linux-kernel - Re: [PATCH] arm64: bpf: fix signedness bug in loading 64-bit immediate

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Date:	Fri, 8 May 2015 01:45:59 -0700
From:	Xi Wang <xi.wang@...il.com>
To:	Will Deacon <will.deacon@....com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Zi Shen Lim <zlim.lnx@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Catalin Marinas <Catalin.Marinas@....com>
Subject: Re: [PATCH] arm64: bpf: fix signedness bug in loading 64-bit immediate

On Fri, May 8, 2015 at 1:38 AM, Will Deacon <will.deacon@....com> wrote:
>> -             imm64 = (u64)insn1.imm << 32 | imm;
>> +             imm64 = ((u64)(u32)insn1.imm) << 32 | (u64)(u32)imm;
>
> This seems a bit convoluted to me. Don't you just need to add a (u32)
> cast to imm and that's it? The (u64)(u32) looks redundant.

You're right -  the second (u64) is redundant; the hope was to make
the code easier to understand.  It's from the interpreter code in
kernel/core/bpf.c, which uses (u64)(u32) as well.

>> -                     BPF_ALU64_IMM(BPF_MOV, R0, 1),
>> +                     BPF_LD_IMM64(R0, 0x1ffffffffLL),
>> +                     BPF_ALU64_IMM(BPF_RSH, R0, 32), /* R0 = 1 */
>>                       BPF_EXIT_INSN(),
>
> This hunk should probably be a separate patch, unless you get Alexei's ack
> for me to take it via the arm64 tree too.

I would be happy to split this into a separate patch if that works
better, or simply drop this part.

- xi
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