Re: [PATCH bpf] arm: bpf: Fix bugs with ALU64 {RSH, ARSH} BPF_K shift by 0

[Daniel Borkmann]

On 4/8/20 8:12 PM, Luke Nelson wrote:
> The current arm BPF JIT does not correctly compile RSH or ARSH when the
> immediate shift amount is 0. This causes the "rsh64 by 0 imm" and "arsh64
> by 0 imm" BPF selftests to hang the kernel by reaching an instruction
> the verifier determines to be unreachable.
>
> The root cause is in how immediate right shifts are encoded on arm.
> For LSR and ASR (logical and arithmetic right shift), a bit-pattern
> of 00000 in the immediate encodes a shift amount of 32. When the BPF
> immediate is 0, the generated code shifts by 32 instead of the expected
> behavior (a no-op).
>
> This patch fixes the bugs by adding an additional check if the BPF
> immediate is 0. After the change, the above mentioned BPF selftests pass.
>
> Fixes: 39c13c204bb11 ("arm: eBPF JIT compiler")
> Co-developed-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Xi Wang <xi.wang@xxxxxxxxx>
> Signed-off-by: Luke Nelson <luke.r.nels@xxxxxxxxx>

Yikes, thanks for fixing, applied. Looks like noone was running BPF kselftests
on arm32 for quite a while. :(