linux-kernel - Re: [PATCH] bpf, btf: Make if test explicit to fix Coccinelle error

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Date: Mon, 24 Jun 2024 16:08:13 -0700
From: Thorsten Blum <thorsten.blum@...lux.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: martin.lau@...ux.dev,
 ast@...nel.org,
 daniel@...earbox.net,
 andrii@...nel.org,
 song@...nel.org,
 john.fastabend@...il.com,
 kpsingh@...nel.org,
 sdf@...ichev.me,
 haoluo@...gle.com,
 jolsa@...nel.org,
 yonghong.song@...ux.dev,
 bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bpf, btf: Make if test explicit to fix Coccinelle error

On 24. Jun 2024, at 13:16, Eduard Zingerman <eddyz87@...il.com> wrote:
> On Mon, 2024-06-24 at 21:54 +0200, Thorsten Blum wrote:
>> Explicitly test the iterator variable i > 0 to fix the following
>> Coccinelle/coccicheck error reported by itnull.cocci:
>> 
>> ERROR: iterator variable bound on line 4688 cannot be NULL
>> 
>> Compile-tested only.
>> 
>> Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>
>> ---
>> kernel/bpf/btf.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>> 
>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>> index 821063660d9f..7720f8967814 100644
>> --- a/kernel/bpf/btf.c
>> +++ b/kernel/bpf/btf.c
>> @@ -4687,7 +4687,7 @@ static void btf_datasec_show(const struct btf *btf,
>>    __btf_name_by_offset(btf, t->name_off));
>> for_each_vsi(i, t, vsi) {
>> var = btf_type_by_id(btf, vsi->type);
>> - if (i)
>> + if (i > 0)
>> btf_show(show, ",");
>> btf_type_ops(var)->show(btf, var, vsi->type,
>> data + vsi->offset, bits_offset, show);
> 
> Could you please elaborate a bit?
> Here is for_each_vsi is defined:
> 
> #define for_each_vsi(i, datasec_type, member) \
> for (i = 0, member = btf_type_var_secinfo(datasec_type); \
>     i < btf_type_vlen(datasec_type); \
>     i++, member++)
> 
> Here it sets 'i' to zero for the first iteration.
> Why would the tool report that 'i' can't be zero?

Coccinelle thinks i can't be a NULL pointer (not the number zero). It's
essentially a false-positive warning, but since there are only 4 such
warnings under kernel/, I thought it would be worthwhile to remove some
of them by making the tests explicit.