Re: [PATCH bpf-next v3 1/3] bpf: Report maximum combined stack depth

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2026-05-13 at 21:34 +0200, Paul Chaignon wrote:
> 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 <paul.chaignon@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

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