[PATCH 3/4] export/fuse: Let permissions be adjustable

[Max Reitz]

Allow changing the file mode, UID, and GID through SETATTR.

This only really makes sense with allow-other, though (because without
it, the effective access mode is fixed to be 0600 (u+rw) with qemu's
user being the file's owner), so changing these stat fields is not
allowed without allow-other.

Signed-off-by: Max Reitz <mreitz@redhat.com>
---
 block/export/fuse.c | 48 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 37 insertions(+), 11 deletions(-)

diff --git a/block/export/fuse.c b/block/export/fuse.c
index 1d54286d90..742e0af657 100644
--- a/block/export/fuse.c
+++ b/block/export/fuse.c
@@ -47,6 +47,10 @@ typedef struct FuseExport {
     bool writable;
     bool growable;
     bool allow_other;
+
+    mode_t st_mode;
+    uid_t st_uid;
+    gid_t st_gid;
 } FuseExport;
 
 static GHashTable *exports;
@@ -120,6 +124,13 @@ static int fuse_export_create(BlockExport *blk_exp,
     exp->growable = args->growable;
     exp->allow_other = args->allow_other;
 
+    exp->st_mode = S_IFREG | S_IRUSR;
+    if (exp->writable) {
+        exp->st_mode |= S_IWUSR;
+    }
+    exp->st_uid = getuid();
+    exp->st_gid = getgid();
+
     ret = setup_fuse_export(exp, args->mountpoint, errp);
     if (ret < 0) {
         goto fail;
@@ -329,7 +340,6 @@ static void fuse_getattr(fuse_req_t req, fuse_ino_t inode,
     int64_t length, allocated_blocks;
     time_t now = time(NULL);
     FuseExport *exp = fuse_req_userdata(req);
-    mode_t mode;
 
     length = blk_getlength(exp->common.blk);
     if (length < 0) {
@@ -344,17 +354,12 @@ static void fuse_getattr(fuse_req_t req, fuse_ino_t inode,
         allocated_blocks = DIV_ROUND_UP(allocated_blocks, 512);
     }
 
-    mode = S_IFREG | S_IRUSR;
-    if (exp->writable) {
-        mode |= S_IWUSR;
-    }
-
     statbuf = (struct stat) {
         .st_ino     = inode,
-        .st_mode    = mode,
+        .st_mode    = exp->st_mode,
         .st_nlink   = 1,
-        .st_uid     = getuid(),
-        .st_gid     = getgid(),
+        .st_uid     = exp->st_uid,
+        .st_gid     = exp->st_gid,
         .st_size    = length,
         .st_blksize = blk_bs(exp->common.blk)->bl.request_alignment,
         .st_blocks  = allocated_blocks,
@@ -400,15 +405,23 @@ static int fuse_do_truncate(const FuseExport *exp, 
int64_t size,
 }
 
 /**
- * Let clients set file attributes.  Only resizing is supported.
+ * Let clients set file attributes.  With allow_other, only resizing and
+ * changing permissions (st_mode, st_uid, st_gid) is allowed.  Without
+ * allow_other, only resizing is supported.
  */
 static void fuse_setattr(fuse_req_t req, fuse_ino_t inode, struct stat 
*statbuf,
                          int to_set, struct fuse_file_info *fi)
 {
     FuseExport *exp = fuse_req_userdata(req);
+    int supported_attrs;
     int ret;
 
-    if (to_set & ~FUSE_SET_ATTR_SIZE) {
+    supported_attrs = FUSE_SET_ATTR_SIZE;
+    if (exp->allow_other) {
+        supported_attrs |= FUSE_SET_ATTR_MODE | FUSE_SET_ATTR_UID |
+            FUSE_SET_ATTR_GID;
+    }
+    if (to_set & ~supported_attrs) {
         fuse_reply_err(req, ENOTSUP);
         return;
     }
@@ -426,6 +439,19 @@ static void fuse_setattr(fuse_req_t req, fuse_ino_t inode, 
struct stat *statbuf,
         }
     }
 
+    if (to_set & FUSE_SET_ATTR_MODE) {
+        /* Only allow changing the file mode, not the type */
+        exp->st_mode = (statbuf->st_mode & 07777) | S_IFREG;
+    }
+
+    if (to_set & FUSE_SET_ATTR_UID) {
+        exp->st_uid = statbuf->st_uid;
+    }
+
+    if (to_set & FUSE_SET_ATTR_GID) {
+        exp->st_gid = statbuf->st_gid;
+    }
+
     fuse_getattr(req, inode, fi);
 }
 
-- 
2.31.1