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


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

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")

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

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


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

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(-)


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