[PATCH bpf-next v6 0/2] bpf: Add multi-level pointer parameter support for trampolines

Slava Imameev
2026-03-14T08:22:26Z

This is v6 of a series adding support for new pointer types for
trampoline parameters.

Originally, only support for multi-level pointers was proposed.
As suggested during review, it was extended to single-level pointers.
During discussion, it was proposed to add support for any single or
multi-level pointer type that is not a single-level pointer to a
structure, with the condition if (!btf_type_is_struct(t)). The safety
of this condition is based on BTF data verification performed for
modules and programs, and vmlinux BTF being trusted to not contain
invalid types, so it is not possible for invalid types, like
PTR->DATASEC, PTR->FUNC, PTR->VAR and corresponding multi-level
pointers, to reach btf_ctx_access.

These changes appear to be a safe extension since any future support
for arrays and output values would require annotation (similar to
Microsoft SAL), which differentiates between current unannotated
scalar cases and new annotated cases.

This series adds BPF verifier support for single- and multi-level
pointer parameters and return values in BPF trampolines. The
implementation treats these parameters as SCALAR_VALUE.

This is consistent with existing pointers to int and void that are
already treated as SCALAR.

This provides consistent logic for single- and multi-level pointers:
if the type is treated as SCALAR for a single-level pointer, the
same applies to multi-level pointers, except for pointers to structs
which are currently PTR_TO_BTF_ID. However, in the case of
multi-level pointers, they are treated as scalar since the verifier
lacks the context to infer the size of their target memory regions.

Background:

Prior to these changes, accessing multi-level pointer parameters or
return values through BPF trampoline context arrays resulted in
verification failures in btf_ctx_access, producing errors such as:

func '%s' arg%d type %s is not a struct

For example, consider a BPF program that logs an input parameter of
type struct posix_acl **:

SEC("fentry/__posix_acl_chmod")
int BPF_PROG(trace_posix_acl_chmod, struct posix_acl **ppacl, gfp_t gfp,
umode_t mode)
{
bpf_printk("__posix_acl_chmod ppacl = %px\n", ppacl);
return 0;
}

This program failed BPF verification with the following error:

libbpf: prog 'trace_posix_acl_chmod': -- BEGIN PROG LOAD LOG --
0: R1=ctx() R10=fp0
; int BPF_PROG(trace_posix_acl_chmod, struct posix_acl **ppacl,
gfp_t gfp, umode_t mode) @ posix_acl_monitor.bpf.c:23
0: (79) r6 = *(u64 *)(r1 +16) ; R1=ctx() R6_w=scalar()
1: (79) r1 = *(u64 *)(r1 +0)
func '__posix_acl_chmod' arg0 type PTR is not a struct
invalid bpf_context access off=0 size=8
processed 2 insns (limit 1000000) max_states_per_insn 0 total_states 0
peak_states 0 mark_read 0
-- END PROG LOAD LOG --

The common workaround involved using helper functions to fetch
parameter values by passing the address of the context array entry:

SEC("fentry/__posix_acl_chmod")
int BPF_PROG(trace_posix_acl_chmod, struct posix_acl **ppacl, gfp_t gfp,
umode_t mode)
{
struct posix_acl **pp;
bpf_probe_read_kernel(&pp, sizeof(ppacl), &ctx[0]);
bpf_printk("__posix_acl_chmod %px\n", pp);
return 0;
}

This approach introduced helper call overhead and created
inconsistency with parameter access patterns.

Improvements:

With this patch, trampoline programs can directly access multi-level
pointer parameters, eliminating helper call overhead and explicit ctx
access while ensuring consistent parameter handling. For example, the
following ctx access with a helper call:

SEC("fentry/__posix_acl_chmod")
int BPF_PROG(trace_posix_acl_chmod, struct posix_acl **ppacl, gfp_t gfp,
umode_t mode)
{
struct posix_acl **pp;
bpf_probe_read_kernel(&pp, sizeof(pp), &ctx[0]);
bpf_printk("__posix_acl_chmod %px\n", pp);
...
}

is replaced by a load instruction:

SEC("fentry/__posix_acl_chmod")
int BPF_PROG(trace_posix_acl_chmod, struct posix_acl **ppacl, gfp_t gfp,
umode_t mode)
{
bpf_printk("__posix_acl_chmod %px\n", ppacl);
...
}

The bpf_core_cast macro can be used for deeper level dereferences.

v1 -> v2:
* corrected maintainer's email
v2 -> v3:
* Addressed reviewers' feedback:
* Changed the register type from PTR_TO_MEM to SCALAR_VALUE.
* Modified tests to accommodate SCALAR_VALUE handling.
* Fixed a compilation error for loongarch
* https://lore.kernel.org/oe-kbuild-all/202602181710.tEK6nOl6-lkp@intel.com/
* Addressed AI bot review
* Added a commentary to address a NULL pointer case
* Removed WARN_ON
* Fixed a commentary
v3 -> v4:
* Added more consistent support for single and multi-level pointers
as suggested by reviewers.
* added single level pointers to enum 32 and 64
* added single level pointers to functions
* harmonized support for single and multi-level pointer types
* added new tests to support the above changes
* Removed create_bad_kaddr that allocated and invalidated kernel VA
for tests, and replaced it with hardcoded values similar to
bpf_testmod_return_ptr as suggested by reviewers.
v4 -> v5:
* As suggested, extended support to single-level pointers and
covered all supported valid pointer (single- and multi-level) types
with a wider condition if (!btf_type_is_struct(t)).
* As requested, simplified tests by keeping only tests that check
the verifier log for scalar().
v5 -> v6:
* Fixed the test message based on the bot's feedback.

Slava Imameev (2):
bpf: Support pointer param types via SCALAR_VALUE for trampolines
selftests/bpf: Add trampolines single and multi-level pointer params
test coverage

kernel/bpf/btf.c | 17 ++---
net/bpf/test_run.c | 17 +++++
.../selftests/bpf/prog_tests/verifier.c | 2 +
.../bpf/progs/verifier_ctx_ptr_param.c | 68 +++++++++++++++++++
4 files changed, 94 insertions(+), 10 deletions(-)
create mode 100644 tools/testing/selftests/bpf/progs/verifier_ctx_ptr_param.c

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2.34.1