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

[Brian Gerst]

On Tue, Apr 21, 2020 at 3:32 PM 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.

You should explain the reason it is invalid, ie. the instruction
encoding needs a 32-bit immediate but the current code only emits an
8-bit immediate.

--
Brian Gerst