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Document why the tiny example gets a ud2 now. #166

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May 15, 2025
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Document why the tiny example gets a ud2 now. #166

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28 changes: 20 additions & 8 deletions example-crates/tiny/README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -181,20 +181,27 @@ does take a few extra bytes.
With all these optimizations, the generated code looks like this:

```asm
00000000004000b0 <.text>:
4000b0: 48 89 e7 mov %rsp,%rdi
4000b3: 55 push %rbp
4000b4: e9 00 00 00 00 jmp 0x4000b9
4000b9: 6a 2a push $0x2a
4000bb: 5f pop %rdi
4000bc: b8 e7 00 00 00 mov $0xe7,%eax
4000c1: 0f 05 syscall
00000000002000cc <.text>:
2000cc: 48 89 e7 mov %rsp,%rdi
2000cf: 55 push %rbp
2000d0: e9 00 00 00 00 jmp 0x2000d5
2000d5: 6a 2a push $0x2a
2000d7: 5f pop %rdi
2000d8: b8 e7 00 00 00 mov $0xe7,%eax
2000dd: 0f 05 syscall
2000df: 0f 0b ud2
```

Those first 3 instructions are origin's `_start` function. The next 5
instructions are `origin::program::entry` and everything, including the user
`origin_main` function and the `exit_group` syscall inlined into it.

Even though we added "trap-unreachable=no", we still have a ud2 instruction
after the syscall. It's added by rustix because in theory users could run
the program under a seccomp configuration in which `exit_group` does return,
and rustix needs to be completely sure that execution won't fall through into
whatever instructions happen to appear next in memory.

## Optimizations not done

In theory this code be made even smaller.
Expand Down Expand Up @@ -233,6 +240,11 @@ saving 2 bytes. In theory origin could have a feature to enable this, however
it's a very minor optimization, and it would introduce undefined behavior if
somehow some thread got created outside of origin, so I chose not to add it.

We could also add an option to rustix to have it omit the `ud2` after the
`exit_group` syscall for users willing to promise that they won't run the
program under a pathological seccomp configuration, however it'd only save
2 bytes in an uncommon situation.

## Sources

Many of these optimizations came from the following websites:
Expand Down