[PATCH bpf-next] libbpf: don't allocate 16M for log buffer by default

[Stanislav Fomichev <sdf@xxxxxxxxxx>]

For each prog/btf load we allocate and free 16 megs of verifier buffer.
On production systems it doesn't really make sense because the
programs/btf have gone through extensive testing and (mostly) guaranteed
to successfully load.

Let's switch to a much smaller buffer by default (128 bytes, sys_bpf
doesn't accept smaller log buffer) and resize it if the kernel returns
ENOSPC. On the first ENOSPC error we resize the buffer to BPF_LOG_BUF_SIZE
and then, on each subsequent ENOSPC, we keep doubling the buffer.

Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
---
 tools/lib/bpf/btf.c             | 10 +++++++++-
 tools/lib/bpf/libbpf.c          | 10 ++++++++--
 tools/lib/bpf/libbpf_internal.h |  2 ++
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index 3d1c25fc97ae..53c7efc3b347 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -657,13 +657,14 @@ int btf__finalize_data(struct bpf_object *obj, struct btf *btf)
 
 int btf__load(struct btf *btf)
 {
-	__u32 log_buf_size = BPF_LOG_BUF_SIZE;
+	__u32 log_buf_size = BPF_MIN_LOG_BUF_SIZE;
 	char *log_buf = NULL;
 	int err = 0;
 
 	if (btf->fd >= 0)
 		return -EEXIST;
 
+retry_load:
 	log_buf = malloc(log_buf_size);
 	if (!log_buf)
 		return -ENOMEM;
@@ -673,6 +674,13 @@ int btf__load(struct btf *btf)
 	btf->fd = bpf_load_btf(btf->data, btf->data_size,
 			       log_buf, log_buf_size, false);
 	if (btf->fd < 0) {
+		if (errno == ENOSPC) {
+			log_buf_size = max((__u32)BPF_LOG_BUF_SIZE,
+					   log_buf_size << 1);
+			free(log_buf);
+			goto retry_load;
+		}
+
 		err = -errno;
 		pr_warn("Error loading BTF: %s(%d)\n", strerror(errno), errno);
 		if (*log_buf)
diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 085e41f9b68e..793c81b35ccc 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -4855,7 +4855,7 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 {
 	struct bpf_load_program_attr load_attr;
 	char *cp, errmsg[STRERR_BUFSIZE];
-	int log_buf_size = BPF_LOG_BUF_SIZE;
+	size_t log_buf_size = BPF_MIN_LOG_BUF_SIZE;
 	char *log_buf;
 	int btf_fd, ret;
 
@@ -4911,7 +4911,13 @@ load_program(struct bpf_program *prog, struct bpf_insn *insns, int insns_cnt,
 	}
 
 	if (errno == ENOSPC) {
-		log_buf_size <<= 1;
+		if (errno == ENOSPC) {
+			log_buf_size = max((size_t)BPF_LOG_BUF_SIZE,
+					   log_buf_size << 1);
+			free(log_buf);
+			goto retry_load;
+		}
+
 		free(log_buf);
 		goto retry_load;
 	}
diff --git a/tools/lib/bpf/libbpf_internal.h b/tools/lib/bpf/libbpf_internal.h
index 8c3afbd97747..2720f3366798 100644
--- a/tools/lib/bpf/libbpf_internal.h
+++ b/tools/lib/bpf/libbpf_internal.h
@@ -23,6 +23,8 @@
 #define BTF_PARAM_ENC(name, type) (name), (type)
 #define BTF_VAR_SECINFO_ENC(type, offset, size) (type), (offset), (size)
 
+#define BPF_MIN_LOG_BUF_SIZE 128
+
 #ifndef min
 # define min(x, y) ((x) < (y) ? (x) : (y))
 #endif
-- 
2.25.1.696.g5e7596f4ac-goog