diff --git a/HANDOFF-x86-aggregates.md b/HANDOFF-x86-aggregates.md index 1dc9a9b..17d41a2 100644 --- a/HANDOFF-x86-aggregates.md +++ b/HANDOFF-x86-aggregates.md @@ -4,46 +4,156 @@ Branch `dev-loop`, from `957ba07`. This closes item 4 of `HANDOFF-x86-redef.md`: takes and returns a struct, which is the one case the two backends' conventions disagree about and the reason `X86.redefinition` exists at all rather than reusing `Emit.redefinition`. -*Status: in progress. This file is written early and on purpose, so that a lane that picks the work up mid-way -has the plan and not just the diff.* +**The claim is now measured and it holds.** An `--x86` host dlopening `--x86` modules gets the right answer +when four redefined functions each take a struct and return one. The same fixture, built and reloaded entirely +through LLVM, prints the same transcript. And the crossed pair — an `--x86` host given LLVM-built modules — +dies with SIGSEGV on the first such call, which is the measurement the "same-convention-by-construction" +argument was standing in for. -## The plan +## What was built -1. A fixture pair, `test/programs/reload-agg.flan` and `reload-agg-v2.flan`, beside the existing `reload.flan` - rather than inside it — `reload.flan`'s transcript is shared with the LLVM path and must not move. - No `main`; the host is `test/reload_host.c` unchanged, which wants `outer : () -> i64` and a `counter` - global. Everything aggregate happens *inside* Flan, between the host's compiled-once call site and the - redefined body, because the C boundary is where the two conventions are required to agree and is not what - is under test. -2. Four struct shapes, because SysV treats them in four different ways and `x86.ml` treats all four the same: - - `≤16 bytes, all integer` — SysV passes in `rdi`:`rsi` and returns in `rax`:`rdx`; `x86.ml` passes a - pointer and returns through a hidden `sret`. Diverges in both directions. - - `>16 bytes` — SysV copies the argument onto the stack, which `x86.ml` does not; the *return*, though, - is a hidden pointer in the first integer register for SysV too, so that half may well coincide. Worth - knowing, and worth not assuming either way. - - `{f64, f64}` — SysV classifies both eightbytes SSE and uses `xmm0`:`xmm1`. - - `{i64, f64}` — one INTEGER and one SSE eightbyte, so `rax` and `xmm0`, which is a third register pattern - again. -3. Every field weighted by position in the printed answer (`a + 100*b + 10000*c + …`), so that a scrambled - field order, a wrong base pointer or a half-loaded eightbyte prints a visibly wrong integer rather than the - right one by symmetry. A test whose answer is a symmetric sum of the fields passes under a convention - mismatch that merely permutes them. -4. Run it on both backends: the LLVM `Emit.redefinition` path as the control, and the `X86.redefinition` path - as the claim. The same transcript from both is the measurement. - -## Open questions recorded rather than asked - -- Whether the MEMORY-return half of the matrix genuinely coincides between the two conventions, or only looks - as though it should. Measured by the mismatch spike rather than argued. - -## Baseline - -| | before | +| file | what | |---|---| -| `spike/x86/survey.sh` | 99 MATCH / 0 DIFFER / 0 refused | -| `spike/x86/cells.sh` | 4/4 ok | -| `dune test --root .` | exit 0 | +| `test/programs/reload-agg.flan` | the fixture. No `main`, so `test/reload_host.c` is the host, unchanged | +| `test/programs/reload-agg-v2.flan` | the same four signatures with different arithmetic | +| `test/test_reload.ml` | the aggregate section, run on both backends against one transcript, plus the two refusal checks | +| `lib/build.ml` — `shared`, `shared_x86` | a symmetric refusal when the option record names the other backend. Two `failwith`s and their comments; nothing else in the file moved | -Note in advance: `survey.sh` globs `test/programs/*.flan`, so each new no-`main` fixture adds one to its -`no-main` skip count. That number moving is the fixtures existing and is not a regression; the MATCH count is -what must not move. +Nothing in `lib/x86.ml` was touched. + +## The fixture, and why it is shaped this way + +Everything aggregate happens *inside* Flan, between the host's compiled-once call site and the redefined body. +The C boundary stays scalar on purpose: that is where the two conventions are required to agree, `check.ml` +already refuses an aggregate in a `declare` signature, and it is not what is under test. `outer` returns `i64` +and `counter` is `i64`, so `reload_host.c` needed no change at all. + +Four struct shapes, because SysV treats them four different ways and `x86.ml` treats all four the same: + +- `Pair` — two INTEGER eightbytes; SysV passes in `rdi`:`rsi` and returns in `rax`:`rdx`. +- `Quad` — thirty-two bytes, so MEMORY. +- `Duo` — two SSE eightbytes, `xmm0`:`xmm1`. +- `Mix` — one INTEGER and one SSE. + +Each of the four redefined `step-*` takes its struct and returns it, so a single call crosses the boundary in +both directions. Each