Re: [PATCH bpf v3 2/5] bpf: Fix NULL-ptr-deref when showing a void BTF type

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 8/31/26 4:01 AM, Jiayuan Chen wrote:
> btf_modifier_show() resolves the modifier and then calls
> btf_type_ops(t)->show() unconditionally. For the void type (type_id 0,
> BTF_KIND_UNKN) kind_ops[] has no entry, so ->show is NULL.
> 
> The map dump path cannot reach a void type (a map key/value must have a
> size and void has none), but bpf_snprintf_btf() takes a type_id straight
> from the BPF program, and a "const void" (a modifier that resolves to
> void, present in the vmlinux BTF) NULL-derefs there:
> 
> KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
> RIP: 0010:btf_modifier_show (kernel/bpf/btf.c:2914)
> Call Trace:
>  <TASK>
>  btf_type_show (kernel/bpf/btf.c:8251)
>  btf_type_snprintf_show (kernel/bpf/btf.c:8321)
>  bpf_snprintf_btf (kernel/trace/bpf_trace.c:1047)
>  bpf_prog_test_run_raw_tp (net/bpf/test_run.c:829)
>  __sys_bpf (kernel/bpf/syscall.c:4804)
>  do_syscall_64 (arch/x86/entry/syscall_64.c:94)
>  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
>  </TASK>
> 
> void has no size, so there is nothing to render - not even a byte length
> to fall back to dumping as raw hex. btf_df_show() is already the ->show
> for the other kinds that carry no value to print - FWD, FUNC, FUNC_PROTO,
> FLOAT and DECL_TAG - and emits an "<unsupported kind:N>" placeholder;
> void belongs to the same group and only lacks a show op because it has
> no kind_ops[] entry at all. Fall back to btf_df_show() in
> btf_modifier_show() when the resolved type has no show op, so a void type
> prints that placeholder instead of crashing; bpf_snprintf_btf() then
> returns the length as usual.

nit: The explanation is a bit too verbose, I'd try simplifying. And the
comment in the code as well.

> 
> Fixes: c4d0bfb45068 ("bpf: Add bpf_snprintf_btf helper")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  kernel/bpf/btf.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 91b8ce77f699..6e267c4c4e1f 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -2911,7 +2911,15 @@ static void btf_modifier_show(const struct btf *btf,
>  	else
>  		t = btf_type_skip_modifiers(btf, type_id, NULL);
>  
> -	btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);
> +	/*
> +	 * A modifier can resolve to the void type (e.g. "const void"), which
> +	 * has no show op (kind_ops[BTF_KIND_UNKN] is NULL). Print a placeholder
> +	 * instead of dereferencing NULL.
> +	 */
> +	if (!btf_type_ops(t))
> +		btf_df_show(btf, t, type_id, data, bits_offset, show);
> +	else
> +		btf_type_ops(t)->show(btf, t, type_id, data, bits_offset, show);

Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>

>  }
>  
>  static void btf_var_show(const struct btf *btf, const struct btf_type *t,