[PATCH bpf-next] tools: bpftool: fix warning on ignored return value for 'read'

[Quentin Monnet <quentin.monnet@netronome.com>]

When building bpftool, a warning was introduced by commit a94364603610
("bpftool: Allow to read btf as raw data"), because the return value
from a call to 'read()' is ignored. Let's address it.

Cc: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
---
 tools/bpf/bpftool/btf.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
index a7b8bf233cf5..e5bc97b71ceb 100644
--- a/tools/bpf/bpftool/btf.c
+++ b/tools/bpf/bpftool/btf.c
@@ -428,15 +428,15 @@ static struct btf *btf__parse_raw(const char *file)
 static bool is_btf_raw(const char *file)
 {
 	__u16 magic = 0;
-	int fd;
+	int fd, nb_read;
 
 	fd = open(file, O_RDONLY);
 	if (fd < 0)
 		return false;
 
-	read(fd, &magic, sizeof(magic));
+	nb_read = read(fd, &magic, sizeof(magic));
 	close(fd);
-	return magic == BTF_MAGIC;
+	return nb_read == sizeof(magic) && magic == BTF_MAGIC;
 }
 
 static int do_dump(int argc, char **argv)
-- 
2.17.1