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Message-ID: <1467994665.27907.28.camel@poochiereds.net> Date: Fri, 08 Jul 2016 12:17:45 -0400 From: Jeff Layton <jlayton@...chiereds.net> To: Oleg Drokin <green@...uxhacker.ru> Cc: "J. Bruce Fields" <bfields@...ldses.org>, linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] nfsd: Make creates return EEXIST correctly instead of EPERM On Fri, 2016-07-08 at 11:59 -0400, Oleg Drokin wrote: > On Jul 8, 2016, at 11:53 AM, 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? > > "If path names a symbolic link, mkdir() shall fail and set errno to [EEXIST]." > > This sounds pretty straightforward to me, no? > Since it does not matter that we do not have write permissions here, because > the name already exists. > > (also there are tons of applications that make this assumption when > badly reimplementing their mkdir -p thing, I imagine they also have this same > reading of the man page - this is why I even care about it). > I always have trouble with this sort of thing. Just because it's in DESCRIPTION, does that make it supersede the part in ERRORS? IOW, I think that's just telling you how to handle a symlink as the last component, not that you have to do that before the permissions check. Now that said, as a practical matter I do agree that EEXIST _is_ probably the more helpful error message. If there are applications that rely on this then we probably should just take your patch. Reviewed-by: Jeff Layton <jlayton@...chiereds.net>
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