mount: "mount -o rw ...." should not fall back to RO mount

The reported case was an attempt to remount,rw a CD-ROM:

  mount -o remount,rw /mnt/sr0

which "succeeded" by falling back to RO:

  mount("/dev/sr0", "/mnt/sr0", 0x412862, MS_REMOUNT|MS_SILENT|MS_RELATIME, "nojoliet,check=s,map=n,blocksize"...) = -1 EROFS (Read-only file system)
  ...
  mount("/dev/sr0", "/mnt/sr0", 0x412862, MS_RDONLY|MS_REMOUNT|MS_SILENT|MS_RELATIME, "nojoliet,check=s,map=n,blocksize"...) = 0

Clearly, not what was intended!

function                                             old     new   delta
parse_mount_options                                  241     267     +26
mount_main                                          1198    1211     +13
singlemount                                         1301    1313     +12
inetd_main                                          1919    1911      -8
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/1 up/down: 51/-8)              Total: 43 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 13 insertions, 8 deletions

diff --git a/util-linux/mount.c b/util-linux/mount.c
index 44afdbcff..4e65b6b46 100644
--- a/util-linux/mount.c
+++ b/util-linux/mount.c
@@ -589,7 +589,7 @@ static void append_mount_options(char **oldopts, const char *newopts)
 
 // Use the mount_options list to parse options into flags.
 // Also update list of unrecognized options if unrecognized != NULL
-static unsigned long parse_mount_options(char *options, char **unrecognized)
+static unsigned long parse_mount_options(char *options, char **unrecognized, uint32_t *opt)
 {
         unsigned long flags = MS_SILENT;
 
@@ -617,6 +617,11 @@ static unsigned long parse_mount_options(char *options, char **unrecognized)
                                         flags &= fl;
                                 else
                                         flags |= fl;
+                                /* If we see "-o rw" on command line, it's the same as -w:
+                                 * "do not try to fall back to RO mounts"
+                                 */
+                                if (fl == ~MS_RDONLY && opt)
+                                        (*opt) |= OPT_w;
                                 goto found;
                         }
                         option_str += opt_len + 1;
@@ -1973,7 +1978,7 @@ static int singlemount(struct mntent *mp, int ignore_busy)
 
         errno = 0;
 
-        vfsflags = parse_mount_options(mp->mnt_opts, &filteropts);
+        vfsflags = parse_mount_options(mp->mnt_opts, &filteropts, NULL);
 
         // Treat fstype "auto" as unspecified
         if (mp->mnt_type && strcmp(mp->mnt_type, "auto") == 0)
@@ -2047,7 +2052,7 @@ static int singlemount(struct mntent *mp, int ignore_busy)
                                 len, share,
                                 share + len + 1  /* "dir1/dir2" */
                         );
-                        parse_mount_options(unc, &filteropts);
+                        parse_mount_options(unc, &filteropts, NULL);
                         if (ENABLE_FEATURE_CLEAN_UP) free(unc);
                 }
 
@@ -2073,7 +2078,7 @@ static int singlemount(struct mntent *mp, int ignore_busy)
 // (instead of _numeric_ iface_id) with glibc.
 // This probably should be fixed in glibc, not here.
 // The workaround is to manually specify correct "ip=ADDR%n" option.
-                        parse_mount_options(ip, &filteropts);
+                        parse_mount_options(ip, &filteropts, NULL);
                         if (ENABLE_FEATURE_CLEAN_UP) free(ip);
                 }
 
@@ -2355,7 +2360,7 @@ int mount_main(int argc UNUSED_PARAM, char **argv)
         // Past this point, we are handling either "mount -a [opts]"
         // or "mount [opts] single_param"
 
-        cmdopt_flags = parse_mount_options(cmdopts, NULL);
+        cmdopt_flags = parse_mount_options(cmdopts, NULL, &option_mask32);
         if (nonroot && (cmdopt_flags & ~MS_SILENT)) // Non-root users cannot specify flags
                 bb_simple_error_msg_and_die(bb_msg_you_must_be_root);
 
@@ -2429,7 +2434,7 @@ int mount_main(int argc UNUSED_PARAM, char **argv)
                                 continue;
 
                         // Skip noauto and swap anyway
-                        if ((parse_mount_options(mtcur->mnt_opts, NULL) & (MOUNT_NOAUTO | MOUNT_SWAP))
+                        if ((parse_mount_options(mtcur->mnt_opts, NULL, NULL) & (MOUNT_NOAUTO | MOUNT_SWAP))
                         // swap is bogus "fstype", parse_mount_options can't check fstypes
                          || strcasecmp(mtcur->mnt_type, "swap") == 0
                         ) {
@@ -2490,7 +2495,7 @@ int mount_main(int argc UNUSED_PARAM, char **argv)
                 // exit_group(32)                          = ?
 #if 0
                 // In case we want to simply skip swap partitions:
-                l = parse_mount_options(mtcur->mnt_opts, NULL);
+                l = parse_mount_options(mtcur->mnt_opts, NULL, NULL);
                 if ((l & MOUNT_SWAP)
                 // swap is bogus "fstype", parse_mount_options can't check fstypes
                  || strcasecmp(mtcur->mnt_type, "swap") == 0
@@ -2500,7 +2505,7 @@ int mount_main(int argc UNUSED_PARAM, char **argv)
 #endif
                 if (nonroot) {
                         // fstab must have "users" or "user"
-                        l = parse_mount_options(mtcur->mnt_opts, NULL);
+                        l = parse_mount_options(mtcur->mnt_opts, NULL, NULL);
                         if (!(l & MOUNT_USERS))
                                 bb_simple_error_msg_and_die(bb_msg_you_must_be_root);
                 }