/proc/mounts and /proc/net are links into /proc/self. Like /proc/self
itself they resolve differently for every process. A mount on one of the
links redirects the lookup for all processes in that mount namespace.

Add proc_self_symlink() for links that point into /proc/self. It creates
the link with PROC_ENTRY_CANT_MOUNT and proc_lookup_de() splices such an
entry with proc_splice_unmountable(). Use it for /proc/mounts and
/proc/net.

Mounting through the links keeps working. Such a mount lands on the
caller's own /proc/<pid>/mounts or /proc/<pid>/net, not on the link.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/proc/generic.c       | 26 ++++++++++++++++++++++++--
 fs/proc/proc_net.c      |  2 +-
 fs/proc/root.c          |  2 +-
 include/linux/proc_fs.h |  5 +++++
 4 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 4c3fcaf68ef4..484d7dc123eb 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -260,6 +260,10 @@ struct dentry *proc_lookup_de(struct inode *dir, struct dentry *dentry,
 		if (de->flags & PROC_ENTRY_FORCE_LOOKUP)
 			return proc_splice_unmountable(inode, dentry,
 						       &proc_net_dentry_ops);
+		/* a mount on a link into /proc/self redirects every process */
+		if (de->flags & PROC_ENTRY_CANT_MOUNT)
+			return proc_splice_unmountable(inode, dentry,
+						       &proc_misc_dentry_ops);
 		return d_splice_alias_ops(inode, dentry,
 					  &proc_misc_dentry_ops);
 	}
@@ -487,8 +491,8 @@ static struct proc_dir_entry *__proc_create(struct proc_dir_entry **parent,
 	return ent;
 }
 
-struct proc_dir_entry *proc_symlink(const char *name,
-		struct proc_dir_entry *parent, const char *dest)
+static struct proc_dir_entry *__proc_symlink(const char *name,
+		struct proc_dir_entry *parent, const char *dest, u8 flags)
 {
 	struct proc_dir_entry *ent;
 
@@ -496,6 +500,7 @@ struct proc_dir_entry *proc_symlink(const char *name,
 			  (S_IFLNK | S_IRUGO | S_IWUGO | S_IXUGO),1);
 
 	if (ent) {
+		ent->flags |= flags;
 		ent->size = strlen(dest);
 		ent->data = kmemdup(dest, ent->size + 1, GFP_KERNEL);
 		if (ent->data) {
@@ -508,8 +513,25 @@ struct proc_dir_entry *proc_symlink(const char *name,
 	}
 	return ent;
 }
+
+struct proc_dir_entry *proc_symlink(const char *name,
+		struct proc_dir_entry *parent, const char *dest)
+{
+	return __proc_symlink(name, parent, dest, 0);
+}
 EXPORT_SYMBOL(proc_symlink);
 
+/*
+ * A link into /proc/self resolves differently for every process, so a
+ * mount on the link itself would redirect all of them.
+ */
+struct proc_dir_entry *proc_self_symlink(const char *name,
+		struct proc_dir_entry *parent, const char *dest)
+{
+	return __proc_symlink(name, parent, dest, PROC_ENTRY_CANT_MOUNT);
+}
+EXPORT_SYMBOL_GPL(proc_self_symlink);
+
 struct proc_dir_entry *_proc_mkdir(const char *name, umode_t mode,
 		struct proc_dir_entry *parent, void *data, bool force_lookup)
 {
diff --git a/fs/proc/proc_net.c b/fs/proc/proc_net.c
index b1f5eafb069a..578b1eb081bf 100644
--- a/fs/proc/proc_net.c
+++ b/fs/proc/proc_net.c
@@ -420,7 +420,7 @@ static struct pernet_operations __net_initdata proc_net_ns_ops = {
 
 int __init proc_net_init(void)
 {
-	proc_symlink("net", NULL, "self/net");
+	proc_self_symlink("net", NULL, "self/net");
 
 	return register_pernet_subsys(&proc_net_ns_ops);
 }
diff --git a/fs/proc/root.c b/fs/proc/root.c
index 7fbbe92bf73a..42fdc6ec50a3 100644
--- a/fs/proc/root.c
+++ b/fs/proc/root.c
@@ -380,7 +380,7 @@ void __init proc_root_init(void)
 	set_proc_pid_nlink();
 	proc_self_init();
 	proc_thread_self_init();
-	proc_symlink("mounts", NULL, "self/mounts");
+	proc_self_symlink("mounts", NULL, "self/mounts");
 
 	proc_net_init();
 	proc_mkdir("fs", NULL);
diff --git a/include/linux/proc_fs.h b/include/linux/proc_fs.h
index 47d7deaeed8f..903fc811efd4 100644
--- a/include/linux/proc_fs.h
+++ b/include/linux/proc_fs.h
@@ -29,6 +29,7 @@ enum {
 	PROC_ENTRY_proc_compat_ioctl	= 1U << 2,
 	PROC_ENTRY_proc_lseek		= 1U << 3,
 
+	PROC_ENTRY_CANT_MOUNT		= 1U << 6,
 	PROC_ENTRY_FORCE_LOOKUP		= 1U << 7,
 };
 
@@ -87,6 +88,8 @@ extern void proc_flush_pid(struct pid *);
 
 extern struct proc_dir_entry *proc_symlink(const char *,
 		struct proc_dir_entry *, const char *);
+struct proc_dir_entry *proc_self_symlink(const char *, struct proc_dir_entry *,
+					 const char *);
 struct proc_dir_entry *_proc_mkdir(const char *, umode_t, struct proc_dir_entry *, void *, bool);
 extern struct proc_dir_entry *proc_mkdir(const char *, struct proc_dir_entry *);
 extern struct proc_dir_entry *proc_mkdir_data(const char *, umode_t,
@@ -179,6 +182,8 @@ static inline void proc_flush_pid(struct pid *pid)
 
 static inline struct proc_dir_entry *proc_symlink(const char *name,
 		struct proc_dir_entry *parent,const char *dest) { return NULL;}
+static inline struct proc_dir_entry *proc_self_symlink(const char *name,
+		struct proc_dir_entry *parent, const char *dest) { return NULL; }
 static inline struct proc_dir_entry *proc_mkdir(const char *name,
 	struct proc_dir_entry *parent) {return NULL;}
 static inline struct proc_dir_entry *proc_create_mount_point(const char *name) { return NULL; }

-- 
2.53.0