Re: [PATCH v2] bpf: Fix mask generation for 32-bit narrow loads of 64-bit fields

[Alexei Starovoitov]

On Thu, May 18, 2023 at 3:25 AM Will Deacon <will@xxxxxxxxxx> wrote:
>
> A narrow load from a 64-bit context field results in a 64-bit load
> followed potentially by a 64-bit right-shift and then a bitwise AND
> operation to extract the relevant data.
>
> In the case of a 32-bit access, an immediate mask of 0xffffffff is used
> to construct a 64-bit BPP_AND operation which then sign-extends the mask
> value and effectively acts as a glorified no-op. For example:
>
> 0:      61 10 00 00 00 00 00 00 r0 = *(u32 *)(r1 + 0)
>
> results in the following code generation for a 64-bit field:
>
>         ldr     x7, [x7]        // 64-bit load
>         mov     x10, #0xffffffffffffffff
>         and     x7, x7, x10
>
> Fix the mask generation so that narrow loads always perform a 32-bit AND
> operation:
>
>         ldr     x7, [x7]        // 64-bit load
>         mov     w10, #0xffffffff
>         and     w7, w7, w10
>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> Cc: Krzesimir Nowak <krzesimir@xxxxxxxxxx>
> Cc: Andrey Ignatov <rdna@xxxxxx>
> Acked-by: Yonghong Song <yhs@xxxxxx>
> Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields")
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>

Thanks for the fix! Applied to bpf tree.