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Follow-up for masks on shift operations #105

Merged
merged 2 commits into from
Mar 15, 2024
Merged

Follow-up for masks on shift operations #105

merged 2 commits into from
Mar 15, 2024

Commits on Mar 15, 2024

  1. tests/ubpf_vm.rs: Add tests for arsh offset masking 

    Add tests to make sure we handle the masking of the value (in the source
register or as an immediate) telling by how many bits the value in the
destination register should be shifted.

The context is that we used to do no masking on this value, but the BPF
ISA specification does require one, and the kernel implements it when
the value is passed in the source register (and rejects immediates that
it deems invalid).

Signed-off-by: Quentin Monnet <qmo@qmon.net>
    @qmonnet
    qmonnet committed Mar 15, 2024
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  2. src/interpreter: Remove/explain masks for 32-bit shift operations 

    We recently fixed the 64-bit arithmetic shift-right operations by adding
a mask to the number of bits (as immediates or from the source register)
by which to shift, as per the eBPF ISA specification. The 32-bit
operations must use the same mask, but .wrapping_shr() already takes
care of it for us. Let's add a comment to make it explicit.

As it turns out, masking is just as well unnecessary for the
non-arithmetic left-right shift operations that we tried to fix
recently. Let's also remove the mask there.

Signed-off-by: Quentin Monnet <qmo@qmon.net>
    @qmonnet
    qmonnet committed Mar 15, 2024
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