[PATCH bpf-next 1/5] bpf: Add kfunc bpf_get_file_xattr

[Song Liu <song@kernel.org>]

This kfunc can be used to read xattr of a file.

Since vfs_getxattr() requires null-terminated string as input "name", a new
helper bpf_dynptr_is_string() is added to check the input before calling
vfs_getxattr().

Signed-off-by: Song Liu <song@kernel.org>
---
 include/linux/bpf.h      | 12 +++++++++++
 kernel/trace/bpf_trace.c | 44 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 61bde4520f5c..f14fae45e13d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -2472,6 +2472,13 @@ static inline bool has_current_bpf_ctx(void)
 	return !!current->bpf_ctx;
 }
 
+static inline bool bpf_dynptr_is_string(struct bpf_dynptr_kern *ptr)
+{
+	char *str = ptr->data;
+
+	return str[__bpf_dynptr_size(ptr) - 1] == '\0';
+}
+
 void notrace bpf_prog_inc_misses_counter(struct bpf_prog *prog);
 
 void bpf_dynptr_init(struct bpf_dynptr_kern *ptr, void *data,
@@ -2708,6 +2715,11 @@ static inline bool has_current_bpf_ctx(void)
 	return false;
 }
 
+static inline bool bpf_dynptr_is_string(struct bpf_dynptr_kern *ptr)
+{
+	return false;
+}
+
 static inline void bpf_prog_inc_misses_counter(struct bpf_prog *prog)
 {
 }
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index df697c74d519..946268574e05 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -24,6 +24,7 @@
 #include <linux/key.h>
 #include <linux/verification.h>
 #include <linux/namei.h>
+#include <linux/fileattr.h>
 
 #include <net/bpf_sk_storage.h>
 
@@ -1429,6 +1430,49 @@ static int __init bpf_key_sig_kfuncs_init(void)
 late_initcall(bpf_key_sig_kfuncs_init);
 #endif /* CONFIG_KEYS */
 
+/* filesystem kfuncs */
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+		  "kfuncs which will be used in BPF programs");
+
+/**
+ * bpf_get_file_xattr - get xattr of a file
+ * @name_ptr: name of the xattr
+ * @value_ptr: output buffer of the xattr value
+ *
+ * Get xattr *name_ptr* of *file* and store the output in *value_ptr*.
+ *
+ * Return: 0 on success, a negative value on error.
+ */
+__bpf_kfunc int bpf_get_file_xattr(struct file *file, struct bpf_dynptr_kern *name_ptr,
+				   struct bpf_dynptr_kern *value_ptr)
+{
+	if (!bpf_dynptr_is_string(name_ptr))
+		return -EINVAL;
+
+	return vfs_getxattr(mnt_idmap(file->f_path.mnt), file_dentry(file), name_ptr->data,
+			    value_ptr->data, __bpf_dynptr_size(value_ptr));
+}
+
+__diag_pop();
+
+BTF_SET8_START(fs_kfunc_set)
+BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE)
+BTF_SET8_END(fs_kfunc_set)
+
+const struct btf_kfunc_id_set bpf_fs_kfunc_set = {
+	.owner = THIS_MODULE,
+	.set = &fs_kfunc_set,
+};
+
+static int __init bpf_fs_kfuncs_init(void)
+{
+	return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING,
+					 &bpf_fs_kfunc_set);
+}
+
+late_initcall(bpf_fs_kfuncs_init);
+
 static const struct bpf_func_proto *
 bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 {
-- 
2.34.1