[bpf-next,08/11] bpf: add acquire/release based BPF kfuncs for fs_struct's paths

Commit Message

Matt Bobrowski Feb. 20, 2024, 9:28 a.m. UTC

Add the ability to obtain a reference on a common set of path's that
are associated with a task_struct's fs_struct. Both fs_struct's root
and pwd paths are commonly operated on in BPF LSM programs and at
times handed off to BPF helpers and such. There needs to be a
mechanism that supports BPF LSM programs to obtain stable handle to
such in-kernel structures.

We provide that mechanism through the introduction of the following
new BPF kfuncs:

struct path *bpf_get_task_fs_root(struct task_struct *task);
struct path *bpf_get_task_fs_pwd(struct task_struct *task);
void bpf_put_path(struct path *path);

Signed-off-by: Matt Bobrowski <[e-mail]>
---
 kernel/trace/bpf_trace.c | 83 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

Patch

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index fbb252ad1d40..2bb7766337ca 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -10,6 +10,7 @@ 
 #include <linux/bpf_perf_event.h>
 #include <linux/btf.h>
 #include <linux/filter.h>
+#include <linux/fs_struct.h>
 #include <linux/uaccess.h>
 #include <linux/ctype.h>
 #include <linux/kprobes.h>
@@ -1557,6 +1558,83 @@  __bpf_kfunc void bpf_put_file(struct file *f)
 	fput(f);
 }
 
+/**
+ * bpf_get_task_fs_root - get a reference on the fs_struct's root for the
+ * 			  supplied task_struct
+ * @task: task_struct of which the fs_struct's root path to get a reference on
+ *
+ * Get a reference on the root path associated with the supplied *task*. The
+ * referenced path retruned from this kfunc must be released using
+ * bpf_put_path().
+ *
+ * Return: A referenced path pointer to the fs_struct's root of the supplied
+ * *task*, or NULL.
+ */
+__bpf_kfunc struct path *bpf_get_task_fs_root(struct task_struct *task)
+{
+	struct path *root;
+	struct fs_struct *fs;
+
+	task_lock(task);
+	fs = task->fs;
+	if (unlikely(fs)) {
+		task_unlock(task);
+		return NULL;
+	}
+
+	spin_lock(&fs->lock);
+	root = &fs->root;
+	path_get(root);
+	spin_unlock(&fs->lock);
+	task_unlock(task);
+
+	return root;
+}
+
+/**
+ * bpf_get_task_fs_pwd - get a reference on the fs_struct's pwd for the supplied
+ * 			 task_struct
+ * @task: task_struct of which the fs_struct's pwd path to get a reference on
+ *
+ * Get a reference on the pwd path associated with the supplied *task*. A
+ * referenced path returned from this kfunc must be released using
+ * bpf_put_path().
+ *
+ * Return: A referenced path pointer to the fs_struct's pwd of the supplied
+ * *task*, or NULL.
+ */
+__bpf_kfunc struct path *bpf_get_task_fs_pwd(struct task_struct *task)
+{
+	struct path *pwd;
+	struct fs_struct *fs;
+
+	task_lock(task);
+	fs = task->fs;
+	if (unlikely(fs)) {
+		task_unlock(task);
+		return NULL;
+	}
+
+	spin_lock(&fs->lock);
+	pwd = &fs->pwd;
+	path_get(pwd);
+	spin_unlock(&fs->lock);
+	task_unlock(task);
+
+	return pwd;
+}
+
+/**
+ * bpf_put_path - put the reference on the supplied path
+ * @path: path of which to put a reference on
+ *
+ * Put a reference on the supplied *path*.
+  */
+__bpf_kfunc void bpf_put_path(struct path *path)
+{
+	path_put(path);
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(lsm_kfunc_set_ids)
@@ -1568,6 +1646,11 @@  BTF_ID_FLAGS(func, bpf_get_task_exe_file,
 BTF_ID_FLAGS(func, bpf_get_mm_exe_file,
 	     KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE | KF_SLEEPABLE)
+BTF_ID_FLAGS(func, bpf_get_task_fs_root,
+	     KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_get_task_fs_pwd,
+	     KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_put_path, KF_RELEASE | KF_SLEEPABLE)
 BTF_KFUNCS_END(lsm_kfunc_set_ids)
 
 static int bpf_lsm_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id)