linux-kernel - Re: [PATCH bpf-next v3 2/2] bpf: Check return from set_memory

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Date: Fri, 15 Mar 2024 11:34:06 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: Kui-Feng Lee <sinquersw@...il.com>
Cc: bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
 Kees Cook <keescook@...omium.org>,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Zi Shen Lim <zlim.lnx@...il.com>,
 Catalin Marinas <catalin.marinas@....com>, Will Deacon <will@...nel.org>,
 "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Kui-Feng Lee <thinker.li@...il.com>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH bpf-next v3 2/2] bpf: Check return from set_memory_rox()

On 3/15/24 11:11 AM, Kui-Feng Lee wrote:
>> --- a/kernel/bpf/bpf_struct_ops.c
>> +++ b/kernel/bpf/bpf_struct_ops.c
>> @@ -742,8 +742,11 @@ static long bpf_struct_ops_map_update_elem(struct bpf_map 
>> *map, void *key,
>>           if (err)
>>               goto reset_unlock;
>>       }
>> -    for (i = 0; i < st_map->image_pages_cnt; i++)
>> -        arch_protect_bpf_trampoline(st_map->image_pages[i], PAGE_SIZE);
>> +    for (i = 0; i < st_map->image_pages_cnt && !err; i++)
>> +        err = arch_protect_bpf_trampoline(st_map->image_pages[i], PAGE_SIZE);
>> +
>> +    if (err)
> 
> nit: Can it be more specific? I mean to check err < 0, so we can reason
> that this function never returns a positive value other than 0.

I think "if (err)" is fine. It is pretty common in other places of the kernel.

Checking "(err < 0)" may actually mean the return value could be positive. At 
least it is how bpf_struct_ops.c is using "(err < 0)".

[ An unrelated side note is another (err < 0) check in bpf_struct_ops.c could 
have been changed after the recent changes in bpf_struct_ops_prepare_trampoline 
which no longer return +val ].