (u32)
    + ticks = (u64)
    @@ -140,7 +140,7 @@ static ssize_t timerfd_read(struct file
    if (ticks)
    - res = put_user(ticks, buf) ? -EFAULT: sizeof(ticks);
    + res = put_user(ticks, (u64 __user *) buf) ? -EFAULT: sizeof(ticks);
    return res;

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