From f41f34ec64748e16e5a90ab391cec39e30942f32 Mon Sep 17 00:00:00 2001 From: Paul Chaignon Date: Wed, 13 May 2026 21:34:50 +0200 Subject: bpf: Report maximum combined stack depth 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 Acked-by: Eduard Zingerman Link: https://lore.kernel.org/r/d3d23a0410f87f116f3bbaa98a815dbae113bda2.1778700777.git.paul.chaignon@gmail.com Signed-off-by: Alexei Starovoitov --- include/linux/bpf_verifier.h | 2 ++ 1 file changed, 2 insertions(+) (limited to 'include/linux') diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h index 6f12fc40b682..20c421b43849 100644 --- a/include/linux/bpf_verifier.h +++ b/include/linux/bpf_verifier.h @@ -989,6 +989,8 @@ 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 */ -- cgit v1.2.3