rofiles-fuse: Fix lchown() and hardlink verification for symlinks

If you lchown("symlink") then we were incorrectly trying to chown the
symlink target, rather than the symlink itself. In particular, this cause
cp -a to fail for a broken symlink. Additionally, it was using the
symlink target when verifying writability, rather than the symlink
itself.

To fix this, we need pass AT_SYMLINK_NOFOLLOW in these cases.

In general, the kernel itself will always resolve any symlinks for us
before calling into the fuse backend, so we should really never do any
symlink following in the fuse fs itself. So, we pro-actively add
NOFOLLOW flags to a few other places:

 truncate:
      In reality this will never be hit, because
      the kernel will resolve symlinks before calling us.
 access:
      It seems the current fuse implementation never calls this
      (faccessat w/AT_SYMLINK_NOFOLLOW never reaches the fuse fs)
      but if this ever is implemented this is the correct behaviour.

We would ideally do `chmod` but this is not implemented on current kernels.
Because we're not multi-threaded, this is OK anyways.

Further, our write verification wasn't correctly handling the case of hardlinked
symlinks, which can occur for `bare` checkouts but *not* `bare-user` which the
tests were using. Change to `bare` mode to verify that.

src/rofiles-fuse/main.c

@@ -198,26 +198,42 @@ callback_link (const char *from, const char *to)
   return 0;
 }
 
-static gboolean
-stbuf_is_regfile_hardlinked (struct stat *stbuf)
+/* Check whether @stbuf refers to a hardlinked regfile or symlink, and if so
+ * return -EROFS. Otherwise return 0.
+ */
+static int
+can_write_stbuf (struct stat *stbuf)
 {
-  return S_ISREG (stbuf->st_mode) && stbuf->st_nlink > 1;
+  /* If it's not a regular file or symlink, ostree won't hardlink it, so allow
+   * writes - it might be a FIFO or device that somehow
+   * ended up underneath our mount.
+   */
+  if (!(S_ISREG (stbuf->st_mode) || S_ISLNK (stbuf->st_mode)))
+    return 0;
+  /* If the object isn't hardlinked, it's OK to write */
+  if (stbuf->st_nlink <= 1)
+    return 0;
+  /* Otherwise, it's a hardlinked file or symlink; it must be
+   * immutable.
+   */
+  return -EROFS;
 }
 
+/* Check whether @path refers to a hardlinked regfile or symlink, and if so
+ * return -EROFS. Otherwise return 0.
+ */
 static int
 can_write (const char *path)
 {
   struct stat stbuf;
-  if (fstatat (basefd, path, &stbuf, 0) == -1)
+  if (fstatat (basefd, path, &stbuf, AT_SYMLINK_NOFOLLOW) == -1)
     {
       if (errno == ENOENT)
         return 0;
       else
         return -errno;
     }
-  if (stbuf_is_regfile_hardlinked (&stbuf))
-    return -EROFS;
-  return 0;
+  return can_write_stbuf (&stbuf);
 }
 
 #define VERIFY_WRITE(path) do {                 \
@@ -231,6 +247,10 @@ callback_chmod (const char *path, mode_t mode)
 {
   path = ENSURE_RELPATH (path);
   VERIFY_WRITE(path);
+  /* Note we can't use AT_SYMLINK_NOFOLLOW yet;
+   * https://marc.info/?l=linux-kernel&m=148830147803162&w=2
+   * https://marc.info/?l=linux-fsdevel&m=149193779929561&w=2
+   */
   if (fchmodat (basefd, path, mode, 0) != 0)
     return -errno;
   return 0;
@@ -241,7 +261,7 @@ callback_chown (const char *path, uid_t uid, gid_t gid)
 {
   path = ENSURE_RELPATH (path);
   VERIFY_WRITE(path);
-  if (fchownat (basefd, path, uid, gid, 0) != 0)
+  if (fchownat (basefd, path, uid, gid, AT_SYMLINK_NOFOLLOW) != 0)
     return -errno;
   return 0;
 }
@@ -254,7 +274,7 @@ callback_truncate (const char *path, off_t size)
   path = ENSURE_RELPATH (path);
   VERIFY_WRITE(path);
 
-  fd = openat (basefd, path, O_WRONLY);
+  fd = openat (basefd, path, O_NOFOLLOW|O_WRONLY);
   if (fd == -1)
     return -errno;
 
@@ -312,10 +332,11 @@ do_open (const char *path, mode_t mode, struct fuse_file_info *finfo)
           return -errno;
         }
 
-      if (stbuf_is_regfile_hardlinked (&stbuf))
+      int r = can_write_stbuf (&stbuf);
+      if (r != 0)
         {
           (void) close (fd);
-          return -EROFS;
+          return r;
         }
 
