linux-kernel - Re: [PATCH] bpf: Fix mask generation for 32-bit narrow loads of 64-bit fields

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Date:   Fri, 5 May 2023 09:34:21 -0700
From:   Yonghong Song <yhs@...a.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Will Deacon <will@...nel.org>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Krzesimir Nowak <krzesimir@...volk.io>,
        Yonghong Song <yhs@...com>, Andrey Ignatov <rdna@...com>
Subject: Re: [PATCH] bpf: Fix mask generation for 32-bit narrow loads of
 64-bit fields



On 5/5/23 8:30 AM, Alexei Starovoitov wrote:
> On Thu, May 4, 2023 at 1:18 PM Yonghong Song <yhs@...a.com> wrote:
>>
>>
>>
>> On 5/2/23 9:57 AM, Will Deacon wrote:
>>> A narrow load from a 64-bit context field results in a 64-bit load
>>> followed potentially by a 64-bit right-shift and then a bitwise AND
>>> operation to extract the relevant data.
>>>
>>> In the case of a 32-bit access, an immediate mask of 0xffffffff is used
>>> to construct a 64-bit BPP_AND operation which then sign-extends the mask
>>> value and effectively acts as a glorified no-op.
>>>
>>> Fix the mask generation so that narrow loads always perform a 32-bit AND
>>> operation.
>>>
>>> Cc: Alexei Starovoitov <ast@...nel.org>
>>> Cc: Daniel Borkmann <daniel@...earbox.net>
>>> Cc: John Fastabend <john.fastabend@...il.com>
>>> Cc: Krzesimir Nowak <krzesimir@...volk.io>
>>> Cc: Yonghong Song <yhs@...com>
>>> Cc: Andrey Ignatov <rdna@...com>
>>> Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
>>> Signed-off-by: Will Deacon <will@...nel.org>
>>
>>
>> Thanks for the fix! You didn't miss anything. It is a bug and we did not
>> find it probably because user always use 'u64 val = ctx->u64_field' in
>> their bpf code...
>>
>> But I think the commit message can be improved. An example to show the
>> difference without and with this patch can explain the issue much better.
>>
>> Acked-by: Yonghong Song <yhs@...com>
> 
> If I'm reading it correctly it's indeed a bug.
> alu64(and, 0xffffFFFF) is a nop
> but it should have been
> alu32(and, 0xffffFFFF) which will clear upper 32-bit, right?

Right. This is my understanding as well.

> Would be good to have a selftest for this.