[PATCH bpf-next] bpf: add new helper fd2path for mapping a file descriptor to a pathname

[Wenbo Zhang <ethercflow@gmail.com>]

When people want to identify which file system files are being opened,
read, and written to, they can use this helper with file descriptor as
input to achieve this goal. Other pseudo filesystems are also supported.

Signed-off-by: Wenbo Zhang <ethercflow@gmail.com>
---
 include/linux/bpf.h            |  1 +
 include/uapi/linux/bpf.h       | 12 ++++++++++-
 kernel/bpf/core.c              |  1 +
 kernel/bpf/helpers.c           | 39 ++++++++++++++++++++++++++++++++++
 kernel/trace/bpf_trace.c       |  2 ++
 tools/include/uapi/linux/bpf.h | 12 ++++++++++-
 6 files changed, 65 insertions(+), 2 deletions(-)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2c2c29b49845..d73314a7e674 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1082,6 +1082,7 @@ extern const struct bpf_func_proto bpf_get_local_storage_proto;
 extern const struct bpf_func_proto bpf_strtol_proto;
 extern const struct bpf_func_proto bpf_strtoul_proto;
 extern const struct bpf_func_proto bpf_tcp_sock_proto;
+extern const struct bpf_func_proto bpf_fd2path_proto;
 
 /* Shared helpers among cBPF and eBPF. */
 void bpf_user_rnd_init_once(void);
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 4af8b0819a32..fdb37740951f 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -2773,6 +2773,15 @@ union bpf_attr {
  *
  * 		This helper is similar to **bpf_perf_event_output**\ () but
  * 		restricted to raw_tracepoint bpf programs.
+ *
+ * int bpf_fd2path(char *path, u32 size, int fd)
+ *	Description
+ *		Get **file** atrribute from the current task by *fd*, then call
+ *		**d_path** to get it's absolute path and copy it as string into
+ *		*path* of *size*. The **path** also support pseudo filesystems
+ *		(whether or not it can be mounted). The *size* must be strictly
+ *		positive. On success, the helper makes sure that the *path* is
+ *		NUL-terminated. On failure, it is filled with zeroes.
  * 	Return
  * 		0 on success, or a negative error in case of failure.
  */
@@ -2888,7 +2897,8 @@ union bpf_attr {
 	FN(sk_storage_delete),		\
 	FN(send_signal),		\
 	FN(tcp_gen_syncookie),		\
-	FN(skb_output),
+	FN(skb_output),			\
+	FN(fd2path),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index 673f5d40a93e..6b44ed804280 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2079,6 +2079,7 @@ const struct bpf_func_proto bpf_get_current_uid_gid_proto __weak;
 const struct bpf_func_proto bpf_get_current_comm_proto __weak;
 const struct bpf_func_proto bpf_get_current_cgroup_id_proto __weak;
 const struct bpf_func_proto bpf_get_local_storage_proto __weak;
+const struct bpf_func_proto bpf_fd2path_proto __weak;
 
 const struct bpf_func_proto * __weak bpf_get_trace_printk_proto(void)
 {
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 5e28718928ca..0832536c7ddb 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -487,3 +487,42 @@ const struct bpf_func_proto bpf_strtoul_proto = {
 	.arg4_type	= ARG_PTR_TO_LONG,
 };
 #endif
+
+BPF_CALL_3(bpf_fd2path, char *, dst, u32, size, int, fd)
+{
+	struct fd f;
+	int ret;
+	char *p;
+
+	ret = security_locked_down(LOCKDOWN_BPF_READ);
+	if (ret < 0)
+		goto out;
+
+	f = fdget_raw(fd);
+	if (!f.file)
+		goto out;
+
+	p = d_path(&f.file->f_path, dst, size);
+	if (IS_ERR_OR_NULL(p))
+		ret = PTR_ERR(p);
+	else {
+		ret = strlen(p);
+		memmove(dst, p, ret);
+		dst[ret] = 0;
+	}
+
+	if (unlikely(ret < 0))
+out:
+		memset(dst, '0', size);
+
+	return ret;
+}
+
+const struct bpf_func_proto bpf_fd2path_proto = {
+	.func       = bpf_fd2path,
+	.gpl_only   = true,
+	.ret_type   = RET_INTEGER,
+	.arg1_type  = ARG_PTR_TO_UNINIT_MEM,
+	.arg2_type  = ARG_CONST_SIZE,
+	.arg3_type  = ARG_ANYTHING,
+};
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 52f7e9d8c29b..23cc3a955e59 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -735,6 +735,8 @@ tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
 #endif
 	case BPF_FUNC_send_signal:
 		return &bpf_send_signal_proto;
+	case BPF_FUNC_fd2path:
+		return &bpf_fd2path_proto;
 	default:
 		return NULL;
 	}
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 4af8b0819a32..fdb37740951f 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -2773,6 +2773,15 @@ union bpf_attr {
  *
  * 		This helper is similar to **bpf_perf_event_output**\ () but
  * 		restricted to raw_tracepoint bpf programs.
+ *
+ * int bpf_fd2path(char *path, u32 size, int fd)
+ *	Description
+ *		Get **file** atrribute from the current task by *fd*, then call
+ *		**d_path** to get it's absolute path and copy it as string into
+ *		*path* of *size*. The **path** also support pseudo filesystems
+ *		(whether or not it can be mounted). The *size* must be strictly
+ *		positive. On success, the helper makes sure that the *path* is
+ *		NUL-terminated. On failure, it is filled with zeroes.
  * 	Return
  * 		0 on success, or a negative error in case of failure.
  */
@@ -2888,7 +2897,8 @@ union bpf_attr {
 	FN(sk_storage_delete),		\
 	FN(send_signal),		\
 	FN(tcp_gen_syncookie),		\
-	FN(skb_output),
+	FN(skb_output),			\
+	FN(fd2path),
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper
  * function eBPF program intends to call
-- 
2.17.1