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Date: Tue, 21 Apr 2020 10:39:00 -0700 From: "H. Peter Anvin" <hpa@...or.com> To: Luke Nelson <lukenels@...washington.edu>, bpf@...r.kernel.org Cc: Luke Nelson <luke.r.nels@...il.com>, Xi Wang <xi.wang@...il.com>, Wang YanQing <udknight@...il.com>, "David S. Miller" <davem@...emloft.net>, Alexey Kuznetsov <kuznet@....inr.ac.ru>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, x86@...nel.org, Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, Martin KaFai Lau <kafai@...com>, Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>, Andrii Nakryiko <andriin@...com>, John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...omium.org>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH bpf 1/2] bpf, x32: Fix invalid instruction in BPF_LDX zero-extension On 2020-04-21 10:15, Luke Nelson wrote: > The current JIT uses the following sequence to zero-extend into the > upper 32 bits of the destination register for BPF_LDX BPF_{B,H,W}, > when the destination register is not on the stack: > > EMIT3(0xC7, add_1reg(0xC0, dst_hi), 0); > > However, this is not a valid instruction on x86. > > This patch fixes the problem by instead emitting "xor dst_hi,dst_hi" > to clear the upper 32 bits. x32 is not x86-32. In Linux we generally call the latter "i386". 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. -hpa
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