[PATCH RFC 1/3] fs: Introduce task path helpers

[Chen Linxuan via B4 Relay <devnull+me.black-desk.cn@kernel.org>]

From: Chen Linxuan <me@black-desk.cn>

Introduce helpers that acquire a referenced struct path for a task's
executable, root, and working directory. Reuse them for procfs task
links and AppArmor executable-path handling instead of duplicating file
and path reference handling at each call site.

Assisted-by: codex:glm-5.3
Signed-off-by: Chen Linxuan <me@black-desk.cn>
---
 fs/fs_struct.c            | 44 ++++++++++++++++++++++++++++++++++++++++++++
 fs/proc/base.c            | 34 ++--------------------------------
 include/linux/fs_struct.h |  3 +++
 include/linux/mm.h        |  1 +
 kernel/fork.c             | 21 +++++++++++++++++++++
 security/apparmor/task.c  | 12 +++---------
 6 files changed, 74 insertions(+), 41 deletions(-)

diff --git a/fs/fs_struct.c b/fs/fs_struct.c
index 34699f3b6f88..5c772896260a 100644
--- a/fs/fs_struct.c
+++ b/fs/fs_struct.c
@@ -10,6 +10,50 @@
 #include "internal.h"
 #include "mount.h"
 
+/**
+ * get_task_root - acquire a reference to the task's root path
+ * @task: The task.
+ * @root: The task's root path.
+ *
+ * Returns 0 if the task has a root path, or -ENOENT if it does not. The
+ * caller must release the path through path_put() on success.
+ */
+int get_task_root(struct task_struct *task, struct path *root)
+{
+	int ret = -ENOENT;
+
+	task_lock(task);
+	if (task->real_fs) {
+		get_fs_root(task->real_fs, root);
+		ret = 0;
+	}
+	task_unlock(task);
+
+	return ret;
+}
+
+/**
+ * get_task_pwd - acquire a reference to the task's working directory
+ * @task: The task.
+ * @pwd: The task's working directory.
+ *
+ * Returns 0 if the task has a working directory, or -ENOENT if it does not.
+ * The caller must release the path through path_put() on success.
+ */
+int get_task_pwd(struct task_struct *task, struct path *pwd)
+{
+	int ret = -ENOENT;
+
+	task_lock(task);
+	if (task->real_fs) {
+		get_fs_pwd(task->real_fs, pwd);
+		ret = 0;
+	}
+	task_unlock(task);
+
+	return ret;
+}
+
 /*
  * Replace the fs->{rootmnt,root} with {mnt,dentry}. Put the old values.
  * It can block.
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 6a39de424f62..7e2c0538323c 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -206,31 +206,10 @@ static unsigned int __init pid_entry_nlink(const struct pid_entry *entries,
 	return count;
 }
 
-static int get_task_root(struct task_struct *task, struct path *root)
-{
-	int result = -ENOENT;
-
-	task_lock(task);
-	if (task->real_fs) {
-		get_fs_root(task->real_fs, root);
-		result = 0;
-	}
-	task_unlock(task);
-	return result;
-}
-
 static int proc_cwd_link(struct dentry *dentry, struct path *path,
 			 struct task_struct *task)
 {
-	int result = -ENOENT;
-
-	task_lock(task);
-	if (task->real_fs) {
-		get_fs_pwd(task->real_fs, path);
-		result = 0;
-	}
-	task_unlock(task);
-	return result;
+	return get_task_pwd(task, path);
 }
 
 static int proc_root_link(struct dentry *dentry, struct path *path,
@@ -1761,16 +1740,7 @@ static const struct file_operations proc_pid_set_comm_operations = {
 static int proc_exe_link(struct dentry *dentry, struct path *exe_path,
 			 struct task_struct *task)
 {
-	struct file *exe_file;
-
-	exe_file = get_task_exe_file(task);
-	if (exe_file) {
-		*exe_path = exe_file->f_path;
-		path_get(&exe_file->f_path);
-		fput(exe_file);
-		return 0;
-	} else
-		return -ENOENT;
+	return get_task_exe_path(task, exe_path);
 }
 
 static int call_proc_get_link(struct dentry *dentry, struct inode *inode, struct path *path_out)
diff --git a/include/linux/fs_struct.h b/include/linux/fs_struct.h
index 97eef8d3863d..fb725707754d 100644
--- a/include/linux/fs_struct.h
+++ b/include/linux/fs_struct.h
@@ -42,6 +42,9 @@ static inline void get_fs_pwd(struct fs_struct *fs, struct path *pwd)
 	read_sequnlock_excl(&fs->seq);
 }
 
+int get_task_root(struct task_struct *task, struct path *root);
+int get_task_pwd(struct task_struct *task, struct path *pwd);
+
 struct fs_struct *switch_fs_struct(struct fs_struct *new_fs);
 
 extern bool current_chrooted(void);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 485df9c2dbdd..7610eb579b41 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4124,6 +4124,7 @@ extern int set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file);
 extern int replace_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file);
 extern struct file *get_mm_exe_file(struct mm_struct *mm);
 extern struct file *get_task_exe_file(struct task_struct *task);
+int get_task_exe_path(struct task_struct *task, struct path *exe_path);
 
 extern void vm_stat_account(struct mm_struct *, vm_flags_t, long npages);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 1e68404bd773..16aa4c82b9c7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1371,6 +1371,27 @@ struct file *get_task_exe_file(struct task_struct *task)
 	return exe_file;
 }
 
+/**
+ * get_task_exe_path - acquire a reference to the task's executable path
+ * @task: The task.
+ * @exe_path: The task's executable path.
+ *
+ * Returns 0 if the task has an executable path, or -ENOENT if it does not.
+ * The caller must release the path through path_put() on success.
+ */
+int get_task_exe_path(struct task_struct *task, struct path *exe_path)
+{
+	struct file *exe_file = get_task_exe_file(task);
+
+	if (!exe_file)
+		return -ENOENT;
+
+	*exe_path = exe_file->f_path;
+	path_get(exe_path);
+	fput(exe_file);
+	return 0;
+}
+
 /**
  * get_task_mm - acquire a reference to the task's mm
  * @task: The task.
diff --git a/security/apparmor/task.c b/security/apparmor/task.c
index b9fb3738124e..b4a4019d77c7 100644
--- a/security/apparmor/task.c
+++ b/security/apparmor/task.c
@@ -13,6 +13,7 @@
  */
 
 #include <linux/gfp.h>
+#include <linux/mm.h>
 #include <linux/ptrace.h>
 
 #include "include/path.h"
@@ -303,22 +304,15 @@ int aa_may_ptrace(const struct cred *tracer_cred, struct aa_label *tracer,
 
 static const char *get_current_exe_path(char *buffer, int buffer_size)
 {
-	struct file *exe_file;
-	struct path p;
+	struct path p __free(path_put) = {};
 	const char *path_str;
 
-	exe_file = get_task_exe_file(current);
-	if (!exe_file)
+	if (get_task_exe_path(current, &p))
 		return ERR_PTR(-ENOENT);
-	p = exe_file->f_path;
-	path_get(&p);
 
 	if (aa_path_name(&p, FLAG_VIEW_SUBNS, buffer, &path_str, NULL, NULL))
 		path_str = ERR_PTR(-ENOMEM);
 
-	fput(exe_file);
-	path_put(&p);
-
 	return path_str;
 }
 

-- 
2.53.0