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

[Steve Dickson <SteveD@redhat.com>]

On 10/19/2011 01:22 PM, Jeff Layton wrote:
> On Wed, 19 Oct 2011 13:10:19 -0400
> Steve Dickson <SteveD@redhat.com> 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! 
>>
> 
> Ahh, then you have another problem here too then. Most likely it was
> crashing because it caught a SIGXFSZ. Writing out the mtab should not
> be affected by signals.
So calling fflush() generates a SIGXFSZ and call fsync() does not... 
I really don't see what the problem is is call simply calling fsync()
which clearly works?

steved.

> 
> In the mount.cifs helper, I have it do the following before altering
> the mtab (with appropriate error handling):
> 
>         rc = setreuid(geteuid(), -1);
>         rc = sigfillset(&mask);
>         rc = sigprocmask(SIG_SETMASK, &mask, &oldmask);
> 
> 
> IOW, set the real uid to the effective UID to ensure that an
> unprivileged user can't signal the process if it was run as a setuid
> root program and the real UID isn't root. It then masks off all
> signals. That leaves SIGKILL by root as a way to interrupt it but
> there's really nothing you can do about that.
> 
>>
>>>
>>>>  		if (res < 0)
>>>>  			/* Avoid leaving a corrupt mtab file */
>>>>  			ftruncate(fileno(mfp->mntent_fp), length);
>>>
>>>
> 
>