Re: [PATCH bpf 1/2] bpf, x32: Fix invalid instruction in BPF_LDX zero-extension

[hpa]

On April 21, 2020 12:26:12 PM PDT, Xi Wang <xi.wang@xxxxxxxxx> wrote:
>On Tue, Apr 21, 2020 at 10:39 AM H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> x32 is not x86-32.  In Linux we generally call the latter "i386".
>
>Agreed.  Most of the previous patches to this file use "x32" and this
>one just wanted to be consistent.
>
>> C7 /0 imm32 is a valid instruction on i386. However, it is also
>> inefficient when the destination is a register, because B8+r imm32 is
>> equivalent, and when the value is zero, XOR is indeed more efficient.
>>
>> The real error is using EMIT3() instead of EMIT2_off32(), but XOR is
>> more efficient. However, let's make the bug statement *correct*, or
>it
>> is going to confuse the Hades out of people in the future.
>
>I don't see how the bug statement is incorrect, which merely points
>out that "C7 C0 0" is an invalid instruction, regardless of whether
>the JIT intended to emit C7 /0 imm32, B8+r imm32, 31 /r, 33 /r, or any
>other equivalent form.

C7 C0 0 is *not* an invalid instruction, although it is incomplete. It is a different, but arguably even more serious, problem.
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