Re: [PATCH bpf] libbpf: Reject excessive program counts in generated loaders

[Alexei Starovoitov]

On Wed, Oct 28, 2026 at 11:39 AM Mahe Tardy <mahe.tardy@xxxxxxxxx> wrote:
> @@ -122,6 +122,11 @@ void bpf_gen__init(struct bpf_gen *gen, int log_level, int nr_progs, int nr_maps
> +	if (nr_progs > MAX_USED_PROGS) {
> +		pr_warn("Total programs exceeds %d\n", MAX_USED_PROGS);
> +		gen->error = -E2BIG;
> +		return;
> +	}

nr_progs is obj->nr_programs. It counts subprogs and progs with
autoload == false too. See commit 259172bb6514
("libbpf: Fix gen_loader assumption on number of programs.").
Only a prog that reaches bpf_gen__prog_load() takes a slot in prog_fd[].

Two objects with obj->nr_programs == 33:

  object               today            this patch     should be
  -------------------  ---------------  -------------  -------------
  33 progs             -EACCES at load  -E2BIG at gen  -E2BIG at gen
  1 prog, 32 subprogs  -EACCES at load  -E2BIG at gen  loads

The second object fails today because of the cleanup loop in
bpf_gen__init(). It closes nr_progs slots of prog_fd[], and
prog_fd[32] is at r10 + 0:

  r1 = *(u32 *)(r10 +0)
  invalid read from stack R10 off=0 size=4

Lets fix both: clamp the loop and check the limit where a prog takes
the slot, like add_map_fd() does.
Something like this (not even compile tested):

diff --git a/tools/lib/bpf/gen_loader.c b/tools/lib/bpf/gen_loader.c
--- a/tools/lib/bpf/gen_loader.c
+++ b/tools/lib/bpf/gen_loader.c
@@ -139,6 +139,8 @@ void bpf_gen__init(struct bpf_gen *gen, int log_level, int nr_progs, int nr_maps
 	emit(gen, BPF_MOV64_IMM(BPF_REG_3, 0));
 	emit(gen, BPF_EMIT_CALL(BPF_FUNC_probe_read_kernel));
 
+	/* nr_progs includes subprogs and progs that are not loaded */
+	nr_progs = min(nr_progs, MAX_USED_PROGS);
 	/* amount of stack actually used, only used to calculate iterations, not stack offset */
 	nr_progs_sz = offsetof(struct loader_stack, prog_fd[nr_progs]);
 	/* jump over cleanup code */
@@ -1134,6 +1136,11 @@ void bpf_gen__prog_load(struct bpf_gen *gen,
 		gen->attach_kind = 0;
 	}
 	emit_check_err(gen);
+	if (gen->nr_progs == MAX_USED_PROGS) {
+		pr_warn("Total programs exceeds %d\n", MAX_USED_PROGS);
+		gen->error = -E2BIG;
+		return;
+	}
 	/* remember prog_fd in the stack, if successful */
 	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_10, BPF_REG_7,
 			      stack_off(prog_fd[gen->nr_progs])));

since it was already applied. pls send a follow up.