[PATCH] virtiofsd: xattr mapping add a new type "unsupported"

[Vivek Goyal]

Right now for xattr remapping, we support types of "prefix", "ok" or "bad".
Type "bad" returns -EPERM on setxattr and hides xattr in listxattr. For
getxattr, mapping code returns -EPERM but getxattr code converts it to -ENODATA.

I need a new semantics where if an xattr is unsupported, then
getxattr()/setxattr() return -ENOTSUP and listxattr() should hide the xattr.
This is needed to simulate that security.selinux is not supported by
virtiofs filesystem and in that case client falls back to some default
label specified by policy.

So add a new type "unsupported" which returns -ENOTSUP on getxattr() and
setxattr() and hides xattrs in listxattr().

For example, one can use following mapping rule to not support
security.selinux xattr and allow others.

"-o xattrmap=/unsupported/all/security.selinux/security.selinux//ok/all///"

Suggested-by: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 docs/tools/virtiofsd.rst         |    6 ++++++
 tools/virtiofsd/passthrough_ll.c |   17 ++++++++++++++---
 2 files changed, 20 insertions(+), 3 deletions(-)

Index: rhvgoyal-qemu/tools/virtiofsd/passthrough_ll.c
===================================================================
--- rhvgoyal-qemu.orig/tools/virtiofsd/passthrough_ll.c 2021-09-22 
08:37:16.070377732 -0400
+++ rhvgoyal-qemu/tools/virtiofsd/passthrough_ll.c      2021-09-22 
14:17:09.543016250 -0400
@@ -2465,6 +2465,11 @@ static void lo_flock(fuse_req_t req, fus
  * Automatically reversed on read
  */
 #define XATTR_MAP_FLAG_PREFIX  (1 <<  2)
+/*
+ * The attribute is unsupported;
+ * ENOTSUP on write, hidden on read.
+ */
+#define XATTR_MAP_FLAG_UNSUPPORTED     (1 <<  3)
 
 /* scopes */
 /* Apply rule to get/set/remove */
@@ -2636,6 +2641,8 @@ static void parse_xattrmap(struct lo_dat
             tmp_entry.flags |= XATTR_MAP_FLAG_OK;
         } else if (strstart(map, "bad", &map)) {
             tmp_entry.flags |= XATTR_MAP_FLAG_BAD;
+        } else if (strstart(map, "unsupported", &map)) {
+            tmp_entry.flags |= XATTR_MAP_FLAG_UNSUPPORTED;
         } else if (strstart(map, "map", &map)) {
             /*
              * map is sugar that adds a number of rules, and must be
@@ -2646,8 +2653,8 @@ static void parse_xattrmap(struct lo_dat
         } else {
             fuse_log(FUSE_LOG_ERR,
                      "%s: Unexpected type;"
-                     "Expecting 'prefix', 'ok', 'bad' or 'map' in rule %zu\n",
-                     __func__, lo->xattr_map_nentries);
+                     "Expecting 'prefix', 'ok', 'bad', 'unsupported' or 'map'"
+                     " in rule %zu\n", __func__, lo->xattr_map_nentries);
             exit(1);
         }
 
@@ -2749,6 +2756,9 @@ static int xattr_map_client(const struct
             if (cur_entry->flags & XATTR_MAP_FLAG_BAD) {
                 return -EPERM;
             }
+            if (cur_entry->flags & XATTR_MAP_FLAG_UNSUPPORTED) {
+                return -ENOTSUP;
+            }
             if (cur_entry->flags & XATTR_MAP_FLAG_OK) {
                 /* Unmodified name */
                 return 0;
@@ -2788,7 +2798,8 @@ static int xattr_map_server(const struct
 
         if ((cur_entry->flags & XATTR_MAP_FLAG_SERVER) &&
             (strstart(server_name, cur_entry->prepend, &end))) {
-            if (cur_entry->flags & XATTR_MAP_FLAG_BAD) {
+            if (cur_entry->flags & XATTR_MAP_FLAG_BAD ||
+                cur_entry->flags & XATTR_MAP_FLAG_UNSUPPORTED) {
                 return -ENODATA;
             }
             if (cur_entry->flags & XATTR_MAP_FLAG_OK) {
Index: rhvgoyal-qemu/docs/tools/virtiofsd.rst
===================================================================
--- rhvgoyal-qemu.orig/docs/tools/virtiofsd.rst 2021-09-22 08:37:15.938372097 
-0400
+++ rhvgoyal-qemu/docs/tools/virtiofsd.rst      2021-09-22 14:44:09.814188712 
-0400
@@ -183,6 +183,12 @@ Using ':' as the separator a rule is of
   'ok' as either an explicit terminator or for special handling of certain
   patterns.
 
+- 'unsupported' - If a client tries to use a name matching 'key' it's
+  denied using ENOTSUP; when the server passes an attribute
+  name matching 'prepend' it's hidden.  In many ways it's use is very like
+  'ok' as either an explicit terminator or for special handling of certain
+  patterns.
+
 **key** is a string tested as a prefix on an attribute name originating
 on the client.  It maybe empty in which case a 'client' rule
 will always match on client names.