Re: [PATCH bpf-next v3 2/2] bpf: Check return from set_memory_rox()

From: Martin KaFai Lau <martin.lau@linux.dev>
To: Kui-Feng Lee <sinquersw@gmail.com>
Cc: bpf@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	Kees Cook <keescook@chromium.org>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
	Yonghong Song <yonghong.song@linux.dev>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, Zi Shen Lim <zlim.lnx@gmail.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	David Ahern <dsahern@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Kui-Feng Lee <thinker.li@gmail.com>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>
Subject: Re: [PATCH bpf-next v3 2/2] bpf: Check return from set_memory_rox()
Date: Fri, 15 Mar 2024 11:34:06 -0700	[thread overview]
Message-ID: <c8727ba1-0d8b-4c42-a4b1-e98bed061b22@linux.dev> (raw)
In-Reply-To: <81492d37-47b2-4fca-ba2f-9528c2d41029@gmail.com>

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 ].