locks: print a warning when mount fails due to lack of "mand" support
[ Upstream commit df2474a22c42ce419b67067c52d71da06c385501 ]
Since 9e8925b67a ("locks: Allow disabling mandatory locking at compile
time"), attempts to mount filesystems with "-o mand" will fail.
Unfortunately, there is no other indiciation of the reason for the
failure.
Change how the function is defined for better readability. When
CONFIG_MANDATORY_FILE_LOCKING is disabled, printk a warning when
someone attempts to mount with -o mand.
Also, add a blurb to the mandatory-locking.txt file to explain about
the "mand" option, and the behavior one should expect when it is
disabled.
Reported-by: Jan Kara <jack@suse.cz>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
diff --git a/Documentation/filesystems/mandatory-locking.txt b/Documentation/filesystems/mandatory-locking.txt
index 0979d1d..a251ca3 100644
--- a/Documentation/filesystems/mandatory-locking.txt
+++ b/Documentation/filesystems/mandatory-locking.txt
@@ -169,3 +169,13 @@
permissions (remove the setgid bit) before trying to read or write to it.
Of course, that might be a bit tricky if the system is hung :-(
+7. The "mand" mount option
+--------------------------
+Mandatory locking is disabled on all filesystems by default, and must be
+administratively enabled by mounting with "-o mand". That mount option
+is only allowed if the mounting task has the CAP_SYS_ADMIN capability.
+
+Since kernel v4.5, it is possible to disable mandatory locking
+altogether by setting CONFIG_MANDATORY_FILE_LOCKING to "n". A kernel
+with this disabled will reject attempts to mount filesystems with the
+"mand" mount option with the error status EPERM.
diff --git a/fs/namespace.c b/fs/namespace.c
index 9f2390c..68457fa 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1669,13 +1669,18 @@
return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
}
+#ifdef CONFIG_MANDATORY_FILE_LOCKING
static inline bool may_mandlock(void)
{
-#ifndef CONFIG_MANDATORY_FILE_LOCKING
- return false;
-#endif
return capable(CAP_SYS_ADMIN);
}
+#else
+static inline bool may_mandlock(void)
+{
+ pr_warn("VFS: \"mand\" mount option not supported");
+ return false;
+}
+#endif
/*
* Now umount can handle mount points as well as block devices.