RFR: 8354668: Missing REX2 prefix accounting in ZGC barriers leads to incorrect encoding

[Dean Long]

On Tue, 15 Apr 2025 13:50:40 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> ZGC bookkeeps multiple place holders in barrier code snippets through relocations, these are later used to patch appropriate contents (mostly immediate values) in instruction encoding. While most of the relocation records the patching offsets from the end of the instruction, SHL instruction, which is used for pointer coloring, computes the patching offset from the starting address of the instruction.
> 
> Thus, in case the destination register operand of SHL instruction is an extended GPR register, we miss accounting additional REX2 prefix byte in patch offset, thereby corrupting the encoding since runtime patches the primary opcode byte resulting into ILLEGAL instruction exception.
> 
> This patch fixes reported failures by computing the relocation offset of SHL instruction from end of instruction, thereby making the patch offset agnostic to REX/REX2 prefix.
> 
> Please review and share your feedback.
> 
> Best Regards,
> Jatin
> 
> PS: Validation were performed using latest Intel Software Development Emulator after modifying static register allocation order in x86_64.ad file giving preference to EGPRs.

This looks OK, but we could do better.  Instead of making the relocation point to the end of the instruction and then looking up the offset with patch_barrier_relocation_offset(), why not make the offset always 0 and have the relocation point to the data offset inside the instruction?

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PR Comment: https://git.openjdk.org/jdk/pull/24664#issuecomment-2807988702