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From: James Pearson <jcpearson@gmail.com>
Date: Thu, 25 Apr 2024 13:19:12 +0100
Message-ID: <CAK3fRr8N4dNz2+K-BgaZAcswbfXrDem6Z9fRtgTDMJa=Y0R8gA@mail.gmail.com>
Subject: Changing the precedence order of client exports in /etc/exports
To: Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"

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) ?

(I apologize if the formatting of the patch gets mangled by Gmail)

Thanks

James Pearson

diff --git a/support/export/cache.c b/support/export/cache.c
index 6c0a44a3..e9392d8e 100644
--- a/support/export/cache.c
+++ b/support/export/cache.c
@@ -54,6 +54,8 @@ enum nfsd_fsid {
        FSID_UUID16_INUM,
 };

+static int cache_export_ent(char *buf, int buflen, char *domain,
struct exportent *exp, char *path);
+
 #undef is_mountpoint
 static int is_mountpoint(const char *path)
 {
@@ -877,6 +879,14 @@ 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;
@@ -910,6 +920,12 @@ static int nfsd_handle_fh(int f, char *bp, int blen)
                goto out;
        }

+       /* adding this here - to make sure priority export changes are
+        * picked up (this used to be in 1.X versions ?)
+        */
+       if (cache_export_ent(buf, sizeof(buf), dom, found, found_path) < 0)
+               found = 0;
+
        bp = buf; blen = sizeof(buf);
        qword_add(&bp, &blen, dom);
        qword_addint(&bp, &blen, fsidtype);
@@ -1178,6 +1194,12 @@ 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..548063b8 100644
--- a/support/nfs/exports.c
+++ b/support/nfs/exports.c
@@ -374,6 +374,9 @@ 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 +837,14 @@ 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);