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Date: Fri, 8 Jul 2016 16:49:25 -0400 From: "J. Bruce Fields" <bfields@...ldses.org> To: Oleg Drokin <green@...uxhacker.ru> Cc: Jeff Layton <jlayton@...chiereds.net>, linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] nfsd: Make creates return EEXIST correctly instead of EPERM On Fri, Jul 08, 2016 at 12:16:14PM -0400, Oleg Drokin wrote: > > On Jul 8, 2016, at 12:04 PM, J. Bruce Fields wrote: > > > On Fri, Jul 08, 2016 at 11:53:28AM -0400, Jeff Layton wrote: > >> On Fri, 2016-07-08 at 11:14 -0400, Oleg Drokin wrote: > >>> On Jul 8, 2016, at 7:02 AM, Jeff Layton wrote: > >>> > >>>> On Thu, 2016-07-07 at 21:47 -0400, Oleg Drokin wrote: > >>>>> It looks like we are bit overzealous about failing mkdir/create/mknod > >>>>> with permission denied if the parent dir is not writeable. > >>>>> Need to make sure the name does not exist first, because we need to > >>>>> return EEXIST in that case. > >>>>> > >>>>> Signed-off-by: Oleg Drokin <green@...uxhacker.ru> > >>>>> --- > >>>>> A very similar problem exists with symlinks, but the patch is more > >>>>> involved, so assuming this one is ok, I'll send a symlink one separately. > >>>>> fs/nfsd/nfs4proc.c | 6 +++++- > >>>>> fs/nfsd/vfs.c | 11 ++++++++++- > >>>>> 2 files changed, 15 insertions(+), 2 deletions(-) > >>>>> > >>>> > >>>> > >>>> nit: subject says EPERM, but I think you mean EACCES. The mnemonic I've > >>>> always used is that EPERM is "permanent". IOW, changing permissions > >>>> won't ever allow the user to do something. For instance, unprivileged > >>>> users can never chown a file, so they should get back EPERM there. When > >>>> a directory isn't writeable on a create they should get EACCES since > >>>> they could do the create if the directory were writeable. > >>> > >>> Hm, I see, thanks. > >>> Confusing that you get "Permission denied" from perror ;) > >>> > >> > >> Yes indeed. It's a subtle and confusing distinction. > >> > >>>>> diff --git a/fs/nfsd/nfs4proc.c b/fs/nfsd/nfs4proc.c > >>>>> index de1ff1d..0067520 100644 > >>>>> --- a/fs/nfsd/nfs4proc.c > >>>>> +++ b/fs/nfsd/nfs4proc.c > >>>>> @@ -605,8 +605,12 @@ nfsd4_create(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate, > >>>>> > >>>>> fh_init(&resfh, NFS4_FHSIZE); > >>>>> > >>>>> + /* > >>>>> + * We just check thta parent is accessible here, nfsd_* do their > >>>>> + * own access permission checks > >>>>> + */ > >>>>> status = fh_verify(rqstp, &cstate->current_fh, S_IFDIR, > >>>>> - NFSD_MAY_CREATE); > >>>>> + NFSD_MAY_EXEC); > >>>>> if (status) > >>>>> return status; > >>>>> > >>>>> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c > >>>>> index 6fbd81e..6a45ec6 100644 > >>>>> --- a/fs/nfsd/vfs.c > >>>>> +++ b/fs/nfsd/vfs.c > >>>>> @@ -1161,7 +1161,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp, > >>>>> if (isdotent(fname, flen)) > >>>>> goto out; > >>>>> > >>>>> - err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_CREATE); > >>>>> + /* > >>>>> + * Even though it is a create, first we see if we are even allowed > >>>>> + * to peek inside the parent > >>>>> + */ > >>>>> + err = fh_verify(rqstp, fhp, S_IFDIR, NFSD_MAY_EXEC); > >>>>> if (err) > >>>>> goto out; > >>>>> > >>>>> @@ -1211,6 +1215,11 @@ nfsd_create(struct svc_rqst *rqstp, struct svc_fh *fhp, > >>>>> goto out; > >>>>> } > >>>>> > >>>>> + /* Now let's see if we actually have permissions to create */ > >>>>> + err = nfsd_permission(rqstp, fhp->fh_export, dentry, NFSD_MAY_CREATE); > >>>>> + if (err) > >>>>> + goto out; > >>>>> + > >>>>> if (!(iap->ia_valid & ATTR_MODE)) > >>>>> iap->ia_mode = 0; > >>>>> iap->ia_mode = (iap->ia_mode & S_IALLUGO) | type; > >>>> > >>>> > >>>> Ouch. This means two nfsd_permission calls per create operation. If > >>>> it's necessary for correctness then so be it, but is it actually > >>>> documented anywhere (POSIX perhaps?) that we must prefer EEXIST over > >>>> EACCES in this situation? > >>> > >>> Opengroup manpage: http://pubs.opengroup.org/onlinepubs/009695399/functions/mkdir.html > >>> newer version is here: > >>> http://pubs.opengroup.org/onlinepubs/9699919799/ > >>> > >>> They tell us that we absolutely must fail with EEXIST if the name is a symlink > >>> (so we need to lookup it anyway), and also that EEXIST is the failure code > >>> if the path exists. > >>> > >> > >> I'm not sure that that verbiage supersedes the fact that you don't have > >> write permissions on the directory. Does it? > >> > >> ISTM that it's perfectly valid to shortcut looking up the dentry if the > >> user doesn't have write permissions on the directory, even when the > >> target is a symlink. > >> > >> IOW, I'm not sure I see a bug here. > > > > If this is causing real programs to behave incorrectly, then that may > > matter more than the letter of the spec. But I'm a little curious why > > we'd be hearing about that just now--did the client or server's behavior > > change recently? > > We, on the Lustre side, have been hearing about this since 2010, (this optimization > was enabled in 2009). > > I suspect some people just complain in places that not everybody monitors. Sure, but you said "tons of programs" do this, and off hand I can't recall a single report. That's weird. Anyway, I agree that the behavior your want seems more consistent at least. --b. > I tried 3.10 and it has the same problem here. > I just tried on RHEL6 (2.6.32) and the problem is also apparent there. > > Also it's confusing how you get different errors depending on if the cache is hot or not: > [green@...tos6-16 racer]$ mkdir test > mkdir: cannot create directory `test': Permission denied > [green@...tos6-16 racer]$ ls -ld test > drwxr-xr-x 2 root root 4096 Jul 8 12:12 test > [green@...tos6-16 racer]$ mkdir test > mkdir: cannot create directory `test': File exists > > > >>> Are double permission checks really as bad for nfs? it looked like it would > >>> call mostly into VFS so even if first call would be expensive, second call should > >>> be really cheap? > >>> > >> > >> It depends on the underlying fs. In most cases, you're right, but you > >> can export things that overload the ->permission op, and those can be > >> as expensive as they like (within reason of course). > > > > Weird if the expense of a second permission call is significant compared > > to following the mkdir and sync. But, what do I know. > > > > --b.
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