Re: [PATCHSET SLOP RFC 2/6] selftests/bpf: Add kfunc __arena argument tests

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] The kfunc arena argument fixup logic incorrectly translates a valid pointer to the first page of a 4GB-aligned arena as NULL, causing kfuncs to receive a NULL pointer instead of a valid kernel address.
- [Low] Multi-line comment does not follow the mandated BPF subsystem style.
--

--- Patch [2]: [PATCHSET SLOP RFC 2/6] selftests/bpf: Add kfunc __arena argument tests ---
commit c3a66495e39c4d671a1065fb44bc6cef2f26d120
Author: Tejun Heo <[email protected]>

selftests/bpf: Add kfunc __arena argument tests

Add arena-argument kfuncs to bpf_testmod, which also exercises the
argument rebasing on module kfuncs, and tests covering the accepted
argument forms (arena pointer, low 32 bits as a scalar, full user address
as a scalar, NULL), five arena arguments in one call with NULLs mixed in,
a kernel-side dereference of an unpopulated page recovering through the
scratch page, and the rejections (no arena in the program, incompatible
register type).

> diff --git a/tools/testing/selftests/bpf/prog_tests/arena_kfunc.c b/tools/testing/selftests/bpf/prog_tests/arena_kfunc.c
> new file mode 100644
> index 0000000000000..8946184553ea8
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/arena_kfunc.c
> @@ -0,0 +1,14 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2026 Tejun Heo <[email protected]> */
> +#include <test_progs.h>
> +
> +#include "arena_kfunc.skel.h"
> +
> +/* The test kfuncs live in bpf_testmod. Resolving kfuncs against module
> + * BTFs needs CAP_SYS_ADMIN, so run with full capabilities instead of
> + * through the verifier tests' capability-restricted runner.
> + */

[Severity: Low]
This isn't a bug, but does this multi-line comment follow the BPF
subsystem style?

The opening /* should be on its own line:

/*
 * The test kfuncs live in bpf_testmod...
 */

> +void test_arena_kfunc(void)
> +{
> +	RUN_TESTS(arena_kfunc);
> +}

[ ... ]

> diff --git a/tools/testing/selftests/bpf/progs/arena_kfunc.c b/tools/testing/selftests/bpf/progs/arena_kfunc.c
> new file mode 100644
> index 0000000000000..b5996944ed73b
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/arena_kfunc.c

[ ... ]

> +	/* NULL is preserved */
> +	ret = bpf_kfunc_arena_arg_test(NULL);
> +	if (ret != 0xdeadbeef)
> +		return 5;

[Severity: High]
Does the kfunc arena argument fixup logic tested here incorrectly translate
a valid pointer to the first page of a 4GB-aligned arena as NULL?

In kernel/bpf/verifier.c:bpf_fixup_kfunc_call(), if an arena is created with
a 4GB-aligned user_vm_start, and a BPF program passes a pointer to offset 0
within this arena to a kfunc, the fixup logic uses BPF_ZEXT_REG(regno) to
clear the upper 32 bits, making the register exactly 0.

This is then explicitly checked by BPF_JMP_IMM(BPF_JEQ, regno, 0, 1), which
skips the kern_vm_start base addition.

Would this cause kfuncs to receive a NULL pointer instead of a valid kernel
address when passed a pointer to offset 0?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=2