[PATCH v2 bpf-next 10/14] libbpf: add user_ctx support to bpf_link_create() API

[Andrii Nakryiko <andrii@kernel.org>]

Add ability to specify user_ctx value when creating BPF perf link with
bpf_link_create() low-level API.

Given BPF_LINK_CREATE command is growing and keeps getting new fields that are
specific to the type of BPF_LINK, extend libbpf side of bpf_link_create() API
and corresponding OPTS struct to accomodate such changes. Add extra checks to
prevent using incompatible/unexpected combinations of fields.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/bpf.c             | 32 +++++++++++++++++++++++++-------
 tools/lib/bpf/bpf.h             |  8 +++++++-
 tools/lib/bpf/libbpf_internal.h | 32 ++++++++++++++++++++++----------
 3 files changed, 54 insertions(+), 18 deletions(-)

diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index 86dcac44f32f..8dcbee80ced7 100644
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c
@@ -684,8 +684,13 @@ int bpf_link_create(int prog_fd, int target_fd,
 	iter_info_len = OPTS_GET(opts, iter_info_len, 0);
 	target_btf_id = OPTS_GET(opts, target_btf_id, 0);
 
-	if (iter_info_len && target_btf_id)
-		return libbpf_err(-EINVAL);
+	/* validate we don't have unexpected combinations of non-zero fields */
+	if (iter_info_len || target_btf_id) {
+		if (iter_info_len && target_btf_id)
+			return libbpf_err(-EINVAL);
+		if (!OPTS_ZEROED(opts, target_btf_id))
+			return libbpf_err(-EINVAL);
+	}
 
 	memset(&attr, 0, sizeof(attr));
 	attr.link_create.prog_fd = prog_fd;
@@ -693,14 +698,27 @@ int bpf_link_create(int prog_fd, int target_fd,
 	attr.link_create.attach_type = attach_type;
 	attr.link_create.flags = OPTS_GET(opts, flags, 0);
 
-	if (iter_info_len) {
-		attr.link_create.iter_info =
-			ptr_to_u64(OPTS_GET(opts, iter_info, (void *)0));
-		attr.link_create.iter_info_len = iter_info_len;
-	} else if (target_btf_id) {
+	if (target_btf_id) {
 		attr.link_create.target_btf_id = target_btf_id;
+		goto proceed;
 	}
 
+	switch (attach_type) {
+	case BPF_TRACE_ITER:
+		attr.link_create.iter_info = ptr_to_u64(OPTS_GET(opts, iter_info, (void *)0));
+		attr.link_create.iter_info_len = iter_info_len;
+		break;
+	case BPF_PERF_EVENT:
+		attr.link_create.perf_event.user_ctx = OPTS_GET(opts, perf_event.user_ctx, 0);
+		if (!OPTS_ZEROED(opts, perf_event))
+			return libbpf_err(-EINVAL);
+		break;
+	default:
+		if (!OPTS_ZEROED(opts, flags))
+			return libbpf_err(-EINVAL);
+		break;
+	}
+proceed:
 	fd = sys_bpf(BPF_LINK_CREATE, &attr, sizeof(attr));
 	return libbpf_err_errno(fd);
 }
diff --git a/tools/lib/bpf/bpf.h b/tools/lib/bpf/bpf.h
index 4f758f8f50cd..49d5d08c3832 100644
--- a/tools/lib/bpf/bpf.h
+++ b/tools/lib/bpf/bpf.h
@@ -177,8 +177,14 @@ struct bpf_link_create_opts {
 	union bpf_iter_link_info *iter_info;
 	__u32 iter_info_len;
 	__u32 target_btf_id;
+	union {
+		struct {
+			__u64 user_ctx;
+		} perf_event;
+	};
+	size_t :0;
 };
-#define bpf_link_create_opts__last_field target_btf_id
+#define bpf_link_create_opts__last_field perf_event
 
 LIBBPF_API int bpf_link_create(int prog_fd, int target_fd,
 			       enum bpf_attach_type attach_type,
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 016ca7cb4f8a..e371188c8f87 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -195,6 +195,17 @@ void *libbpf_add_mem(void **data, size_t *cap_cnt, size_t elem_sz,
 		     size_t cur_cnt, size_t max_cnt, size_t add_cnt);
 int libbpf_ensure_mem(void **data, size_t *cap_cnt, size_t elem_sz, size_t need_cnt);
 
+static inline bool libbpf_is_mem_zeroed(const char *p, ssize_t len)
+{
+	while (len > 0) {
+		if (*p)
+			return false;
+		p++;
+		len--;
+	}
+	return true;
+}
+
 static inline bool libbpf_validate_opts(const char *opts,
 					size_t opts_sz, size_t user_sz,
 					const char *type_name)
@@ -203,16 +214,9 @@ static inline bool libbpf_validate_opts(const char *opts,
 		pr_warn("%s size (%zu) is too small\n", type_name, user_sz);
 		return false;
 	}
-	if (user_sz > opts_sz) {
-		size_t i;
-
-		for (i = opts_sz; i < user_sz; i++) {
-			if (opts[i]) {
-				pr_warn("%s has non-zero extra bytes\n",
-					type_name);
-				return false;
-			}
-		}
+	if (!libbpf_is_mem_zeroed(opts + opts_sz, (ssize_t)user_sz - opts_sz)) {
+		pr_warn("%s has non-zero extra bytes\n", type_name);
+		return false;
 	}
 	return true;
 }
@@ -232,6 +236,14 @@ static inline bool libbpf_validate_opts(const char *opts,
 			(opts)->field = value;	\
 	} while (0)
 
+#define OPTS_ZEROED(opts, last_nonzero_field)				      \
+({									      \
+	ssize_t __off = offsetofend(typeof(*(opts)), last_nonzero_field);     \
+	!(opts) || libbpf_is_mem_zeroed((const void *)opts + __off,	      \
+					(opts)->sz - __off);		      \
+})
+
+
 int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz);
 int parse_cpu_mask_file(const char *fcpu, bool **mask, int *mask_sz);
 int libbpf__load_raw_btf(const char *raw_types, size_t types_len,
-- 
2.30.2