Re: [PATCH bpf v2 0/2] Keep dynamic inner array lookups nullable

[Jiri Olsa <olsajiri@gmail.com>]

On Sun, Jun 07, 2026 at 09:24:12PM +0800, Nuoqi Gui wrote:
> An ARRAY_OF_MAPS can use an array created with BPF_F_INNER_MAP as its
> inner map template. The flag allows a concrete inner array with a
> different max_entries value to replace the template.
> 
> The verifier currently uses the template's max_entries to elide
> nullness for a constant-key lookup through the inner map pointer. At
> runtime, the lookup uses the concrete inner array's max_entries instead.
> The verifier can therefore accept an unchecked dereference even though
> the runtime helper returns NULL.
> 
> Patch 1 keeps lookups through BPF_F_INNER_MAP array templates nullable.
> Patch 2 adds a verifier regression test for the unchecked dereference.
> 
> Before the fix, the regression program is accepted and the runtime
> reproducer triggers a NULL dereference. With the fix, both programs are
> rejected with an invalid map_value_or_null access.
> 
> Tested by compiling kernel/bpf/verifier.o and
> verifier_map_in_map.bpf.o, and by running the regression program and
> runtime reproducer in QEMU before and after the fix.
> 
> Signed-off-by: Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
> ---
> v1->v2:
> - Update the can_elide_value_nullness() comment to match the changed
>   parameter (const struct bpf_map *map).

Acked-by: Jiri Olsa <jolsa@kernel.org>

jirka

> 
> v1: https://patch.msgid.link/20260604151153.2488051-1-gnq25@mails.tsinghua.edu.cn
> 
> To: Alexei Starovoitov <ast@kernel.org>
> To: Daniel Borkmann <daniel@iogearbox.net>
> To: Andrii Nakryiko <andrii@kernel.org>
> Cc: Daniel Xu <dxu@dxuuu.xyz>
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> Cc: Song Liu <song@kernel.org>
> Cc: Yonghong Song <yonghong.song@linux.dev>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Ihor Solodrai <isolodrai@meta.com>
> Cc: bpf@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-kselftest@vger.kernel.org
> 
> ---
> Nuoqi Gui (2):
>       bpf: Keep dynamic inner array lookups nullable
>       selftests/bpf: Cover dynamic inner array lookup nullability
> 
>  kernel/bpf/verifier.c                              | 15 ++++----
>  .../selftests/bpf/progs/verifier_map_in_map.c      | 40 ++++++++++++++++++++++
>  2 files changed, 49 insertions(+), 6 deletions(-)
> ---
> base-commit: e7ae89a0c97ce2b68b0983cd01eda67cf373517d
> change-id: 20260606-f01-v2-324fb92185a2
> 
> Best regards,
> --  
> Nuoqi Gui <gnq25@mails.tsinghua.edu.cn>
>