[PATCH bpf-next v4 00/18] bpf: Support stack arguments for BPF functions and kfuncs

Yonghong Song
2026-04-12T04:58:38Z

Currently, bpf function calls and kfunc's are limited by 5 reg-level
parameters. For function calls with more than 5 parameters,
developers can use always inlining or pass a struct pointer
after packing more parameters in that struct. But there is
no workaround for kfunc if more than 5 parameters is needed.

This patch set lifts the 5-argument limit by introducing stack-based
argument passing for BPF functions and kfunc's, coordinated with
compiler support in LLVM [1]. The compiler emits loads/stores through
a new bpf register r12 (BPF_REG_STACK_ARG_BASE) to pass arguments beyond
the 5th, keeping the stack arg area separate from the r10-based program
stack. The maximum number of arguments is capped at MAX_BPF_FUNC_ARGS
(12), which is sufficient for the vast majority of use cases.

The x86_64 JIT translates r12-relative accesses to RBP-relative
native instructions. Each function's stack allocation is extended
by 'max_outgoing' bytes to hold the outgoing arg area below the
callee-saved registers. This makes implementation easier as the r10 can be
reused for stack argument access. At both BPF-to-BPF and kfunc calls,
outgoing args are pushed onto the expected calling convention locations
directly. The incoming parameters can directly get the value from
caller.

To support kfunc stack arguments, before doing any stack arguments,
existing codes are refactored/modified to use bpf_reg_state as much
as possible instead of using regno, and to pass regno/argno in a
single variable where non-negative means regno and negative means argno.

Global subprogs with >5 args are not yet supported. Only x86_64
is supported for now.

For the rest of patches, patches 1-6 make changes to make it
easy for future stack arguments for kfuncs. Patches 7-10
supports bpf-to-bpf stack arguments. Patch 11 rejects interpreter
for stack arguments. Patch 12 rejects subprogs if tailcall reachable.
Patch 13 adds stack argument support for kfuncs. Patch 14 enables
stack arguments for x86_64 and Patch 15 implements the x86_64 JIT.
Patches 16-18 are some test cases.

[1] https://github.com/llvm/llvm-project/pull/189060

Note:
- The patch set is on top of the following commit:
2ec74a053611 bpf: Simplify do_check_insn()
- This patch set requires latest llvm23 compiler. It is possible that a build
failure may appear:
/home/yhs/work/bpf-next/scripts/mod/modpost.c:59:13: error: variable 'extra_warn' set but not used [-Werror,-Wunused-but-set-global]
59 | static bool extra_warn;
| ^
1 error generated.
In this case, the following hack can workaround the build issue:
--- a/Makefile
+++ b/Makefile
@@ -467,7 +467,7 @@ KERNELDOC = $(srctree)/tools/docs/kernel-doc
export KERNELDOC

KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
- -O2 -fomit-frame-pointer -std=gnu11
+ -O2 -fomit-frame-pointer -std=gnu11 -Wno-unused-but-set-global
KBUILD_USERCFLAGS := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
KBUILD_USERLDFLAGS := $(USERLDFLAGS)

Changelogs:
v3 -> v4:
- v3: https://lore.kernel.org/bpf/20260405172505.1329392-1-yonghong.song@linux.dev/
- Refactor/Modify codes to make it easier for later kfunc stack argument support
- Invalidate outgoing slots immediately after the call to prevent reuse
- Fix interaction between stack argument PTR_TO_STACK and dead slot poisoning
- Reject stack arguments if tail call reachable
- Disable private stack if stack argument is used
- Allocate outgoing stack argument region after callee saved registers, and this
simplifies the JITed code a lot.
v2 -> v3:
- v2: https://lore.kernel.org/bpf/20260405165300.826241-1-yonghong.song@linux.dev/
- Fix selftest stack_arg_gap_at_minus8().
- Fix a few 'UTF-8' issues.
v1 -> v2:
- v1: https://lore.kernel.org/bpf/20260402012727.3916819-1-yonghong.song@linux.dev/
- Add stack_arg_safe() to do pruning for stack arguments.
- Fix an issue with KF_ARG_PTR_TO_MEM_SIZE. Since a faked register is
used, added verification log to indicate the start and end of such
faked register usage.
- For x86_64 JIT, copying incoming parameter values directly from caller's stack.
- Add test cases with stack arguments e.g. mem, mem+size, dynptr, iter, etc.

