Re: [BUG] Is casefold unreported in /proc/mounts?

[Gabriel Krisman Bertazi <krisman@suse.de>]

[-- Attachment #1: Type: text/plain, Size: 1474 bytes --]

Daniel Tang <danielzgtg.opensource@gmail.com> writes:

> Upon successfully mounting via `sudo mount -t tmpfs -o casefold test /mnt`, I
> cannot find a read-only way to determine whether this option is enabled.
> `mount`, `cat /proc/mounts`, and `cat /proc/mountinfo` only display "rw",
> "relatime", and "inode64", never "casefold" for both with and without
> `-o casefold`. The only way on 7.0.0-34-generic to test I found is
> `mkdir test ; chattr +F test ; echo $?`, but that isn't atomic. Which command
> can determine, without writing anything, whether someone or some program
> specified `-o casefold`? If absent, I suggest adding the missing info to
> `/proc/mounts`, likely `mm/shmem.c:shmem_show_options`.

Yeah.  For device-backed filesystems it is not a mount parameter, but it
is for shmem/ovl and should show in /proc/mounts.

Either way, there are two ways to check:

1) Parse dmesg after the mount. It is easily overwritten and probably
worse than just trying;

2) better way: do FS_IOC_SETFLAGS on the mountpoint (with chattr
+F).  If casefold is supported, you'll get -ENOTEMPTY, otherwise you get
-EOPNOTSUPP.  Since it is the root of the filesystem and a single
syscall, unless its remounted from under you (in which case /proc/mounts
doesn't help either), you are safe.

It is worth mentioning you can check kernel support with
/sys/fs/*/features/casefold.  We probably should add the same for
/sys/fs/ext4/<disk>/.

This fixes /proc/mounts for shmem: 


[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #2: 0001-shmem-Report-casefold-setting-in-mount-options.patch --]
[-- Type: text/x-patch, Size: 1414 bytes --]

From c4b198424fac49e10e32553b223b7615febc6513 Mon Sep 17 00:00:00 2001
From: Gabriel Krisman Bertazi <krisman@suse.de>
Date: Sun, 27 Sep 2026 02:49:26 -0400
Subject: [PATCH] shmem: Report casefold setting in mount options

For not device-backed filesystems, casefolding is a mount option.
Report the current encoding and casefolding flags in /proc/mounts so
userspace can easily check if casefolding is supported.

Reported-by: Daniel Tang <danielzgtg.opensource@gmail.com>
Link: https://lore.kernel.org/linux-fsdevel/5193732.31r3eYUQgx@daniel-desktop3/#R
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
---
 mm/shmem.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/mm/shmem.c b/mm/shmem.c
index 897fa2b61346..2b93fcbf5127 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -4916,6 +4916,17 @@ static int shmem_show_options(struct seq_file *seq, struct dentry *root)
 	if (sbinfo->qlimits.grpquota_ihardlimit)
 		seq_printf(seq, ",grpquota_inode_hardlimit=%lld",
 			   sbinfo->qlimits.grpquota_ihardlimit);
+#endif
+#if IS_ENABLED(CONFIG_UNICODE)
+	if (root->d_sb->s_encoding) {
+		unsigned int version = root->d_sb->s_encoding->version;
+
+		seq_printf(seq, ",casefold=utf8-%u.%u.%u",
+			   unicode_major(version), unicode_minor(version),
+			   unicode_rev(version));
+		if (sb_has_strict_encoding(root->d_sb))
+			seq_puts(seq, ",strict_encoding");
+	}
 #endif
 	return 0;
 }
-- 
2.55.0