Messages in this thread | | | Subject | Re: [PATCH bpf 1/2] bpf, x32: Fix invalid instruction in BPF_LDX zero-extension | From | "H. Peter Anvin" <> | Date | Tue, 21 Apr 2020 10:39:00 -0700 |
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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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