[PATCH RFC 0/3] A few round_pipe_size() and pipe-max-size fixups

[Joe Lawrence]

While backporting Michael's "pipe: fix limit handling" [1] patchset to a
distro-kernel, Mikulas noticed that current upstream pipe limit handling
contains a few problems:

  1 - round_pipe_size() nr_pages overflow on 32bit:  this would
      subsequently try roundup_pow_of_two(0), which is undefined.

  2 - visible non-rounded pipe-max-size value: there is no mutual
      exclusion or protection between the time pipe_max_size is assigned
      a raw value from proc_dointvec_minmax() and when it is rounded.

  3 - procfs signed wrap: echo'ing a large number into
      /proc/sys/fs/pipe-max-size and then cat'ing it back out shows a
      negative value.


This RFC serves as a bug report and a contains a few possible fixes.
There may be better / more consistent ways to fix the overflows and
procfs bugs, but I figured I'd throw an RFC w/code out there for initial
conversation.  Suggestions welcome!

-- Joe


Testing
=======

Patch 1 - 32bit overflow
------------------------
>From userspace:

  fcntl(fd, F_SETPIPE_SZ, 0xffffffff);

- Before the fix, return value was 4096 as pipe size overflowed and
  was set to 4096

- After the fix, returns -1 and sets errno EINVAL, pipe size remains
  untouched


Patch 2 - non-rounded pipe-max-size value
-----------------------------------------
Keep plugging in values that need to be rounded:

  while (true); do echo 1048570 > /proc/sys/fs/pipe-max-size; done

and in another terminal, loop around reading the value:

  time (while (true); do SIZE=$(cat /proc/sys/fs/pipe-max-size); [[ $(( $SIZE % 4096 )) -ne 0 ]] && break; done; echo "$SIZE")
  1048570

  real    0m46.213s
  user    0m29.688s
  sys     0m20.042s

after the fix, the test loop never encountered a non-page-rounded value.


Patch 3 - procfs signed wrap
----------------------------
Before:

  % echo 2147483647 >/proc/sys/fs/pipe-max-size
  % cat /proc/sys/fs/pipe-max-size
  -2147483648

After:

  % echo 2147483647 >/proc/sys/fs/pipe-max-size
  % cat /proc/sys/fs/pipe-max-size
  2147483648


Joe Lawrence (3):
  pipe: avoid round_pipe_size() nr_pages overflow on 32-bit
  pipe: protect pipe_max_size access with a mutex
  pipe: match pipe_max_size data type with procfs

 fs/pipe.c       | 48 +++++++++++++++++++++++++++++++++++++++++-------
 kernel/sysctl.c |  2 +-
 2 files changed, 42 insertions(+), 8 deletions(-)

-- 
1.8.3.1