Yonghong Song (18):
bpf: Remove unused parameter from check_map_kptr_access()
bpf: Change from "arg #%d" to "arg#%d" in verifier log
bpf: Refactor to avoid redundant calculation of bpf_reg_state
bpf: Refactor to handle memory and size together
bpf: Change some regno type from u32 to int type
bpf: Use argument index instead of register index in kfunc verifier
logs
bpf: Introduce bpf register BPF_REG_STACK_ARG_BASE
bpf: Reuse MAX_BPF_FUNC_ARGS for maximum number of arguments
bpf: Support stack arguments for bpf functions
bpf: Fix interaction between stack argument PTR_TO_STACK and dead slot
poisoning
bpf: Reject stack arguments in non-JITed programs
bpf: Reject stack arguments if tail call reachable
bpf: Support stack arguments for kfunc calls
bpf: Enable stack argument support for x86_64
bpf,x86: Implement JIT support for stack arguments
selftests/bpf: Add tests for BPF function stack arguments
selftests/bpf: Add negative test for greater-than-8-byte kfunc stack
argument
selftests/bpf: Add verifier tests for stack argument validation

arch/x86/net/bpf_jit_comp.c | 177 ++-
include/linux/bpf.h | 6 +
include/linux/bpf_verifier.h | 32 +-
include/linux/filter.h | 4 +-
kernel/bpf/btf.c | 21 +-
kernel/bpf/core.c | 12 +-
kernel/bpf/verifier.c | 1038 ++++++++++++-----
.../selftests/bpf/prog_tests/cb_refs.c | 2 +-
.../selftests/bpf/prog_tests/linked_list.c | 4 +-
.../selftests/bpf/prog_tests/stack_arg.c | 132 +++
.../selftests/bpf/prog_tests/stack_arg_fail.c | 24 +
.../selftests/bpf/prog_tests/verifier.c | 2 +
.../selftests/bpf/progs/cpumask_failure.c | 4 +-
.../testing/selftests/bpf/progs/dynptr_fail.c | 26 +-
.../selftests/bpf/progs/file_reader_fail.c | 4 +-
.../selftests/bpf/progs/iters_state_safety.c | 14 +-
.../selftests/bpf/progs/iters_testmod.c | 6 +-
.../selftests/bpf/progs/iters_testmod_seq.c | 4 +-
.../bpf/progs/local_kptr_stash_fail.c | 2 +-
.../selftests/bpf/progs/map_kptr_fail.c | 4 +-
.../bpf/progs/mem_rdonly_untrusted.c | 2 +-
.../bpf/progs/nested_trust_failure.c | 2 +-
.../selftests/bpf/progs/res_spin_lock_fail.c | 2 +-
tools/testing/selftests/bpf/progs/stack_arg.c | 212 ++++
.../selftests/bpf/progs/stack_arg_fail.c | 32 +
.../selftests/bpf/progs/stack_arg_kfunc.c | 164 +++
.../testing/selftests/bpf/progs/stream_fail.c | 2 +-
.../selftests/bpf/progs/task_kfunc_failure.c | 4 +-
.../selftests/bpf/progs/verifier_bits_iter.c | 4 +-
.../bpf/progs/verifier_cgroup_storage.c | 4 +-
.../selftests/bpf/progs/verifier_ctx.c | 2 +-
.../bpf/progs/verifier_ref_tracking.c | 2 +-
.../selftests/bpf/progs/verifier_sock.c | 6 +-
.../selftests/bpf/progs/verifier_stack_arg.c | 316 +++++
.../selftests/bpf/progs/verifier_unpriv.c | 4 +-
.../selftests/bpf/progs/verifier_vfs_reject.c | 8 +-
.../testing/selftests/bpf/progs/wq_failures.c | 4 +-
.../selftests/bpf/test_kmods/bpf_testmod.c | 73 ++
.../bpf/test_kmods/bpf_testmod_kfunc.h | 26 +
tools/testing/selftests/bpf/verifier/calls.c | 6 +-
.../testing/selftests/bpf/verifier/map_kptr.c | 10 +-
41 files changed, 1996 insertions(+), 407 deletions(-)
create mode 100644 tools/testing/selftests/bpf/prog_tests/stack_arg.c
create mode 100644 tools/testing/selftests/bpf/prog_tests/stack_arg_fail.c
create mode 100644 tools/testing/selftests/bpf/progs/stack_arg.c
create mode 100644 tools/testing/selftests/bpf/progs/stack_arg_fail.c
create mode 100644 tools/testing/selftests/bpf/progs/stack_arg_kfunc.c
create mode 100644 tools/testing/selftests/bpf/progs/verifier_stack_arg.c

--
2.52.0