[PATCH] proc: allow to mark /proc files permanent outside of fs/proc/

[Alexey Dobriyan <adobriyan@gmail.com>]

From 06e2ff406942fef65b9c397a7f44478dd4b61451 Mon Sep 17 00:00:00 2001
From: Alexey Dobriyan <adobriyan@gmail.com>
Date: Sat, 5 Apr 2025 14:50:10 +0300
Subject: [PATCH 1/1] proc: allow to mark /proc files permanent outside of
 fs/proc/

From: Mateusz Guzik <mjguzik@gmail.com>

Add proc_make_permanent() function to mark PDE as permanent to speed up
open/read/close (one alloc/free and lock/unlock less).

Enable it for built-in code and for compiled-in modules.
This function becomes nop magically in modular code.

Use it on /proc/filesystems to add a user.

		Note, note, note!

If built-in code creates and deletes PDEs dynamically (not in init
hook), then proc_make_permanent() must not be used.

It is intended for simple code:

	static int __init xxx_module_init(void)
	{
		g_pde = proc_create_single();
		proc_make_permanent(g_pde);
		return 0;
	}
	static void __exit xxx_module_exit(void)
	{
		remove_proc_entry(g_pde);
	}

If module is built-in then exit hook never executed and PDE is
permanent so it is OK to mark it as such.

If module is module then rmmod will yank PDE, but proc_make_permanent()
is nop and core /proc code will do everything right.

[adobriyan@gmail.com: unexport function (usual exporting is a bug)]
[adobriyan@gmail.com: rewrite changelog]

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---
 fs/filesystems.c        |  4 +++-
 fs/proc/generic.c       | 12 ++++++++++++
 fs/proc/internal.h      |  3 +++
 include/linux/proc_fs.h | 10 ++++++++++
 4 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/fs/filesystems.c b/fs/filesystems.c
index 58b9067b2391..81dcd0ddadb6 100644
--- a/fs/filesystems.c
+++ b/fs/filesystems.c
@@ -252,7 +252,9 @@ static int filesystems_proc_show(struct seq_file *m, void *v)
 
 static int __init proc_filesystems_init(void)
 {
-	proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
+	struct proc_dir_entry *pde =
+		proc_create_single("filesystems", 0, NULL, filesystems_proc_show);
+	proc_make_permanent(pde);
 	return 0;
 }
 module_init(proc_filesystems_init);
diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index a3e22803cddf..0342600c0172 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -826,3 +826,15 @@ ssize_t proc_simple_write(struct file *f, const char __user *ubuf, size_t size,
 	kfree(buf);
 	return ret == 0 ? size : ret;
 }
+
+/*
+ * Not exported to modules:
+ * modules' /proc files aren't permanent because modules aren't permanent.
+ */
+void impl_proc_make_permanent(struct proc_dir_entry *pde);
+void impl_proc_make_permanent(struct proc_dir_entry *pde)
+{
+	if (pde) {
+		pde_make_permanent(pde);
+	}
+}
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 96122e91c645..885b1cd38020 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -80,8 +80,11 @@ static inline bool pde_is_permanent(const struct proc_dir_entry *pde)
 	return pde->flags & PROC_ENTRY_PERMANENT;
 }
 
+/* This is for builtin code, not even for modules which are compiled in. */
 static inline void pde_make_permanent(struct proc_dir_entry *pde)
 {
+	/* Ensure magic flag does something. */
+	static_assert(PROC_ENTRY_PERMANENT != 0);
 	pde->flags |= PROC_ENTRY_PERMANENT;
 }
 
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index ea62201c74c4..2d59f29b49eb 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -247,4 +247,14 @@ static inline struct pid_namespace *proc_pid_ns(struct super_block *sb)
 
 bool proc_ns_file(const struct file *file);
 
+static inline void proc_make_permanent(struct proc_dir_entry *pde)
+{
+	/* Don't give matches to modules. */
+#if defined CONFIG_PROC_FS && !defined MODULE
+	/* This mess is created by defining "struct proc_dir_entry" elsewhere. */
+	void impl_proc_make_permanent(struct proc_dir_entry *pde);
+	impl_proc_make_permanent(pde);
+#endif
+}
+
 #endif /* _LINUX_PROC_FS_H */
-- 
2.47.0