libbpf: Streamline error reporting for high-level APIs

Implement changes to error reporting for high-level libbpf APIs to make them
less surprising and less error-prone to users:
  - in all the cases when error happens, errno is set to an appropriate error
    value;
  - in libbpf 1.0 mode, all pointer-returning APIs return NULL on error and
    error code is communicated through errno; this applies both to APIs that
    already returned NULL before (so now they communicate more detailed error
    codes), as well as for many APIs that used ERR_PTR() macro and encoded
    error numbers as fake pointers.
  - in legacy (default) mode, those APIs that were returning ERR_PTR(err),
    continue doing so, but still set errno.

With these changes, errno can be always used to extract actual error,
regardless of legacy or libbpf 1.0 modes. This is utilized internally in
libbpf in places where libbpf uses it's own high-level APIs.
libbpf_get_error() is adapted to handle both cases completely transparently to
end-users (and is used by libbpf consistently as well).

More context, justification, and discussion can be found in "Libbpf: the road
to v1.0" document ([0]).

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

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210525035935.1461796-5-andrii@kernel.org

1 files changed, 11 insertions, 11 deletions

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 1dca41a24f75..10911a8cad0f 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -220,16 +220,16 @@ struct bpf_linker *bpf_linker__new(const char *filename, struct bpf_linker_opts
 	int err;
 
 	if (!OPTS_VALID(opts, bpf_linker_opts))
-		return NULL;
+		return errno = EINVAL, NULL;
 
 	if (elf_version(EV_CURRENT) == EV_NONE) {
 		pr_warn_elf("libelf initialization failed");
-		return NULL;
+		return errno = EINVAL, NULL;
 	}
 
 	linker = calloc(1, sizeof(*linker));
 	if (!linker)
-		return NULL;
+		return errno = ENOMEM, NULL;
 
 	linker->fd = -1;
 
@@ -241,7 +241,7 @@ struct bpf_linker *bpf_linker__new(const char *filename, struct bpf_linker_opts
 
 err_out:
 	bpf_linker__free(linker);
-	return NULL;
+	return errno = -err, NULL;
 }
 
 static struct dst_sec *add_dst_sec(struct bpf_linker *linker, const char *sec_name)
@@ -444,10 +444,10 @@ int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
 	int err = 0;
 
 	if (!OPTS_VALID(opts, bpf_linker_file_opts))
-		return -EINVAL;
+		return libbpf_err(-EINVAL);
 
 	if (!linker->elf)
-		return -EINVAL;
+		return libbpf_err(-EINVAL);
 
 	err = err ?: linker_load_obj_file(linker, filename, opts, &obj);
 	err = err ?: linker_append_sec_data(linker, &obj);
@@ -467,7 +467,7 @@ int bpf_linker__add_file(struct bpf_linker *linker, const char *filename,
 	if (obj.fd >= 0)
 		close(obj.fd);
 
-	return err;
+	return libbpf_err(err);
 }
 
 static bool is_dwarf_sec_name(const char *name)
@@ -2548,11 +2548,11 @@ int bpf_linker__finalize(struct bpf_linker *linker)
 	int err, i;
 
 	if (!linker->elf)
-		return -EINVAL;
+		return libbpf_err(-EINVAL);
 
 	err = finalize_btf(linker);
 	if (err)
-		return err;
+		return libbpf_err(err);
 
 	/* Finalize strings */
 	strs_sz = strset__data_size(linker->strtab_strs);
@@ -2584,14 +2584,14 @@ int bpf_linker__finalize(struct bpf_linker *linker)
 	if (elf_update(linker->elf, ELF_C_NULL) < 0) {
 		err = -errno;
 		pr_warn_elf("failed to finalize ELF layout");
-		return err;
+		return libbpf_err(err);
 	}
 
 	/* Write out final ELF contents */
 	if (elf_update(linker->elf, ELF_C_WRITE) < 0) {
 		err = -errno;
 		pr_warn_elf("failed to write ELF contents");
-		return err;
+		return libbpf_err(err);
 	}
 
 	elf_end(linker->elf);