calls a `weigh-*` the module does *not* define, which hands an aggregate the other way — +module to host — and doubles as the tripwire `reload.flan`'s `helper` is: v2's text for all four `weigh` +functions multiplies by ten, and since a module declares a sibling rather than defining it, that text has to be +dead. A module that grew its own copy would print 12411 where the expected transcript says 1611. + +Every field is weighted by position in the answer (`a + 100b + 10000c + 1000000d`) rather than summed. A +symmetric sum would print the right number under a convention mismatch that merely permuted the fields, which +is the way a test like this passes while proving nothing. + +The `defstruct` blocks are byte-identical between the two files and must stay that way. Layout is computed per +module, so a field reordered in v2 would make host and module disagree about offsets — a real bug, but one +wearing this test's clothes and indistinguishable in the transcript from the thing the fixture is for. + +The arithmetic in the test is derived in a comment rather than read off a run, and both backends are held to +the same string. The LLVM row is not decoration: a wrong expected number would otherwise be indistinguishable +from a backend that is right, and two independently-built agreements on one transcript are what rule that out. + +## The transcript + +``` +a1 +host 54063108 +a1 +after1 54063108 +a2 +after2 104337044 +counter 12 +``` + +Identical from `Emit.redefinition` + `Build.shared` and from `X86.redefinition` + `Build.shared_x86`. +`counter` is stepped from inside the redefined body — by one in v1, by ten in v2 — so 1 + 1 + 10 = 12 is the +host's global written by three different bodies in turn. + +## The mismatch, measured + +The same run crossed: an `--x86` host, LLVM-built modules. + +``` +FAIL cross aggregate reload + got: "a1\nhost 54063108\na1\n" (exit 139) + wanted: "a1\nhost 54063108\na1\nafter1 54063108\na2\nafter2 104337044\ncounter 12\n" +``` + +Exit 139 is SIGSEGV. It dies on the first call into a redefined aggregate body, before `after1` is printed. + +Note what this does and does not say. It says the crossed pair is fatal. It does **not** give a per-shape +breakdown: `step-pair` is called first and kills the process, so `step-quad`, `step-duo` and `step-mix` never +run under the crossed pair at all. Whether the MEMORY-return half of the matrix happens to coincide between the +two conventions is still unmeasured, and would need four crossed runs isolating one `step` at a time. Nothing +in this lane needs the answer; the four shapes earn their place as *all four cross correctly on a matched +pair*, which is what was asked for. + +This case is not in `dune test`. It is undefined behaviour, and what it prints is a property of whichever LLVM +is installed; a test that pinned it would be pinning the shape of a crash. + +## The serious finding: nothing refuses a mismatched pair, and the CLI can build one + +There is no check anywhere that a redefinition module was built by the same backend as its host. +`Build.shared` and `Build.shared_x86` are unrelated functions, neither looked at `opts.x86`, and the loaded +object carries no mark saying which backend made it. + +**It is reachable from the CLI, by a user doing nothing unusual.** `bin/main.ml:441` reads `--x86` for +`flan build` only. `flan reload` (`bin/main.ml:506`–523) hardcodes `Build.shared` with +`{ default with dev = true; debug }` and has no `--x86` spelling at all. So: + +``` +flan build game.flan --x86 --dev -o game # an --x86 host, with cells +flan reload game.flan changed.flan -o v2.so # an LLVM module, silently +``` + +and the agent in `vendor/agent/flan_agent.c` dlopens `v2.so` into that host. If any redefined function in +`changed.flan` takes or returns a struct, that is the exit-139 above, at a call site, in a running game, with +nothing anywhere having said a word. + +### What was done about it + +`Build.shared` now refuses an option record with `x86` set, and `Build.shared_x86` refuses one without it, each +naming the reason. Both refusals are asserted in `test/test_reload.ml`. + +**That guard does not close the hole, and it was re-measured after landing to be sure.** The crossed run above +passes both refusals — it hands `Build.shared` an honest LLVM option record and simply loads the result into an +x86 host — and it still segfaults. The guard catches a caller holding *one* option record and reaching for the +wrong builder, which is the accident a lane wiring item 3 would make. It cannot catch a caller holding two, and +`flan reload` is exactly that caller. + +### The recommended complete fix, not done here + +A marker symbol. `X86.program` defines `flan.abi.x86` and `X86.redefinition` emits an undefined reference to +it; `Emit.program` defines `flan.abi.llvm` and `Emit.redefinition` references that. A crossed pair then fails +at `dlopen` with an undefined-symbol message naming the ABI, before a single instruction of the new body runs — +the loader refusing the