api: fix seccomp_export_bpf_mem out-of-bounds read

*len is the length of the destination buffer, but program->blks is
probably not anywhere near that long.  It's already been checked above
that BPF_PGM_SIZE(program) is less than or equal to *len, so that's
the correct value to use here to avoid either reading or writing too
much.

I noticed this because tests/11-basic-basic_errors started failing on
musl after e797591 ("all: add seccomp_precompute() functionality").

Signed-off-by: Alyssa Ross <[email protected]>

src/api.c

@@ -786,7 +786,7 @@ API int seccomp_export_bpf_mem(const scmp_filter_ctx ctx, void *buf,
 		if (BPF_PGM_SIZE(program) > *len)
 			rc = _rc_filter(-ERANGE);
 		else
-			memcpy(buf, program->blks, *len);
+			memcpy(buf, program->blks, BPF_PGM_SIZE(program));
 	}
 	*len = BPF_PGM_SIZE(program);