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In-Reply-To: <171410661177.7600.9594587292479704884@noble.neil.brown.name>
From: James Pearson <jcpearson@gmail.com>
Date: Fri, 26 Apr 2024 17:08:24 +0100
Message-ID: <CAK3fRr9kngizdGsAU8NpzBjpPSh4k6-23wpwfpZUHJeEoU7yVg@mail.gmail.com>
Subject: Re: Changing the precedence order of client exports in /etc/exports
To: NeilBrown <neilb@suse.de>
Cc: Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"

On Fri, 26 Apr 2024 at 05:43, NeilBrown <neilb@suse.de> wrote:
>
> On Thu, 25 Apr 2024, James Pearson wrote:
> > Many years ago, I asked on this list if it was possible to change the
> > precedence order of exports listed in /etc/exports where there is more
> > than one possible match (see the thread at:
> > https://marc.info/?l=linux-nfs&m=130565040627856&w=2) - and answer was
> > 'No'
> >
> > At that time, I used a simple hack to force the precedence order I
> > required (by modifying the 'MCL' enum order in nfs-utils
> > support/include/exportfs.h)
> >
> > However, the issue has come up again for me, so I thought I would see
> > if I could alter the precedence order by adding an exports 'priority='
> > option as suggested later in the above thread
> >
> > I started with the nfs-utils supplied with CentOS 7 (based on 1.3.0) -
> > and added logic to lookup_export() to check for client specifications
> > with a higher priority - but this didn't work - so looking for other
> > places that looped through MCL types, I added similar logic in
> > nfsd_fh() - which seems to work as I expected (I'm using NFSv3)
> >
> > However, adding similar logic to the nfs-utils supplied with Rocky 9
> > (based on 2.5.4) didn't work ...
> >
> > But comparing the code in nfsd_fh() in v1.3.0 and nfsd_handle_fh() in
> > v2.5.4, nfsd_fh() in v1.3.0 does the following towards the end of the
> > function - whereas nfsd_handle_fh() in v2.5.4 doesn't:
> >
> >         if (cache_export_ent(buf, sizeof(buf), dom, found, found_path) < 0)
> >                 found = 0;
> >
> > By adding the above lines at a similar place in nfsd_handle_fh() in
> > v2.5.4, seems to 'fix' the issue and all works as I expected
> >
> > I don't fully understand what is going on under the hood with all
> > this, so no idea if what I've done is 'correct', or if there is a
> > better way of doing what I'm trying to achieve ?
> >
> > Below is a patch (made against the latest git nfs-utils) of what I've
> > done - could anyone let me know if I'm going along the right lines (or
> > not) ?
>
> The restored cache_export_ent() call has to go.
> You need to update init_exportent() to initialise the new field.
> You probably need to make some changes to auth_authenticate_newcache().
> Probably let the loop run all the way to MCL_MAXTYPES, and do a priority
> comparison if you find a new possible match.
> export_find() probably need some attention too.
>
> If you it still doesn't work after addressing those, I'll have a look
> and see if I can beat it into shape.

Thanks for the pointers - new patch below

I don't quite understand what export_find() is actually doing ?

As far as I can tell, it is only used by exportfs when an export is
given on the command line - and only if that export is of type
MCL_FQDN - so I'm not sure it needs any changes to support these
priority additions ? (I might be completely wrong here ...)

Thanks

James Pearson

diff --git a/support/export/auth.c b/support/export/auth.c
index 2d7960f1..3d9e07b5 100644
--- a/support/export/auth.c
+++ b/support/export/auth.c
@@ -175,7 +175,7 @@ auth_authenticate_newcache(const struct sockaddr *caller,
                           const char *path, struct addrinfo *ai,
                           enum auth_error *error)
 {
-       nfs_export *exp;
+       nfs_export *exp, *found;
        int i;

        free(my_client.m_hostname);
@@ -189,6 +189,7 @@ auth_authenticate_newcache(const struct sockaddr *caller,
        my_exp.m_client = &my_client;

