[PATCH bpf-next v6 0/2] bpf: Relax 8 frame limitation for global subprogs

[Emil Tsalapatis <emil@etsalapatis.com>]

The BPF verifier currently limits the maximum runtime call stack to
8 frames. Larger BPF programs like sched-ext schedulers routinely
fail verification because they exceed this limit, even as they use
very little actual stack space for each frame.

Relax the verifier to permit call stacks > 8 frames deep when the
call stacks include global subprogs. The old 8 stack frame limit now
only applies to call stacks composed entirely of static function calls.
This works because global functions are each verified in isolation, so
the verifier does not need to cross-reference verification state across
the function call boundary, which has been the reason for limiting the
call stack size in the first place.

This patch does not change the verification time limit of 8 stack
frames. Static functions that are inlined for verification purposes
still only go 8 frames deep to avoid changing the verifier's internal
data structures used for verification. These data structures only
support holding information on up to 8 stack frames.

This patch also does not adjust the actual maximum stack size of 512.

CHANGELOG
=========

v5 -> v6 (https://lore.kernel.org/bpf/20260311182831.91219-1-emil@etsalapatis.com/)
- Make bpf_subprog_call_depth_info internal to verifier.c (Alexei)

v4 -> v5 (https://lore.kernel.org/bpf/20260309204430.201219-1-emil@etsalapatis.com/)
- Move depth tracking state to verifier (Eduard) and free it after verification (Alexei)
- Fix selftest patch title and formatting errors (Yonghong)

v3 -> v4 (https://lore.kernel.org/bpf/20260303043106.406099-1-emil@etsalapatis.com/)
- Factor out temp call depth tracking info into its own struct (Eduard)
- Bring depth calculation loop in line with the other instances (Mykyta)
- Add comment on why selftest call stack is 16 bytes/frame (Eduard)
- Rename "cidx" to "caller" for clarity (Mykyta, Eduard)

v2 -> v3 (https://lore.kernel.org/bpf/20260210213606.475415-1-emil@etsalapatis.com/)
- Change logic to remove arbitrary limit on call depth (Eduard)
- Add additional selftests (Eduard)

v1 -> v2 (https://lore.kernel.org/bpf/20260202233716.835638-1-emil@etsalapatis.com)
- Adjust patch to only increase the runtime stack depth, leaving the
verification-time stack depth unchanged (Alexei)

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>

Emil Tsalapatis (2):
  bpf: Only enforce 8 frame call stack limit for all-static stacks
  selftests/bpf: Add deep call stack selftests

 kernel/bpf/verifier.c                         | 76 ++++++++++-----
 .../bpf/prog_tests/test_global_funcs.c        |  2 +
 .../selftests/bpf/progs/test_global_func3.c   | 18 ++--
 .../bpf/progs/test_global_func_deep_stack.c   | 95 +++++++++++++++++++
 4 files changed, 160 insertions(+), 31 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_global_func_deep_stack.c

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2.49.0