pair rather than the processor refusing it at a call. It costs one symbol in each of +four functions and nothing at run time. + +It was not done here because it needs edits in `lib/x86.ml`, which two other lanes are in, and because a +refusal at the wrong moment is worse than a loud one slightly later. It should be done next, together with item +3 of `HANDOFF-x86-redef.md` — when `flan dev` and `flan reload` learn to choose host and module backend +together, the marker is what makes the choice checkable rather than merely intended. + +## Baseline, before and after + +| | before | after | +|---|---|---| +| `spike/x86/survey.sh` | 99 MATCH / 0 DIFFER / 0 REFUSED / 0 NOX86 | **99 / 0 / 0 / 0** | +| skip breakdown | 28 does-not-compile / 6 no-main / 2 runs-forever | 28 / **8** / 2 | +| `spike/x86/cells.sh` | 4/4 ok | **4/4 ok** | +| `dune test --root .` | exit 0 | **exit 0** | + +The `no-main` count moving from 6 to 8 is the two new fixtures existing: `survey.sh` globs +`test/programs/*.flan`, and a program with no `main` fails the LLVM link and is classified there, before the +x86 build is attempted. The MATCH count is what must not move, and it did not. + +One thing to know if you re-run the survey to check this: the shell expands that glob once, when the loop +starts. A survey launched before the fixtures were written will report 6 and look like a contradiction. + +## What remains + +- The marker symbol above, and item 3 of `HANDOFF-x86-redef.md` with it. +- A per-shape crossed measurement, if anyone ever wants to know *which* of the four disagree and how. Four runs + isolating one `step` at a time. Nothing depends on it. +- Items 1, 2 and 5 of `HANDOFF-x86-redef.md`, untouched: the new-name path, the transient thunk, and items 2–7 + of `HANDOFF-x86-rt.md` §6. diff --git a/lib/build.ml b/lib/build.ml index e0726d3..f9d35e5 100644 --- a/lib/build.ml +++ b/lib/build.ml @@ -887,12 +887,16 @@ let shared ?(opts = default) ~ir ~out () : timing = option record is where the backend choice lives, so a builder handed the other backend's record refuses by name rather than producing that object. - This is a guard and not a proof: a caller holding two option records can - still pick the wrong one, and the loaded object carries no mark saying - which backend made it. The complete answer is a marker symbol the host - defines and a module references, so the loader refuses the pair at dlopen - rather than the processor refusing it at a call. See - HANDOFF-x86-aggregates.md. *) + Be clear about how little this catches, because a guard that reads wider + than it is is worse than none. It catches a caller holding one option + record and reaching for the wrong builder. It does not catch a caller + holding two and picking the wrong one — the crossed pair that was measured + segfaulting passes this check and still segfaults, because it hands an + LLVM record to the LLVM builder and simply loads the result into an x86 + host. [flan reload] is precisely that caller. The complete answer is a + marker symbol the host defines and a module references, so the loader + refuses the pair at dlopen rather than the processor refusing it at a + call. See HANDOFF-x86-aggregates.md. *) if opts.x86 then failwith "--x86: Build.shared is the LLVM redefinition path, and an --x86 host \ diff --git a/test/test_reload.ml b/test/test_reload.ml index f690f5f..083489a 100644 --- a/test/test_reload.ml +++ b/test/test_reload.ml @@ -355,17 +355,16 @@ let () = That is not asserted here. It is undefined behaviour and what it prints is a property of whichever LLVM is installed; a test that pins it would - be pinning the shape of a crash. What is asserted is that the two - builders refuse to be crossed when the option record says which backend - is in play — see below. *) + be pinning the shape of a crash. - (* Which is the only thing that stands between a future caller and that - segfault. [Build.opts] is where the backend choice lives, so a builder - handed an option record belonging to the other backend refuses rather - than producing an object that links, loads, and then dies at a call - site. It is not a complete defence and must not be read as one: a caller - that keeps two option records and picks the wrong one by hand defeats - it, and the CLI can still do exactly that — see + What is asserted is narrower, and the gap between the two is the finding + rather than a caveat on it: [Build.opts] is where the backend choice + lives, so a builder handed the *other* backend's option record refuses. + The crossed run above passes both refusals — it hands [Build.shared] an + LLVM record and never calls [Build.shared_x86] at all — and it still + segfaults with this guard in place, re-measured after it landed. The + guard catches a caller holding one option record; it cannot catch a + caller holding two, and [flan reload] is exactly that caller. See HANDOFF-x86-aggregates.md. *) (match Build.shared ~opts:x86 ~ir:"" ~out:(tmp "never.so") () with | _ -> fail "Build.shared accepted an --x86 option record"