libbpf: Skip hash computation when loader generation failed

bpf_gen__finish() calls compute_sha_update_offsets() gated only on
the gen_hash option, without first consulting gen->error. On a failed
generation this is buggy: a failed realloc_data_buf() sets gen->data_start
to NULL (leaving gen->data_cur dangling), so compute_sha_update_offsets()
runs libbpf_sha256() over a NULL buffer with a bogus length; a failed
realloc_insn_buf() likewise sets gen->insn_start to NULL and the hash
immediates get patched through that NULL base.

The computed program is discarded in either case, since the following
"if (!gen->error)" block does not publish opts->insns once an error is
set. Thus, skip the hash pass when generation has already failed.

Fixes: ea92308 ("libbpf: Embed and verify the metadata hash in the loader")
Reported-by: sashiko <sashiko@sashiko.dev>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260529094119.307264-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: borkmann

tools/lib/bpf/gen_loader.c

@@ -397,13 +397,12 @@ int bpf_gen__finish(struct bpf_gen *gen, int nr_progs, int nr_maps)
 			      blob_fd_array_off(gen, i));
 	emit(gen, BPF_MOV64_IMM(BPF_REG_0, 0));
 	emit(gen, BPF_EXIT_INSN());
-	if (OPTS_GET(gen->opts, gen_hash, false))
-		compute_sha_update_offsets(gen);
-
-	pr_debug("gen: finish %s\n", errstr(gen->error));
 	if (!gen->error) {
 		struct gen_loader_opts *opts = gen->opts;
 
+		if (OPTS_GET(opts, gen_hash, false))
+			compute_sha_update_offsets(gen);
+
 		opts->insns = gen->insn_start;
 		opts->insns_sz = gen->insn_cur - gen->insn_start;
 		opts->data = gen->data_start;
@@ -418,6 +417,7 @@ int bpf_gen__finish(struct bpf_gen *gen, int nr_progs, int nr_maps)
 				bpf_insn_bswap(insn++);
 		}
 	}
+	pr_debug("gen: finish %s\n", errstr(gen->error));
 	return gen->error;
 }