Merge branch 'libbpf: error reporting changes for v1.0'

Andrii Nakryiko says:

====================

Implement error reporting changes discussed in "Libbpf: the road to v1.0"
([0]) document.

Libbpf gets a new API, libbpf_set_strict_mode() which accepts a set of flags
that turn on a set of libbpf 1.0 changes, that might be potentially breaking.
It's possible to opt-in into all current and future 1.0 features by specifying
LIBBPF_STRICT_ALL flag.

When some of the 1.0 "features" are requested, libbpf APIs might behave
differently. In this patch set a first set of changes are implemented, all
related to the way libbpf returns errors. See individual patches for details.

Patch #1 adds a no-op libbpf_set_strict_mode() functionality to enable
updating selftests.

Patch #2 gets rid of all the bad code patterns that will break in libbpf 1.0
(exact -1 comparison for low-level APIs, direct IS_ERR() macro usage to check
pointer-returning APIs for error, etc). These changes make selftest work in
both legacy and 1.0 libbpf modes. Selftests also opt-in into 100% libbpf 1.0
mode to automatically gain all the subsequent changes, which will come in
follow up patches.

Patch #3 streamlines error reporting for low-level APIs wrapping bpf() syscall.

Patch #4 streamlines errors for all the rest APIs.

Patch #5 ensures that BPF skeletons propagate errors properly as well, as
currently on error some APIs will return NULL with no way of checking exact
error code.

  [0] https://docs.google.com/document/d/1UyjTZuPFWiPFyKk1tV5an11_iaRuec6U-ZESZ54nNTY

v1->v2:
  - move libbpf_set_strict_mode() implementation to patch #1, where it belongs
    (Alexei);
  - add acks, slight rewording of commit messages.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/bpf/bpftool/gen.c

@@ -713,6 +713,7 @@ static int do_skeleton(int argc, char **argv)
 		#ifndef %2$s						    \n\
 		#define %2$s						    \n\
 									    \n\
+		#include <errno.h>					    \n\
 		#include <stdlib.h>					    \n\
 		#include <bpf/libbpf.h>					    \n\
 									    \n\
@@ -793,18 +794,23 @@ static int do_skeleton(int argc, char **argv)
 		%1$s__open_opts(const struct bpf_object_open_opts *opts)    \n\
 		{							    \n\
 			struct %1$s *obj;				    \n\
+			int err;					    \n\
 									    \n\
 			obj = (struct %1$s *)calloc(1, sizeof(*obj));	    \n\
-			if (!obj)					    \n\
+			if (!obj) {					    \n\
+				errno = ENOMEM;				    \n\
 				return NULL;				    \n\
-			if (%1$s__create_skeleton(obj))			    \n\
-				goto err;				    \n\
-			if (bpf_object__open_skeleton(obj->skeleton, opts)) \n\
-				goto err;				    \n\
+			}						    \n\
+									    \n\
+			err = %1$s__create_skeleton(obj);		    \n\
+			err = err ?: bpf_object__open_skeleton(obj->skeleton, opts);\n\
+			if (err)					    \n\
+				goto err_out;				    \n\
 									    \n\
 			return obj;					    \n\
-		err:							    \n\
+		err_out:						    \n\
 			%1$s__destroy(obj);				    \n\
+			errno = -err;					    \n\
 			return NULL;					    \n\
 		}							    \n\
 									    \n\
@@ -824,12 +830,15 @@ static int do_skeleton(int argc, char **argv)
 		%1$s__open_and_load(void)				    \n\
 		{							    \n\
 			struct %1$s *obj;				    \n\
+			int err;					    \n\
 									    \n\
 			obj = %1$s__open();				    \n\
 			if (!obj)					    \n\
 				return NULL;				    \n\
-			if (%1$s__load(obj)) {				    \n\
+			err = %1$s__load(obj);				    \n\
+			if (err) {					    \n\
 				%1$s__destroy(obj);			    \n\
+				errno = -err;				    \n\
 				return NULL;				    \n\
 			}						    \n\
 			return obj;					    \n\
@@ -860,7 +869,7 @@ static int do_skeleton(int argc, char **argv)
 									    \n\
 			s = (struct bpf_object_skeleton *)calloc(1, sizeof(*s));\n\
 			if (!s)						    \n\
-				return -1;				    \n\
+				goto err;				    \n\
 			obj->skeleton = s;				    \n\
 									    \n\
 			s->sz = sizeof(*s);				    \n\
@@ -949,7 +958,7 @@ static int do_skeleton(int argc, char **argv)
 			return 0;					    \n\
 		err:							    \n\
 			bpf_object__destroy_skeleton(s);		    \n\
-			return -1;					    \n\
+			return -ENOMEM;					    \n\
 		}							    \n\
 									    \n\
 		#endif /* %s */						    \n\

tools/lib/bpf/Makefile

@@ -229,6 +229,7 @@ install_headers: $(BPF_HELPER_DEFS)
 		$(call do_install,libbpf.h,$(prefix)/include/bpf,644); \
 		$(call do_install,btf.h,$(prefix)/include/bpf,644); \
 		$(call do_install,libbpf_common.h,$(prefix)/include/bpf,644); \
+		$(call do_install,libbpf_legacy.h,$(prefix)/include/bpf,644); \
 		$(call do_install,xsk.h,$(prefix)/include/bpf,644); \
 		$(call do_install,bpf_helpers.h,$(prefix)/include/bpf,644); \
 		$(call do_install,$(BPF_HELPER_DEFS),$(prefix)/include/bpf,644); \