Merge patch series "CONFIG_DEBUG_VFS at last"

Mateusz Guzik <mjguzik@gmail.com> says:

This adds a super basic version just to get the mechanism going, along
with sample usage.

The macro set is incomplete (e.g., lack of locking macros) and
dump_inode routine fails to dump any state yet, to be implemented(tm).

I think despite the primitive state this is complete enough to start
sprinkling asserts as necessary.

* patches from https://lore.kernel.org/r/20250209185523.745956-1-mjguzik@gmail.com:
  vfs: use the new debug macros in inode_set_cached_link()
  vfs: catch invalid modes in may_open()
  vfs: add initial support for CONFIG_DEBUG_VFS

Link: https://lore.kernel.org/r/20250209185523.745956-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/inode.c

@@ -2953,3 +2953,18 @@ umode_t mode_strip_sgid(struct mnt_idmap *idmap,
 	return mode & ~S_ISGID;
 }
 EXPORT_SYMBOL(mode_strip_sgid);
+
+#ifdef CONFIG_DEBUG_VFS
+/*
+ * Dump an inode.
+ *
+ * TODO: add a proper inode dumping routine, this is a stub to get debug off the
+ * ground.
+ */
+void dump_inode(struct inode *inode, const char *reason)
+{
+       pr_warn("%s encountered for inode %px", reason, inode);
+}
+
+EXPORT_SYMBOL(dump_inode);
+#endif

fs/namei.c

@@ -3415,6 +3415,8 @@ static int may_open(struct mnt_idmap *idmap, const struct path *path,
 		if ((acc_mode & MAY_EXEC) && path_noexec(path))
 			return -EACCES;
 		break;
+	default:
+		VFS_BUG_ON_INODE(1, inode);
 	}
 
 	error = inode_permission(idmap, inode, MAY_OPEN | acc_mode);

include/linux/fs.h

@@ -2,6 +2,7 @@
 #ifndef _LINUX_FS_H
 #define _LINUX_FS_H
 
+#include <linux/vfsdebug.h>
 #include <linux/linkage.h>
 #include <linux/wait_bit.h>
 #include <linux/kdev_t.h>
@@ -791,6 +792,8 @@ struct inode {
 
 static inline void inode_set_cached_link(struct inode *inode, char *link, int linklen)
 {
+	VFS_WARN_ON_INODE(strlen(link) != linklen, inode);
+	VFS_WARN_ON_INODE(inode->i_opflags & IOP_CACHED_LINK, inode);
 	inode->i_link = link;
 	inode->i_linklen = linklen;
 	inode->i_opflags |= IOP_CACHED_LINK;

include/linux/vfsdebug.h

@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef LINUX_VFS_DEBUG_H
+#define LINUX_VFS_DEBUG_H 1
+
+#include <linux/bug.h>
+
+struct inode;
+
+#ifdef CONFIG_DEBUG_VFS
+void dump_inode(struct inode *inode, const char *reason);
+
+#define VFS_BUG_ON(cond) BUG_ON(cond)
+#define VFS_WARN_ON(cond) (void)WARN_ON(cond)
+#define VFS_WARN_ON_ONCE(cond) (void)WARN_ON_ONCE(cond)
+#define VFS_WARN_ONCE(cond, format...) (void)WARN_ONCE(cond, format)
+#define VFS_WARN(cond, format...) (void)WARN(cond, format)
+
+#define VFS_BUG_ON_INODE(cond, inode)		({			\
+	if (unlikely(!!(cond))) {					\
+		dump_inode(inode, "VFS_BUG_ON_INODE(" #cond")");\
+		BUG_ON(1);						\
+	}								\
+})
+
+#define VFS_WARN_ON_INODE(cond, inode)		({			\
+	int __ret_warn = !!(cond);					\
+									\
+	if (unlikely(__ret_warn)) {					\
+		dump_inode(inode, "VFS_WARN_ON_INODE(" #cond")");\
+		WARN_ON(1);						\
+	}								\
+	unlikely(__ret_warn);						\
+})
+#else
+#define VFS_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)
+#define VFS_WARN_ON(cond) BUILD_BUG_ON_INVALID(cond)
+#define VFS_WARN_ON_ONCE(cond) BUILD_BUG_ON_INVALID(cond)
+#define VFS_WARN_ONCE(cond, format...) BUILD_BUG_ON_INVALID(cond)
+#define VFS_WARN(cond, format...) BUILD_BUG_ON_INVALID(cond)
+
+#define VFS_BUG_ON_INODE(cond, inode) VFS_BUG_ON(cond)
+#define VFS_WARN_ON_INODE(cond, inode)  BUILD_BUG_ON_INVALID(cond)
+#endif /* CONFIG_DEBUG_VFS */
+
+#endif

lib/Kconfig.debug

@@ -808,6 +808,15 @@ config ARCH_HAS_DEBUG_VM_PGTABLE
 	  An architecture should select this when it can successfully
 	  build and run DEBUG_VM_PGTABLE.
 
+config DEBUG_VFS
+	bool "Debug VFS"
+	depends on DEBUG_KERNEL
+	help
+	  Enable this to turn on extended checks in the VFS layer that may impact
+	  performance.
+
+	  If unsure, say N.
+
 config DEBUG_VM_IRQSOFF
 	def_bool DEBUG_VM && !PREEMPT_RT