SIMD-0178: SBPF Static Syscalls #178
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LucasSte
changed the title
| The opcode `0x9D` must represent the return instruction, which supersedes the | ||
| `exit` instruction. The opcode (opcode `0x95`), previously assigned to the | ||
| `exit` instruction, must now be interpreted as the new syscall instruction. |
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What is the motivation behind changing this?
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Also, changing the name from exit to return when it is the same instruction could be confusing. I have already seen this confused in other SIMDs.
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Side note - we should bundle large sets of proposed ISA changes together into the same SBPF version upgrade, so that clients don't have to support a mis-mash of ISAs based on feature flags. I believe this is the intent of #161, but just re-iterating 🙏
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Motivation is that exit was occupying the slot in the instruction class for controlflow with immediate values and it does not take an immediate value. The new syscall opcode however does, so it took its place.
| ## Detailed Design | ||
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| The following must go into effect if and only if a program indicates the SBPF | ||
| version XX or higher in its ELF header e_flags field, according to the |
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Should we specify which version XX is?
| The resolution of syscalls during ELF loading requires relocating addresses, | ||
| which is a performance burden for the validator. Relocations require an entire | ||
| copy of the ELF file in memory to either relocate addresses we fetch from the | ||
| symbol table or offset addresses to after the start of the virtual machine’s | ||
| memory. Moreover, relocations pose security concerns, as they allow the | ||
| arbitrary modification of program headers and programs sections. A new | ||
| separate opcode for syscalls modifies the behavior of the ELF loader, allowing | ||
| us to resolve syscalls without relocations. |
| phase. `call imm` (opcode `0x85`) instructions must only refer to internal | ||
| calls and its immediate field must only be interpreted as a relative address | ||
| to jump from the program counter. |
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Does this mean there is no longer a need to hash the immediates?
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It does and is the intention.
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