libbpf: Fix out-of-bound read in bpf_linker__add_buf()

Fix a potential out-of-bound read in bpf_linker__add_buf() by advancing
the buffer pointer and reducing the remaining buffer size passed to
write() in each iteration. The bug is reported in [0].

[0]: libbpf/libbpf#945

Fixes: 6d5e5e5 ("libbpf: Extend linker API to support in-memory ELF files")
Signed-off-by: Amery Hung <ameryhung@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>

Author: ameryhung

tools/lib/bpf/linker.c

@@ -581,7 +581,7 @@ int bpf_linker__add_buf(struct bpf_linker *linker, void *buf, size_t buf_sz,
 
 	written = 0;
 	while (written < buf_sz) {
-		ret = write(fd, buf, buf_sz);
+		ret = write(fd, buf + written, buf_sz - written);
 		if (ret < 0) {
 			ret = -errno;
 			pr_warn("failed to write '%s': %s\n", filename, errstr(ret));