Messages in this thread | | | From | Alexei Starovoitov <> | Date | Fri, 19 May 2023 11:03:05 -0700 | Subject | Re: [PATCH v2] bpf: Fix mask generation for 32-bit narrow loads of 64-bit fields |
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On Thu, May 18, 2023 at 3:25 AM Will Deacon <will@kernel.org> 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@kernel.org> > Cc: Daniel Borkmann <daniel@iogearbox.net> > Cc: John Fastabend <john.fastabend@gmail.com> > Cc: Krzesimir Nowak <krzesimir@kinvolk.io> > Cc: Andrey Ignatov <rdna@fb.com> > Acked-by: Yonghong Song <yhs@fb.com> > Fixes: 31fd85816dbe ("bpf: permits narrower load from bpf program context fields") > Signed-off-by: Will Deacon <will@kernel.org>
Thanks for the fix! Applied to bpf tree.
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