[PATCH bpf-next 2/3] bpf: Add bpf_kern_path and bpf_path_put kfuncs

[Song Liu]

Add two new kfuncs to fs/bpf_fs_kfuncs.c that wrap kern_path() for use
by BPF LSM programs:

bpf_kern_path():
- Resolves a pathname string to a struct path
- Allocates memory for the path structure
- Returns NULL on error or if the path doesn't exist
- Marked with KF_ACQUIRE | KF_SLEEPABLE | KF_RET_NULL

bpf_path_put():
- Releases the path reference and frees the allocated memory
- Marked with KF_RELEASE to enforce acquire/release semantics

These kfuncs enable BPF LSM programs to resolve pathnames provided by
hook arguments (e.g., dev_name from sb_mount) and validate or inspect
the resolved paths. The verifier enforces proper resource management
through acquire/release tracking.

Example usage:
  struct path *p = bpf_kern_path("/etc/passwd", LOOKUP_FOLLOW);
  if (p) {
      // Use the path...
      bpf_path_put(p);  // Must release
  }

Signed-off-by: Song Liu <song at kernel.org>
---
 fs/bpf_fs_kfuncs.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
index 5ace2511fec5..977f8dcbc208 100644
--- a/fs/bpf_fs_kfuncs.c
+++ b/fs/bpf_fs_kfuncs.c
@@ -11,6 +11,7 @@
 #include <linux/file.h>
 #include <linux/kernfs.h>
 #include <linux/mm.h>
+#include <linux/namei.h>
 #include <linux/xattr.h>
 
 __bpf_kfunc_start_defs();
@@ -96,6 +97,61 @@ __bpf_kfunc int bpf_path_d_path(const struct path *path, char *buf, size_t buf__
 	return len;
 }
 
+/**
+ * bpf_kern_path - resolve a pathname to a struct path
+ * @pathname__str: pathname to resolve
+ * @flags: lookup flags (e.g., LOOKUP_FOLLOW)
+ *
+ * Resolve the pathname for the supplied *pathname__str* and return a pointer
+ * to a struct path. This is a wrapper around kern_path() that allocates and
+ * returns a struct path pointer on success.
+ *
+ * The returned struct path pointer must be released using bpf_path_put().
+ * Failing to call bpf_path_put() on the returned struct path pointer will
+ * result in the BPF program being rejected by the BPF verifier.
+ *
+ * This BPF kfunc may only be called from BPF LSM programs.
+ *
+ * Return: A pointer to an allocated struct path on success, NULL on error.
+ */
+__bpf_kfunc struct path *bpf_kern_path(const char *pathname__str, unsigned int flags)
+{
+	struct path *path;
+	int ret;
+
+	path = kmalloc(sizeof(*path), GFP_KERNEL);
+	if (!path)
+		return NULL;
+
+	ret = kern_path(pathname__str, flags, path);
+	if (ret) {
+		kfree(path);
+		return NULL;
+	}
+
+	return path;
+}
+
+/**
+ * bpf_path_put - release a struct path reference
+ * @path: struct path pointer to release
+ *
+ * Release the struct path pointer that was acquired by bpf_kern_path().
+ * This BPF kfunc calls path_put() on the supplied *path* and then frees
+ * the allocated memory.
+ *
+ * Only struct path pointers acquired by bpf_kern_path() may be passed to
+ * this BPF kfunc. Attempting to pass any other pointer will result in the
+ * BPF program being rejected by the BPF verifier.
+ *
+ * This BPF kfunc may only be called from BPF LSM programs.
+ */
+__bpf_kfunc void bpf_path_put(struct path *path)
+{
+	path_put(path);
+	kfree(path);
+}
+
 static bool match_security_bpf_prefix(const char *name__str)
 {
 	return !strncmp(name__str, XATTR_NAME_BPF_LSM, XATTR_NAME_BPF_LSM_LEN);
@@ -363,6 +419,8 @@ BTF_ID_FLAGS(func, bpf_get_task_exe_file,
 	     KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_put_file, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_path_d_path, KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_kern_path, KF_TRUSTED_ARGS | KF_ACQUIRE | KF_SLEEPABLE | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_path_put, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_get_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_set_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS)
-- 
2.47.3