[PATCH bpf-next] bpftool: avoid leaking the JSON writer prepared for program metadata

Bpftool creates a new JSON object for writing program metadata in plain
text mode, regardless of metadata being present or not. Then this writer
is freed if any metadata has been found and printed, but it leaks
otherwise. We cannot destroy the object unconditionally, because the
destructor prints an undesirable line break. Instead, make sure the
writer is created only after we have found program metadata to print.

Found with valgrind.

Cc: YiFei Zhu <redacted>
Fixes: aff52e685eb3 ("bpftool: Support dumping metadata")
Signed-off-by: Quentin Monnet <redacted>
---
 tools/bpf/bpftool/prog.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
index 277d51c4c5d9..f633299b1261 100644
--- a/tools/bpf/bpftool/prog.c
+++ b/tools/bpf/bpftool/prog.c

@@ -307,18 +307,12 @@ static void show_prog_metadata(int fd, __u32 num_maps)
 		if (printed_header)
 			jsonw_end_object(json_wtr);
 	} else {
-		json_writer_t *btf_wtr = jsonw_new(stdout);
+		json_writer_t *btf_wtr;
 		struct btf_dumper d = {
 			.btf = btf,
-			.jw = btf_wtr,
 			.is_plain_text = true,
 		};
 
-		if (!btf_wtr) {
-			p_err("jsonw alloc failed");
-			goto out_free;
-		}
-
 		for (i = 0; i < vlen; i++, vsi++) {
 			t_var = btf__type_by_id(btf, vsi->type);
 			name = btf__name_by_offset(btf, t_var->name_off);

@@ -328,6 +322,14 @@ static void show_prog_metadata(int fd, __u32 num_maps)
 
 			if (!printed_header) {
 				printf("\tmetadata:");
+
+				btf_wtr = jsonw_new(stdout);
+				if (!btf_wtr) {
+					p_err("jsonw alloc failed");
+					goto out_free;
+				}
+				d.jw = btf_wtr,
+
 				printed_header = true;
 			}
 

-- 
2.30.2

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Andrii Nakryiko [off-list ref]:

On Fri, 22 Oct 2021 10:47:43 +0100 you wrote:

Bpftool creates a new JSON object for writing program metadata in plain
text mode, regardless of metadata being present or not. Then this writer
is freed if any metadata has been found and printed, but it leaks
otherwise. We cannot destroy the object unconditionally, because the
destructor prints an undesirable line break. Instead, make sure the
writer is created only after we have found program metadata to print.

[...]

Here is the summary with links:
  - [bpf-next] bpftool: avoid leaking the JSON writer prepared for program metadata
    https://git.kernel.org/bpf/bpf-next/c/e89ef634f81c

You are awesome, thank you!
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