[0/2] xfs stats fixes

Message

Carlos Maiolino Oct. 3, 2018, 12:35 p.m. UTC

Hi,

this week I found a bug in the code for xqmstat file in procfs, caused by wrong
offsets while reading the __xfsstats structure fields in xqmstat_proc_show.

This patchset aims to fix the bug (patch 1) and add a way to reduce the
likelyhood of creating the same error again.

xqmstat_proc_show() uses a loop to walk through the quota-specific fields in
__xfsstats when printing the information. The problem is, since the start/end
offsets are hardcoded, any update to __xfsstats may also require an update on
the start offset in xqmstat_proc_show().

I decided to split it into two patches, because I think it is easier to let the
bugfix itself in a separated patch, and the approach on how to avoid it in the
future can be discussed in a separated patch.

My first idea was to try to add a method to check it at compile time, and I spent
some time discussing with Eric what would be the best way to avoid this problem
in the future.
We end up with an possible solution using offsetof() and a new constant in
xfs_stats.h to properly check the offsets at compile time.
Although, after giving it some extra thought, I found it better to only use a
new constant to mark the start of xqmstat in xfs_stats.h and not add any fancy
calculation in xqmstat_proc_show itself.
History told me that all updates to __xfsstats and its respective content/uses,
were properly done when needed, only the xqmstat was left behind because it's
unusual direct usage of the offset markers. So, I believe that the addition of a
new marker to specify the start offset of the quota stat fields is enough to
avoid future problems here, once no more changes will be needed in xqmstat code
whenever __xfsstats requires to be updated.

My idea though is still to move this file to sysfs together with the rest of
statistic code, but by now, I believe we should fix this bug.

Comments?

Cheers.


Carlos Maiolino (2):
  xfs: Fix xqmstats offsets in /proc/fs/xfs/xqmstat
  xfs: Add new constant to mark start of xqmstat

 fs/xfs/xfs_stats.c | 2 +-
 fs/xfs/xfs_stats.h | 1 +
 2 files changed, 2 insertions(+), 1 deletion(-)