[PATCH bpf-next 1/2] bpf: Add kfunc bpf_get_dentry_xattr() to read xattr from dentry

[Song Liu <song@xxxxxxxxxx>]

The primary use case here is to read xattr for directories from BPF
programs. This feature enables tagging all the files in a directory
with a xattr on the directory. More specifically, starting from LSM hook
security_file_open(), we can read xattr of the file's parent directories.

To have referenced access to dentry, a few more kfuncs are added:
  - bpf_file_dentry
  - bpf_dget_parent
  - bpf_dput

Both bpf_file_dentry and bpf_dget_parent take a reference to the dentry
(KF_ACQUIRE), which has to be released by bpf_dput (KF_RELEASE). This
makes sure only trusted pointers (KF_TRUSTED_ARGS) can be used for the
dentry.

Note that, file_dentry() doesn't take reference to the dentry, but
bpf_file_dentry() does.

Signed-off-by: Song Liu <song@xxxxxxxxxx>
---
 kernel/trace/bpf_trace.c | 60 ++++++++++++++++++++++++++++++++++------
 1 file changed, 51 insertions(+), 9 deletions(-)

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index cd098846e251..b8e6eeabb773 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1443,22 +1443,21 @@ late_initcall(bpf_key_sig_kfuncs_init);
 __bpf_kfunc_start_defs();
 
 /**
- * bpf_get_file_xattr - get xattr of a file
- * @file: file to get xattr from
+ * bpf_get_dentry_xattr - get xattr of a dentry
+ * @dentry: dentry to get xattr from
  * @name__str: name of the xattr
  * @value_p: output buffer of the xattr value
  *
- * Get xattr *name__str* of *file* and store the output in *value_ptr*.
+ * Get xattr *name__str* of *dentry* and store the output in *value_ptr*.
  *
  * For security reasons, only *name__str* with prefix "user." is allowed.
  *
  * Return: 0 on success, a negative value on error.
  */
-__bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
-				   struct bpf_dynptr *value_p)
+__bpf_kfunc int bpf_get_dentry_xattr(struct dentry *dentry, const char *name__str,
+				     struct bpf_dynptr *value_p)
 {
 	struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p;
-	struct dentry *dentry;
 	u32 value_len;
 	void *value;
 	int ret;
@@ -1471,20 +1470,63 @@ __bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
 	if (!value)
 		return -EINVAL;
 
-	dentry = file_dentry(file);
 	ret = inode_permission(&nop_mnt_idmap, dentry->d_inode, MAY_READ);
 	if (ret)
 		return ret;
 	return __vfs_getxattr(dentry, dentry->d_inode, name__str, value, value_len);
 }
 
+/**
+ * bpf_get_file_xattr - get xattr of a file
+ * @file: file to get xattr from
+ * @name__str: name of the xattr
+ * @value_p: output buffer of the xattr value
+ *
+ * Get xattr *name__str* of *file* and store the output in *value_ptr*.
+ *
+ * For security reasons, only *name__str* with prefix "user." is allowed.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str,
+				   struct bpf_dynptr *value_p)
+{
+	struct dentry *dentry;
+
+	dentry = file_dentry(file);
+	return bpf_get_dentry_xattr(dentry, name__str, value_p);
+}
+
+__bpf_kfunc struct dentry *bpf_file_dentry(const struct file *file)
+{
+	/* file_dentry() does not hold reference to the dentry. We add a
+	 * dget() here so that we can add KF_ACQUIRE flag to
+	 * bpf_file_dentry().
+	 */
+	return dget(file_dentry(file));
+}
+
+__bpf_kfunc struct dentry *bpf_dget_parent(struct dentry *dentry)
+{
+	return dget_parent(dentry);
+}
+
+__bpf_kfunc void bpf_dput(struct dentry *dentry)
+{
+	return dput(dentry);
+}
+
 __bpf_kfunc_end_defs();
 
 BTF_KFUNCS_START(fs_kfunc_set_ids)
+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_file_dentry, KF_TRUSTED_ARGS | KF_ACQUIRE)
+BTF_ID_FLAGS(func, bpf_dget_parent, KF_TRUSTED_ARGS | KF_ACQUIRE)
+BTF_ID_FLAGS(func, bpf_dput, KF_RELEASE)
 BTF_KFUNCS_END(fs_kfunc_set_ids)
 
-static int bpf_get_file_xattr_filter(const struct bpf_prog *prog, u32 kfunc_id)
+static int fs_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id)
 {
 	if (!btf_id_set8_contains(&fs_kfunc_set_ids, kfunc_id))
 		return 0;
@@ -1496,7 +1538,7 @@ static int bpf_get_file_xattr_filter(const struct bpf_prog *prog, u32 kfunc_id)
 static const struct btf_kfunc_id_set bpf_fs_kfunc_set = {
 	.owner = THIS_MODULE,
 	.set = &fs_kfunc_set_ids,
-	.filter = bpf_get_file_xattr_filter,
+	.filter = fs_kfunc_filter,
 };
 
 static int __init bpf_fs_kfuncs_init(void)
-- 
2.43.0