linux-kernel - Re: [PATCH v4 net-next] arm: eBPF JIT compiler

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Date:   Tue, 22 Aug 2017 17:25:54 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Shubham Bansal <illusionist.neo@...il.com>, linux@...linux.org.uk,
        davem@...emloft.net
CC:     netdev@...r.kernel.org, ast@...com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        keescook@...omium.org
Subject: Re: [PATCH v4 net-next] arm: eBPF JIT compiler

On 08/22/2017 05:08 PM, Daniel Borkmann wrote:
> On 08/22/2017 08:36 AM, Shubham Bansal wrote:
> [...]
>> +
>> +static int out_offset = -1; /* initialized on the first pass of build_body() */
>
> Hm, why is this a global var actually? There can be
> multiple parallel calls to bpf_int_jit_compile(), we
> don't take a global lock on this. Unless I'm missing
> something this should really reside in jit_ctx, no?

Hm, okay, it's for generating the out jmp offsets in
tail call emission which are supposed to always be the
same relative offsets; should be fine then.

> Given this is on emit_bpf_tail_call(), did you get
> tail calls working the way I suggested to test?
>
>> +static int emit_bpf_tail_call(struct jit_ctx *ctx)
>>   {
> [...]
>> +    const int idx0 = ctx->idx;
>> +#define cur_offset (ctx->idx - idx0)
>> +#define jmp_offset (out_offset - (cur_offset))
> [...]
>> +
>> +    /* out: */
>> +    if (out_offset == -1)
>> +        out_offset = cur_offset;
>> +    if (cur_offset != out_offset) {
>> +        pr_err_once("tail_call out_offset = %d, expected %d!\n",
>> +                cur_offset, out_offset);
>> +        return -1;
>> +    }
>> +    return 0;
>> +#undef cur_offset
>> +#undef jmp_offset
>>   }