Stream: git-wasmtime

Topic: wasmtime / Issue #2409 Masking of shift amounts

view this post on Zulip Wasmtime GitHub notifications bot (Nov 12 2020 at 22:21):

abrown opened Issue #2409:

As discussed in https://github.com/bytecodealliance/wasmtime/pull/2397#discussion_r522410217, the Cranelift semantics for shift amounts must be masked to the number of bits that are present in the value to be shifted; e.g. if we try to shift a 32-bit integer by 33 we would actually shift by 33 mod 32 = 1. Currently, these semantics (which may the Wasm semantics) are enforced in the translation from Wasm to CLIF (i.e. code_translator.rs`) and not by the compiler backends. This means that users of Cranelift are able to use shifts with the incorrect semantics.

My proposed fix is to remove the masking from code_translator.rs and add it directly to the x64 backend--might need some help for the other backends (cc: @akirilov-arm, @cfallin). Another fix would be to change the semantics, but this seems less desirable so I don't plan to pursue it (let me know in the comments if you disagree!).

view this post on Zulip Wasmtime GitHub notifications bot (Nov 30 2020 at 20:47):

abrown edited Issue #2409:

As discussed in https://github.com/bytecodealliance/wasmtime/pull/2397#discussion_r522410217, the Cranelift semantics for shift amounts must be masked to the number of bits that are present in the value to be shifted; e.g. if we try to shift a 32-bit integer by 33 we would actually shift by 33 mod 32 = 1. Currently, these semantics (which match the Wasm semantics) are enforced in the translation from Wasm to CLIF (i.e. code_translator.rs) and not by the compiler backends. This means that users of Cranelift are able to use shifts with the incorrect semantics.

My proposed fix is to remove the masking from code_translator.rs and add it directly to the x64 backend--might need some help for the other backends (cc: @akirilov-arm, @cfallin). Another fix would be to change the semantics, but this seems less desirable so I don't plan to pursue it (let me know in the comments if you disagree!).

Last updated: Dec 23 2024 at 12:05 UTC