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

[Steve Dickson <SteveD@redhat.com>]

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! 

steved.

> 
>>  		if (res < 0)
>>  			/* Avoid leaving a corrupt mtab file */
>>  			ftruncate(fileno(mfp->mntent_fp), length);
> 
>