Re: [PATCH 1/1] mount.nfs: mtab corruption when RLIMIT_FSIZE causes a partial write

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Oct 19, 2011 at 01:32:41PM -0400, Steve Dickson wrote:
> 
> 
> On 10/19/2011 01:28 PM, J. Bruce Fields wrote:
> > On Wed, Oct 19, 2011 at 01:10:19PM -0400, Steve Dickson wrote:
> >>
> >>
> >> On 10/19/2011 12:36 PM, Jeff Layton wrote:
> >>> On Wed, 19 Oct 2011 11:34:30 -0400
> >>> Steve Dickson <steved@redhat.com> wrote:
> >>>
> >>>> This patch is a following on to commit 7a802337. Using the
> >>>> tool in https://bugzilla.redhat.com/show_bug.cgi?id=695916
> >>>> caused the fflush() and fclose() to fail in turn causing
> >>>> corruption in the mtab.
> >>>>
> >>>> The failures were in the internals of both calls. Switch those
> >>>> calls with the actual system calls eliminated the failures.
> >>>>
> >>>> Signed-off-by: Steve Dickson <steved@redhat.com>
> >>>> ---
> >>>>  support/nfs/nfs_mntent.c |    4 ++--
> >>>>  1 files changed, 2 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/support/nfs/nfs_mntent.c b/support/nfs/nfs_mntent.c
> >>>> index a2118a2..b80f270 100644
> >>>> --- a/support/nfs/nfs_mntent.c
> >>>> +++ b/support/nfs/nfs_mntent.c
> >>>> @@ -117,7 +117,7 @@ void
> >>>>  nfs_endmntent (mntFILE *mfp) {
> >>>>  	if (mfp) {
> >>>>  		if (mfp->mntent_fp)
> >>>> -			fclose(mfp->mntent_fp);
> >>>> +			close(fileno(mfp->mntent_fp));
> >>>>  		if (mfp->mntent_file)
> >>>>  			free(mfp->mntent_file);
> >>>>  		free(mfp);
> >>>> @@ -147,7 +147,7 @@ nfs_addmntent (mntFILE *mfp, struct mntent *mnt) {
> >>>>  	free(m3);
> >>>>  	free(m4);
> >>>>  	if (res >= 0) {
> >>>> -		res = fflush(mfp->mntent_fp);
> >>>> +		res = fsync(fileno(mfp->mntent_fp));
> >>>
> >>> fsync doesn't imply an fflush. With this, I think you may end up
> >>> without everything being committed to disk if part or all of it is
> >>> still in the file stream buffer. You probably want to do an fflush()
> >>> and then an fsync here.
> >> The problem was with the fflush() call. The call was causing the
> >> mount to drop core in turn causing mtab corruption. Changing that
> >> call to a fsync() worked just fine... no corruption... every time! 
> > 
> > Looking at man setrlimit....  If you're worried about RLIMIT_FSIZE, then
> > I guess you need to handle SIGXFSZ and EFBIG returns from write().
> > 
> > It looks like lock_mtab() already sets SIGXFSZ to be ignored.  Is a libc
> > bug crashing fflush on EFBIG?  Or maybe the ftruncate is a problem?
> > Maybe you just want to do away with stream IO completely?
> Calling fsync() clearly works.

Until we understand why it works, it may be just luck.

--b.

> So why are looking for alternative
> solutions??