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From:   "NeilBrown" <neilb@suse.de>
To:     "Petr Vorel" <pvorel@suse.cz>
Cc:     "J . Bruce Fields" <bfields@fieldses.org>,
        linux-nfs@vger.kernel.org, "Steve Dickson" <steved@redhat.com>,
        "Chuck Lever" <chuck.lever@oracle.com>,
        "Alexey Kodanev" <alexey.kodanev@oracle.com>
Subject: [PATCH/RFC nfs-utils] Fix NFSv4 export of tmpfs filesystems.
In-reply-to: <YILQip3nAxhpXP9+@pevik>
References: <20210422191803.31511-1-pvorel@suse.cz>,
 <20210422202334.GB25415@fieldses.org>, <YILQip3nAxhpXP9+@pevik>
Date:   Fri, 07 May 2021 11:48:43 +1000
Message-id: <162035212343.24322.12361160756597283121@noble.neil.brown.name>
Precedence: bulk


[[This is a proposed fix.  It seems to work.  I'd like
  some review comments before it is committed.
  Petr: it would be great if you could test it to confirm
  it actually works in your case.
]]

Some filesystems cannot be exported without an fsid or uuid.
tmpfs is the main example.

When mountd creates nfsv4 pseudo-root exports for the path leading down
to an export point it exports each directory without any fsid or uuid.
If one of these directories is on tmp, that will fail.

The net result is that exporting a subdirectory of a tmpfs filesystem
will not work over NFSv4 as the parents within the filesystem cannot be
exported.  It will either fail, or fall-back to NFSv3 (depending on the
version of the mount.nfs program).

To fix this we need to provide an fsid or uuid for these pseudo-root
exports.  This patch does that by creating a UUID with the first 4 bytes
0xFFFFFFFF and the remaining 12 bytes form from the path name, xoring
bytes together if the path is longer than 12 characters.
Hopefully no filesystem uses a UUID like this....

The patch borrows some code from exportfs.  Maybe that code should be
move to a library..

Signed-off-by: NeilBrown <neilb@suse.de>
---
 support/export/v4root.c | 57 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)

diff --git a/support/export/v4root.c b/support/export/v4root.c
index 3654bd7c10c0..fd36eb704441 100644
--- a/support/export/v4root.c
+++ b/support/export/v4root.c
@@ -11,6 +11,7 @@
 #include <config.h>
 #endif
 
+#include <fcntl.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/queue.h>
@@ -21,6 +22,7 @@
 #include <unistd.h>
 #include <errno.h>
 
+#include "nfsd_path.h"
 #include "xlog.h"
 #include "exportfs.h"
 #include "nfslib.h"
@@ -73,6 +75,38 @@ set_pseudofs_security(struct exportent *pseudo)
 	}
 }
 
+static ssize_t exportfs_write(int fd, const char *buf, size_t len)
+{
+	return nfsd_path_write(fd, buf, len);
+}
+
+static int test_export(struct exportent *eep, int with_fsid)
+{
+	char *path = eep->e_path;
+	int flags = eep->e_flags | (with_fsid ? NFSEXP_FSID : 0);
+	/* beside max path, buf size should take protocol str into account */
+	char buf[NFS_MAXPATHLEN+1+64] = { 0 };
+	char *bp = buf;
+	int len = sizeof(buf);
+	int fd, n;
+
+	n = snprintf(buf, len, "-test-client- ");
+	bp += n;
+	len -= n;
+	qword_add(&bp, &len, path);
+	if (len < 1)
+		return 0;
+	snprintf(bp, len, " 3 %d 65534 65534 0\n", flags);
+	fd = open("/proc/net/rpc/nfsd.export/channel", O_WRONLY);
+	if (fd < 0)
+		return 0;
+	n = exportfs_write(fd, buf, strlen(buf));
+	close(fd);
+	if (n < 0)
+		return 0;
+	return 1;
+}
+
 /*
  * Create a pseudo export
  */
@@ -82,6 +116,7 @@ v4root_create(char *path, nfs_export *export)
 	nfs_export *exp;
 	struct exportent eep;
 	struct exportent *curexp = &export->m_export;
+	char uuid[33];
 
 	dupexportent(&eep, &pseudo_root.m_export);
 	eep.e_ttl = default_ttl;
@@ -89,6 +124,28 @@ v4root_create(char *path, nfs_export *export)
 	strncpy(eep.e_path, path, sizeof(eep.e_path)-1);
 	if (strcmp(path, "/") != 0)
 		eep.e_flags &= ~NFSEXP_FSID;
+	if (strcmp(path, "/") != 0 &&
+	    !test_export(&eep, 0)) {
+		/* Need a uuid - base it on path */
+		char buf[12], *pp = path;
+		unsigned int i = 0;
+
+		memset(buf, 0, sizeof(buf));
+		while (*pp) {
+			buf[i] ^= *pp++;
+			i += 1;
+			if (i >= sizeof(buf))
+				i = 0;
+		}
+		memset(uuid, 'F', 32);
+		uuid[32] = '\0';
+		pp = uuid + 32 - sizeof(buf) * 2;
+		for (i = 0; i < sizeof(buf); i++) {
+			snprintf(pp, 3, "%02X", buf[i]);
+			pp += 2;
+		}
+		eep.e_uuid = uuid;
+	}
 	set_pseudofs_security(&eep);
 	exp = export_create(&eep, 0);
 	if (exp == NULL)
-- 
2.31.1