libbpf: add bpf_object__open_{file, mem} w/ extensible opts

Add new set of bpf_object__open APIs using new approach to optional
parameters extensibility allowing simpler ABI compatibility approach.

This patch demonstrates an approach to implementing libbpf APIs that
makes it easy to extend existing APIs with extra optional parameters in
such a way, that ABI compatibility is preserved without having to do
symbol versioning and generating lots of boilerplate code to handle it.
To facilitate succinct code for working with options, add OPTS_VALID,
OPTS_HAS, and OPTS_GET macros that hide all the NULL, size, and zero
checks.

Additionally, newly added libbpf APIs are encouraged to follow similar
pattern of having all mandatory parameters as formal function parameters
and always have optional (NULL-able) xxx_opts struct, which should
always have real struct size as a first field and the rest would be
optional parameters added over time, which tune the behavior of existing
API, if specified by user.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 32 insertions, 0 deletions

diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 2e83a34f8c79..f51444fc7eb7 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -47,6 +47,38 @@ do {				\
 #define pr_info(fmt, ...)	__pr(LIBBPF_INFO, fmt, ##__VA_ARGS__)
 #define pr_debug(fmt, ...)	__pr(LIBBPF_DEBUG, fmt, ##__VA_ARGS__)
 
+static inline bool libbpf_validate_opts(const char *opts,
+					size_t opts_sz, size_t user_sz,
+					const char *type_name)
+{
+	if (user_sz < sizeof(size_t)) {
+		pr_warning("%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_warning("%s has non-zero extra bytes",
+					   type_name);
+				return false;
+			}
+		}
+	}
+	return true;
+}
+
+#define OPTS_VALID(opts, type)						      \
+	(!(opts) || libbpf_validate_opts((const char *)opts,		      \
+					 offsetofend(struct type,	      \
+						     type##__last_field),     \
+					 (opts)->sz, #type))
+#define OPTS_HAS(opts, field) \
+	((opts) && opts->sz >= offsetofend(typeof(*(opts)), field))
+#define OPTS_GET(opts, field, fallback_value) \
+	(OPTS_HAS(opts, field) ? (opts)->field : fallback_value)
+
 int libbpf__load_raw_btf(const char *raw_types, size_t types_len,
 			 const char *str_sec, size_t str_len);