We've hit the 512 bytes limit on stack depth a few times in Cilium recently. As a result, we started reporting in CI our current maximum stack depth across all configurations for each BPF program. Unfortunately, that is not trivial to compute in userspace. The verifier reports the stack depths of individual subprogs at the end of the logs. However the maximum combined stack depth also depends on the callgraph of those subprogs (the max combined stack depth is the height of the callgraph weighted by per-subprog stack depths). We can compute a callgraph in userspace from the loaded instructions, but it often doesn't match the verifier's own callgraph because of dead code elimination. Our current approach relies on dumping the BPF_LOG_LEVEL2 logs, but this feels overkill considering the verifier already has the information we need. The patch lets the verifier dump the maximum combined stack depth in the logs, on the same line as the per-subprog stack depths: stack depth 16+256 max 272 The per-subprog stack depths and the new max stack depth are not directly comparable. The former is sometimes updated during fixups, while the latter is not. As a result, even with a single subprog, we may end up with two slightly different values. The aim of the new max value is to be closest to what is actually enforced by the verifier. Signed-off-by: Paul Chaignon --- include/linux/bpf_verifier.h | 2 ++ kernel/bpf/verifier.c | 6 +++++- 2 files changed, 7 insertions(+), 1 deletion(-) diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h index 976e2b2f40e8..d91843994c82 100644 --- a/include/linux/bpf_verifier.h +++ b/include/linux/bpf_verifier.h @@ -936,6 +936,8 @@ struct bpf_verifier_env { u32 prev_insn_processed, insn_processed; /* number of jmps, calls, exits analyzed so far */ u32 prev_jmps_processed, jmps_processed; + /* maximum combined stack depth */ + u32 max_stack_depth; /* total verification time */ u64 verification_time; /* maximum number of verifier states kept in 'branching' instructions */ diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 11054ad89c14..896dbb4515d7 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -5045,6 +5045,8 @@ static int check_max_stack_depth_subprog(struct bpf_verifier_env *env, int idx, } } else { depth += subprog_depth; + if (depth > env->max_stack_depth) + env->max_stack_depth = depth; if (depth > MAX_BPF_STACK) { total = 0; for (tmp = idx; tmp >= 0; tmp = dinfo[tmp].caller) @@ -5185,6 +5187,8 @@ static int check_max_stack_depth(struct bpf_verifier_env *env) if (priv_stack_mode == PRIV_STACK_UNKNOWN) priv_stack_mode = bpf_enable_priv_stack(env->prog); + env->max_stack_depth = env->subprog_info[0].stack_depth; + /* All async_cb subprogs use normal kernel stack. If a particular * subprog appears in both main prog and async_cb subtree, that * subprog will use normal kernel stack to avoid potential nesting. @@ -18289,7 +18293,7 @@ static void print_verification_stats(struct bpf_verifier_env *env) verbose(env, "stack depth %d", env->subprog_info[0].stack_depth); for (i = 1; i < subprog_cnt; i++) verbose(env, "+%d", env->subprog_info[i].stack_depth); - verbose(env, "\n"); + verbose(env, " max %d\n", env->max_stack_depth); verbose(env, "insns processed %d", env->subprog_info[0].insn_processed); for (i = 1; i < subprog_cnt; i++) if (bpf_subprog_is_global(env, i)) -- 2.43.0