Re: [PATCH v2] erofs-utils: mount: fix flag-clearing bug and missing error check in parse_flagopts

[Gao Xiang]

On 2026/3/1 20:12, Yifan Zhao wrote:

The MS_* constants in glibc's <sys/mount.h> are defined as members of
an anonymous enum whose underlying type is unsigned int (because the
last member, MS_NOUSER, is initialised with '1U << 31').  Therefore
~MS_RDONLY, ~MS_NOSUID, etc. are unsigned int values that, when stored

The problem seems glibc refines these macros as enum.

into a 'long flags' field, undergo zero-extension, not sign-extension.
As a result every 'clearing' entry (rw, suid, dev, exec, async, atime,
diratime, norelatime, loud) produced a positive long, so the
opts[i].flags < 0 guard in erofsmount_parse_flagopts() was never true
and the corresponding flags were set rather than cleared.

Fix by casting the operand to long before applying bitwise-NOT,
ensuring the result is a negative long with the correct bit pattern.

Also add the missing return-value check for erofsmount_parse_flagopts()
in the '-o' option handler.

Reported-By: rorosen <76747196+roro...@users.noreply.github.com>

It seems the real email is:
Reported-by: Robert Rose <robert.r...@mailbox.org>

Otherwise it looks good to me, will apply:
Reviewed-by: Gao Xiang <hsiang...@linux.alibaba.com>

Thanks,
Gao Xiang

Closes: https://github.com/NixOS/nixpkgs/issues/494653
Signed-off-By: Yifan Zhao <yifan.yfz...@foxmail.com>
---
  mount/main.c | 27 +++++++++++++++------------
  1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/mount/main.c b/mount/main.c
index b04be5d..7c557bd 100644
--- a/mount/main.c
+++ b/mount/main.c
@@ -203,15 +203,15 @@ static long erofsmount_parse_flagopts(char *s, long 
flags, char **more)
        } opts[] = {
                {"defaults", 0}, {"quiet", 0}, // NOPs
                {"user", 0}, {"nouser", 0}, // checked in fstab, ignored in -o
-               {"ro", MS_RDONLY}, {"rw", ~MS_RDONLY},
-               {"nosuid", MS_NOSUID}, {"suid", ~MS_NOSUID},
-               {"nodev", MS_NODEV}, {"dev", ~MS_NODEV},
-               {"noexec", MS_NOEXEC}, {"exec", ~MS_NOEXEC},
-               {"sync", MS_SYNCHRONOUS}, {"async", ~MS_SYNCHRONOUS},
-               {"noatime", MS_NOATIME}, {"atime", ~MS_NOATIME},
-               {"norelatime", ~MS_RELATIME}, {"relatime", MS_RELATIME},
-               {"nodiratime", MS_NODIRATIME}, {"diratime", ~MS_NODIRATIME},
-               {"loud", ~MS_SILENT},
+               {"ro", MS_RDONLY}, {"rw", ~(long)MS_RDONLY},
+               {"nosuid", MS_NOSUID}, {"suid", ~(long)MS_NOSUID},
+               {"nodev", MS_NODEV}, {"dev", ~(long)MS_NODEV},
+               {"noexec", MS_NOEXEC}, {"exec", ~(long)MS_NOEXEC},
+               {"sync", MS_SYNCHRONOUS}, {"async", ~(long)MS_SYNCHRONOUS},
+               {"noatime", MS_NOATIME}, {"atime", ~(long)MS_NOATIME},
+               {"norelatime", ~(long)MS_RELATIME}, {"relatime", MS_RELATIME},
+               {"nodiratime", MS_NODIRATIME}, {"diratime", 
~(long)MS_NODIRATIME},
+               {"loud", ~(long)MS_SILENT},
                {"remount", MS_REMOUNT}, {"move", MS_MOVE},
                // mand dirsync rec iversion strictatime
        };
@@ -281,6 +281,7 @@ static int erofsmount_parse_options(int argc, char **argv)
                {0, 0, 0, 0},
        };
        char *dot;
+       long ret;
        int opt;
        int i;

@@ -305,9 +306,11 @@ static int erofsmount_parse_options(int argc, char **argv)

                        break;
                case 'o':
                        mountcfg.full_options = optarg;
-                       mountcfg.flags =
-                               erofsmount_parse_flagopts(optarg, 
mountcfg.flags,
-                                                         &mountcfg.options);
+                       ret = erofsmount_parse_flagopts(optarg, mountcfg.flags,
+                                                                  
&mountcfg.options);
+                       if (ret < 0)
+                               return (int)ret;
+                       mountcfg.flags = ret;
                        break;
                case 't':
                        dot = strchr(optarg, '.');