Merge patch series "vfs: output mount_too_revealing() errors to fscontext"

Aleksa Sarai <cyphar@cyphar.com> says:

It makes little sense for fsmount() to output the warning message when
mount_too_revealing() is violated to kmsg. Instead, the warning should
be output (with a "VFS" prefix) to the fscontext log. In addition,
include the same log message for mount_too_revealing() when doing a
regular mount for consistency.

With the newest fsopen()-based mount(8) from util-linux, the error
messages now look like

  # mount -t proc proc /tmp
  mount: /tmp: fsmount() failed: VFS: Mount too revealing.
	 dmesg(1) may have more information after failed mount system call.

which could finally result in mount_too_revealing() errors being easier
for users to detect and understand.

* patches from https://lore.kernel.org/20250806-errorfc-mount-too-revealing-v2-0-534b9b4d45bb@cyphar.com:
  vfs: output mount_too_revealing() errors to fscontext
  fscontext: add custom-prefix log helpers

Link: https://lore.kernel.org/20250806-errorfc-mount-too-revealing-v2-0-534b9b4d45bb@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

2 files changed, 18 insertions, 6 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index ddfd4457d338..86d12f88b688 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3724,8 +3724,10 @@ static int do_new_mount_fc(struct fs_context *fc, struct path *mountpoint,
 	int error;
 
 	error = security_sb_kern_mount(sb);
-	if (!error && mount_too_revealing(sb, &mnt_flags))
+	if (!error && mount_too_revealing(sb, &mnt_flags)) {
+		errorfcp(fc, "VFS", "Mount too revealing");
 		error = -EPERM;
+	}
 
 	if (unlikely(error)) {
 		fc_drop_locked(fc);
@@ -4441,7 +4443,7 @@ SYSCALL_DEFINE3(fsmount, int, fs_fd, unsigned int, flags,
 
 	ret = -EPERM;
 	if (mount_too_revealing(fc->root->d_sb, &mnt_flags)) {
-		pr_warn("VFS: Mount too revealing\n");
+		errorfcp(fc, "VFS", "Mount too revealing");
 		goto err_unlock;
 	}
 
diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
index 7773eb870039..671f031be173 100644
--- a/include/linux/fs_context.h
+++ b/include/linux/fs_context.h
@@ -186,10 +186,12 @@ struct fc_log {
 extern __attribute__((format(printf, 4, 5)))
 void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt, ...);
 
-#define __logfc(fc, l, fmt, ...) logfc((fc)->log.log, NULL, \
-					l, fmt, ## __VA_ARGS__)
-#define __plog(p, l, fmt, ...) logfc((p)->log, (p)->prefix, \
-					l, fmt, ## __VA_ARGS__)
+#define __logfc(fc, l, fmt, ...) \
+	logfc((fc)->log.log, NULL, (l), (fmt), ## __VA_ARGS__)
+#define __plogp(p, prefix, l, fmt, ...) \
+	logfc((p)->log, (prefix), (l), (fmt), ## __VA_ARGS__)
+#define __plog(p, l, fmt, ...) __plogp(p, (p)->prefix, l, fmt, ## __VA_ARGS__)
+
 /**
  * infof - Store supplementary informational message
  * @fc: The context in which to log the informational message
@@ -201,6 +203,8 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
 #define infof(fc, fmt, ...) __logfc(fc, 'i', fmt, ## __VA_ARGS__)
 #define info_plog(p, fmt, ...) __plog(p, 'i', fmt, ## __VA_ARGS__)
 #define infofc(fc, fmt, ...) __plog((&(fc)->log), 'i', fmt, ## __VA_ARGS__)
+#define infofcp(fc, prefix, fmt, ...) \
+	__plogp((&(fc)->log), prefix, 'i', fmt, ## __VA_ARGS__)
 
 /**
  * warnf - Store supplementary warning message
@@ -213,6 +217,8 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
 #define warnf(fc, fmt, ...) __logfc(fc, 'w', fmt, ## __VA_ARGS__)
 #define warn_plog(p, fmt, ...) __plog(p, 'w', fmt, ## __VA_ARGS__)
 #define warnfc(fc, fmt, ...) __plog((&(fc)->log), 'w', fmt, ## __VA_ARGS__)
+#define warnfcp(fc, prefix, fmt, ...) \
+	__plogp((&(fc)->log), prefix, 'w', fmt, ## __VA_ARGS__)
 
 /**
  * errorf - Store supplementary error message
@@ -225,6 +231,8 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
 #define errorf(fc, fmt, ...) __logfc(fc, 'e', fmt, ## __VA_ARGS__)
 #define error_plog(p, fmt, ...) __plog(p, 'e', fmt, ## __VA_ARGS__)
 #define errorfc(fc, fmt, ...) __plog((&(fc)->log), 'e', fmt, ## __VA_ARGS__)
+#define errorfcp(fc, prefix, fmt, ...) \
+	__plogp((&(fc)->log), prefix, 'e', fmt, ## __VA_ARGS__)
 
 /**
  * invalf - Store supplementary invalid argument error message
@@ -237,5 +245,7 @@ void logfc(struct fc_log *log, const char *prefix, char level, const char *fmt,
 #define invalf(fc, fmt, ...) (errorf(fc, fmt, ## __VA_ARGS__), -EINVAL)
 #define inval_plog(p, fmt, ...) (error_plog(p, fmt, ## __VA_ARGS__), -EINVAL)
 #define invalfc(fc, fmt, ...) (errorfc(fc, fmt, ## __VA_ARGS__), -EINVAL)
+#define invalfcp(fc, prefix, fmt, ...) \
+	(errorfcp(fc, prefix, fmt, ## __VA_ARGS__), -EINVAL)
 
 #endif /* _LINUX_FS_CONTEXT_H */