        exp = NULL;
+       found = NULL;
        for (i = 0; !exp && i < MCL_MAXTYPES; i++)
                for (exp = exportlist[i].p_head; exp; exp = exp->m_next) {
                        if (strcmp(path, exp->m_export.e_path))
@@ -198,8 +199,11 @@ auth_authenticate_newcache(const struct sockaddr *caller,
                        if (exp->m_export.e_flags & NFSEXP_V4ROOT)
                                /* not acceptable for v[23] export */
                                continue;
-                       break;
+                       /* we have a match - see if it is a higher priority */
+                       if (!found || exp->m_export.e_priority >
found->m_export.e_priority)
+                               found = exp;
                }
+       exp = found;
        *error = not_exported;
        if (!exp)
                return NULL;
diff --git a/support/export/cache.c b/support/export/cache.c
index 6c0a44a3..dfb0051b 100644
--- a/support/export/cache.c
+++ b/support/export/cache.c
@@ -877,6 +877,14 @@ static int nfsd_handle_fh(int f, char *bp, int blen)
                                xlog(L_WARNING, "%s and %s have same
filehandle for %s, using first",
                                     found_path, path, dom);
                        } else {
+                               /* same path, see if this one has a
higher export priority */
+                               if (exp->m_export.e_priority >
found->e_priority) {
+                                       found = &exp->m_export;
+                                       free(found_path);
+                                       found_path = strdup(path);
+                                       if (found_path == NULL)
+                                               goto out;
+                               }
                                /* same path, if one is V4ROOT, choose
the other */
                                if (found->e_flags & NFSEXP_V4ROOT) {
                                        found = &exp->m_export;
@@ -1178,6 +1186,12 @@ lookup_export(char *dom, char *path, struct addrinfo *ai)
                                found_type = i;
                                continue;
                        }
+                       /* see if this one has a higher export priority */
+                       if (exp->m_export.e_priority >
found->m_export.e_priority) {
+                               found = exp;
+                               found_type = i;
+                               continue;
+                       }
                        /* Always prefer non-V4ROOT exports */
                        if (exp->m_export.e_flags & NFSEXP_V4ROOT)
                                continue;
diff --git a/support/include/nfslib.h b/support/include/nfslib.h
index eff2a486..ab22ecaf 100644
--- a/support/include/nfslib.h
+++ b/support/include/nfslib.h
@@ -99,6 +99,7 @@ struct exportent {
        unsigned int    e_ttl;
        char *          e_realpath;
        int             e_reexport;
+       int             e_priority;
 };

 struct rmtabent {
diff --git a/support/nfs/exports.c b/support/nfs/exports.c
index a6816e60..afc139db 100644
--- a/support/nfs/exports.c
+++ b/support/nfs/exports.c
@@ -106,6 +106,7 @@ static void init_exportent (struct exportent *ee,
int fromkernel)
        ee->e_uuid = NULL;
        ee->e_ttl = default_ttl;
        ee->e_reexport = REEXP_NONE;
+       ee->e_priority = 0;
 }

 struct exportent *
@@ -374,6 +375,9 @@ putexportent(struct exportent *ep)
                                fprintf(fp, "%d,", id[i]);
        }
        fprintf(fp, "anonuid=%d,anongid=%d", ep->e_anonuid, ep->e_anongid);
+       if (ep->e_priority) {
+               fprintf(fp, ",priority=%d", ep->e_priority);
+       }
        secinfo_show(fp, ep);
        xprtsecinfo_show(fp, ep);
        fprintf(fp, ")\n");
@@ -834,6 +838,14 @@ bad_option:
                                setflags(NFSEXP_FSID, active, ep);

                        saw_reexport = 1;
+               } else if (strncmp(opt, "priority=", 9) == 0) {
+                       char *oe;
+                       ep->e_priority = strtol(opt+9, &oe, 10);
+                       if (opt[9]=='\0' || *oe != '\0') {
+                               xlog(L_ERROR, "%s: %d: bad priority \"%s\"\n",
+                                    flname, flline, opt);
+                               goto bad_option;
+                       }
                } else {
                        xlog(L_ERROR, "%s:%d: unknown keyword \"%s\"\n",
                                        flname, flline, opt);
diff --git a/utils/exportfs/exportfs.c b/utils/exportfs/exportfs.c
index b03a047b..5e6a64b6 100644
--- a/utils/exportfs/exportfs.c
+++ b/utils/exportfs/exportfs.c
@@ -753,6 +753,8 @@ dump(int verbose, int export_format)
                                break;
 #endif
                        }
+                       if (ep->e_priority)
+                               c = dumpopt(c, "priority=%d", ep->e_priority);
                        secinfo_show(stdout, ep);
                        xprtsecinfo_show(stdout, ep);
                        printf("%c\n", (c != '(')? ')' : ' ');