libbpf: Wire through log_true_size returned from kernel for BPF_PROG_LOAD

Add output-only log_true_size field to bpf_prog_load_opts to return
bpf_attr->log_true_size value back from bpf() syscall.

Note, that we have to drop const modifier from opts in bpf_prog_load().
This could potentially cause compilation error for some users. But
the usual practice is to define bpf_prog_load_ops
as a local variable next to bpf_prog_load() call and pass pointer to it,
so const vs non-const makes no difference and won't even come up in most
(if not all) cases.

There are no runtime and ABI backwards/forward compatibility issues at all.
If user provides old struct bpf_prog_load_opts, libbpf won't set new
fields. If old libbpf is provided new bpf_prog_load_opts, nothing will
happen either as old libbpf doesn't yet know about this new field.

Adding a new variant of bpf_prog_load() just for this seems like a big
and unnecessary overkill. As a corroborating evidence is the fact that
entire selftests/bpf code base required not adjustment whatsoever.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230406234205.323208-16-andrii@kernel.org

tools/lib/bpf/bpf.c

@@ -230,9 +230,9 @@ alloc_zero_tailing_info(const void *orecord, __u32 cnt,
 int bpf_prog_load(enum bpf_prog_type prog_type,
 		  const char *prog_name, const char *license,
 		  const struct bpf_insn *insns, size_t insn_cnt,
-		  const struct bpf_prog_load_opts *opts)
+		  struct bpf_prog_load_opts *opts)
 {
-	const size_t attr_sz = offsetofend(union bpf_attr, fd_array);
+	const size_t attr_sz = offsetofend(union bpf_attr, log_true_size);
 	void *finfo = NULL, *linfo = NULL;
 	const char *func_info, *line_info;
 	__u32 log_size, log_level, attach_prog_fd, attach_btf_obj_fd;
@@ -312,6 +312,7 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 	}
 
 	fd = sys_bpf_prog_load(&attr, attr_sz, attempts);
+	OPTS_SET(opts, log_true_size, attr.log_true_size);
 	if (fd >= 0)
 		return fd;
 
@@ -352,6 +353,7 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 		}
 
 		fd = sys_bpf_prog_load(&attr, attr_sz, attempts);
+		OPTS_SET(opts, log_true_size, attr.log_true_size);
 		if (fd >= 0)
 			goto done;
 	}
@@ -366,6 +368,7 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 		attr.log_level = 1;
 
 		fd = sys_bpf_prog_load(&attr, attr_sz, attempts);
+		OPTS_SET(opts, log_true_size, attr.log_true_size);
 	}
 done:
 	/* free() doesn't affect errno, so we don't need to restore it */

tools/lib/bpf/bpf.h

@@ -96,13 +96,20 @@ struct bpf_prog_load_opts {
 	__u32 log_level;
 	__u32 log_size;
 	char *log_buf;
+	/* output: actual total log contents size (including termintaing zero).
+	 * It could be both larger than original log_size (if log was
+	 * truncated), or smaller (if log buffer wasn't filled completely).
+	 * If kernel doesn't support this feature, log_size is left unchanged.
+	 */
+	__u32 log_true_size;
+	size_t :0;
 };
-#define bpf_prog_load_opts__last_field log_buf
+#define bpf_prog_load_opts__last_field log_true_size
 
 LIBBPF_API int bpf_prog_load(enum bpf_prog_type prog_type,
 			     const char *prog_name, const char *license,
 			     const struct bpf_insn *insns, size_t insn_cnt,
-			     const struct bpf_prog_load_opts *opts);
+			     struct bpf_prog_load_opts *opts);
 
 /* Flags to direct loading requirements */
 #define MAPS_RELAX_COMPAT	0x01