netdev - Re: [PATCH bpf-next] bpf: arm64: fix uninitialized variable

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Message-ID: <68b024dd-4113-8c8a-a606-7b4b0206973d@fb.com>
Date:   Mon, 18 Dec 2017 10:36:47 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>
CC:     Arnd Bergmann <arnd@...db.de>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH bpf-next] bpf: arm64: fix uninitialized variable

On 12/18/17 10:19 AM, Daniel Borkmann wrote:
> On 12/18/2017 07:09 PM, Alexei Starovoitov wrote:
>> From: Alexei Starovoitov <ast@...com>
>>
>> fix the following issue:
>> arch/arm64/net/bpf_jit_comp.c: In function 'bpf_int_jit_compile':
>> arch/arm64/net/bpf_jit_comp.c:982:18: error: 'image_size' may be used
>> uninitialized in this function [-Werror=maybe-uninitialized]
>>
>> Fixes: db496944fdaa ("bpf: arm64: add JIT support for multi-function programs")
>> Reported-by: Arnd Bergmann <arnd@...db.de>
>> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
>> ---
>>  arch/arm64/net/bpf_jit_comp.c | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>> index 396490cf7316..acaa935ed977 100644
>> --- a/arch/arm64/net/bpf_jit_comp.c
>> +++ b/arch/arm64/net/bpf_jit_comp.c
>> @@ -897,6 +897,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>  		image_ptr = jit_data->image;
>>  		header = jit_data->header;
>>  		extra_pass = true;
>> +		image_size = sizeof(u32) * ctx.idx;
>>  		goto skip_init_ctx;
>>  	}
>>  	memset(&ctx, 0, sizeof(ctx));
>>
>
> I don't really mind, but it feels more complex than it needs to be
> imho, since in the initial pass you fetch 'image_size' in fake pass
> from ctx.idx, then we set ctx.idx to 0 again, do another pass and
> use the cached ctx.idx from that second pass instead of the first
> one where we set 'image_size' originally, so we definitely need to
> take that into consideration in future reviews at least.

not sure what you mean.
This check: ctx.idx != jit_data->ctx.idx matters the most.
After first alloc the 'image_size' variable used for dumping only.
That's why the JITing itself worked fine. We could have removed it
since it's computable from idx, but imo it's fine this way.