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Zydis formats extra operand-size suffix for VSIB instructions #550

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recoules opened this issue Feb 11, 2025 · 4 comments · May be fixed by #551
Open

Zydis formats extra operand-size suffix for VSIB instructions #550

recoules opened this issue Feb 11, 2025 · 4 comments · May be fixed by #551
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A-formatter Area: Formatter C-bug Category: This is a bug (or a fix for a bug, when applied to PRs)

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@recoules
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Dear developers,

The FormatterATT module appends the operand-size suffix for instruction using VSIB.

For instance, the opcode c42289914c3afc is disassembled as vpgatherqqq %xmm14, -0x04(%rdx,%xmm15,1), %xmm9.

Yet, in case of vpgather, the operand-size is already part of the mnemonic. The extra suffix makes the printed disassembly invalid.
I am not sure, but I think it is the same for other mnemonics using a VSIB operand.

Would it make sense to remove the second case here ?
https://github.com/zyantific/zydis/blob/bffbb610cfea643b98e87658b9058382f7522807/src/FormatterATT.c#L306C14-L306C15

Regards,
@flobernd
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Hi @recoules, good catch!

This could be a potential fix, but we have to check all VSIB instructions first to be sure this doesn't break anything (aka making sure that the mnemonics always include the size suffix natively).

@mappzor
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mappzor commented Feb 12, 2025

This could be a potential fix, but we have to check all VSIB instructions first to be sure this doesn't break anything (aka making sure that the mnemonics always include the size suffix natively).

This seems to be the case. At least for now all VSIB instructions have mandatory OSIZE prefix.

@flobernd
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Thanks for checking @mappzor 🙂

Do you want to create a PR @recoules ?

@recoules
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recoules commented Feb 12, 2025 •
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Thanks for reactive responses :)

Do you want to create a PR @recoules ?

Yes I can (but will have to wait this evening or tomorrow)

recoules added a commit to recoules/zydis that referenced this issue Feb 12, 2025
@recoules
Fix ATT Formatter VSIB extra operand-size suffix (zyantific#550)
20f013a
@recoules recoules linked a pull request Feb 12, 2025 that will close this issue
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@flobernd flobernd added C-bug Category: This is a bug (or a fix for a bug, when applied to PRs) A-formatter Area: Formatter labels Mar 5, 2025
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Fix ATT Formatter VSIB extra operand-size suffix recoules/zydis
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@flobernd @recoules @mappzor