       /* Handle O_TRUNC here only after verifying hardlink state */
@@ -433,7 +454,7 @@ callback_access (const char *path, int mode)
    * before trying to do an unlink.  So...we'll just lie about
    * writable access here.
    */
-  if (faccessat (basefd, path, mode, 0) == -1)
+  if (faccessat (basefd, path, mode, AT_SYMLINK_NOFOLLOW) == -1)
     return -errno;
   return 0;
 }

tests/test-rofiles-fuse.sh

@@ -24,13 +24,16 @@ set -euo pipefail
 skip_without_fuse
 skip_without_user_xattrs
 
-setup_test_repository "bare-user"
+setup_test_repository "bare"
 
 echo "1..7"
 
+cd ${test_tmpdir}
 mkdir mnt
-
-$OSTREE checkout -H -U test2 checkout-test2
+# The default content set amazingly doesn't have a non-broken link
+ln -s firstfile files/firstfile-link
+$OSTREE commit -b test2 --tree=dir=files
+$OSTREE checkout -H test2 checkout-test2
 
 rofiles-fuse checkout-test2 mnt
 cleanup_fuse() {
@@ -40,22 +43,32 @@ trap cleanup_fuse EXIT
 assert_file_has_content mnt/firstfile first
 echo "ok mount"
 
-if cp /dev/null mnt/firstfile 2>err.txt; then
-    assert_not_reached "inplace mutation"
-fi
-assert_file_has_content err.txt "Read-only file system"
-assert_file_has_content mnt/firstfile first
-assert_file_has_content checkout-test2/firstfile first
-
+# Test open(O_TRUNC) directly and via symlink
+for path in firstfile{,-link}; do
+    if cp /dev/null mnt/${path} 2>err.txt; then
+        assert_not_reached "inplace mutation ${path}"
+    fi
+    assert_file_has_content err.txt "Read-only file system"
+    assert_file_has_content mnt/firstfile first
+    assert_file_has_content checkout-test2/firstfile first
+done
 echo "ok failed inplace mutation (open O_TRUNCATE)"
 
-# Test chmod + chown
+# Test chmod
 if chmod 0600 mnt/firstfile 2>err.txt; then
     assert_not_reached "chmod inplace"
 fi
 assert_file_has_content err.txt "chmod:.*Read-only file system"
-if chown $(id -u) mnt/firstfile 2>err.txt; then
-    assert_not_reached "chown inplace"
+# Test chown with regfiles and symlinks
+for path in firstfile baz/alink; do
+    if chown -h $(id -u) mnt/${path} 2>err.txt; then
+        assert_not_reached "chown inplace ${path}"
+    fi
+    assert_file_has_content err.txt "chown:.*Read-only file system"
+done
+# And test via dereferencing a symlink
+if chown $(id -u) mnt/firstfile-link 2>err.txt; then
+    assert_not_reached "chown inplace firstfile-link"
 fi
 assert_file_has_content err.txt "chown:.*Read-only file system"
 echo "ok failed mutation chmod + chown"
@@ -64,6 +77,13 @@ echo "ok failed mutation chmod + chown"
 echo anewfile-for-fuse > mnt/anewfile-for-fuse
 assert_file_has_content mnt/anewfile-for-fuse anewfile-for-fuse
 assert_file_has_content checkout-test2/anewfile-for-fuse anewfile-for-fuse
+ln -s anewfile-for-fuse mnt/anewfile-for-fuse-link
+# And also test modifications through a symlink
+echo writevialink > mnt/anewfile-for-fuse-link
+for path in anewfile-for-fuse{,-link}; do
+    assert_file_has_content mnt/${path} writevialink
+done
+chown $(id -u) mnt/anewfile-for-fuse-link
 
 mkdir mnt/newfusedir
 for i in $(seq 5); do
@@ -86,7 +106,7 @@ ${CMD_PREFIX} ostree --repo=repo commit -b test2 -s fromfuse --link-checkout-spe
 echo "ok commit"
 
 ${CMD_PREFIX} ostree --repo=repo checkout -U test2 mnt/test2-checkout-copy-fallback
-assert_file_has_content mnt/test2-checkout-copy-fallback/anewfile-for-fuse anewfile-for-fuse
+assert_file_has_content mnt/test2-checkout-copy-fallback/anewfile-for-fuse writevialink
 
 if ${CMD_PREFIX} ostree --repo=repo checkout -UH test2 mnt/test2-checkout-copy-hardlinked 2>err.txt; then
     assert_not_reached "Checking out via hardlinks across mountpoint unexpectedly succeeded!"