6ed29621ad
A program with no 'main' reached the JIT entry-point call with a garbage address (ORC reports lookup success but leaves main_addr degenerate), then called it -> SIGSEGV. Add a pre-JIT entry-point check in main.zig that emits 'error: no main function found' and exits non-zero before codegen, plus a defensive main_addr==0 guard in target.zig runJITFromObject as a backstop. Regression: examples/1188-diagnostics-run-no-main.sx
77 lines
3.5 KiB
Markdown
77 lines
3.5 KiB
Markdown
# 0137 — `sx run` on a program with no `main` segfaults (JIT entry lookup unguarded)
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> **RESOLVED.** A pre-JIT entry-point check in `main.zig` now emits a clean
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> `error: no 'main' function found …` diagnostic and exits non-zero before any
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> codegen/JIT, so a no-main program never reaches the garbage-pointer call. A
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> defensive `main_addr == 0` guard in `target.zig`'s `runJITFromObject` (ORC
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> reports lookup success but leaves the address degenerate) remains as a
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> backstop. Regression test: `examples/1188-diagnostics-run-no-main.sx`.
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## Symptom
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`sx run <file>` on a program that defines no `main` function **crashes**
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(SIGSEGV/abort, "Segmentation fault at address 0x60") instead of emitting a clean
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diagnostic like `error: no 'main' function found`.
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- **Observed:** process crash, exit 134 (abort) / 139 (SIGSEGV); no diagnostic.
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- **Expected:** a normal compile-style error ("no `main` entry point") and a
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clean non-zero exit, the same way any other missing-entry condition reports.
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Independent of inline assembly — surfaced while writing an ASM-stream probe that
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omitted `main`, but reproduces with an ordinary, asm-free program (see below).
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## Reproduction
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A file with only an (uncalled) function and no `main`:
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```sx
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foo :: (n: u64) -> u64 { return n + 1; }
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```
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```sh
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sx run that.sx
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# => "Segmentation fault at address 0x60", exit 134
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# expected: "error: no 'main' function found" (or similar), clean non-zero exit
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```
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## Root cause (suspected)
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`src/target.zig` JIT-run path, ~lines 256–273. After the ORC lookup:
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```zig
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var main_addr: c.LLVMOrcExecutorAddress = 0;
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err = c.LLVMOrcLLJITLookup(jit, &main_addr, "main");
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if (err != null) { /* prints "JIT lookup error" and returns error.CompileError */ }
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// no guard for main_addr == 0 here:
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const main_fn: *const fn () callconv(.c) i32 = @ptrFromInt(main_addr);
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const result = main_fn(); // <- calls a null/garbage pointer when no main
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```
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When the module has no `main` symbol, the lookup leaves `main_addr` at `0` (or
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ORC returns a degenerate success), so `@ptrFromInt(main_addr)` + `main_fn()`
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calls into null → the crash. There is no `main_addr == 0` check.
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## Investigation prompt (paste into a fresh session)
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> `sx run` on a program with no `main` segfaults instead of diagnosing. The JIT
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> run path in `src/target.zig` (~lines 256–273) looks up `"main"` via
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> `LLVMOrcLLJITLookup`, then unconditionally casts `main_addr` to a function
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> pointer and calls it. When the program defines no `main`, `main_addr` is `0`
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> (or the lookup degenerately "succeeds"), so the call dereferences null and
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> crashes.
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>
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> Fix: after the lookup's `err` check, add `if (main_addr == 0) { … }` that emits
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> a clean user-facing error ("no `main` function found" / "program has no entry
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> point") and returns `error.CompileError` (matching the existing
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> `JIT lookup error` style), BEFORE the `@ptrFromInt` + call. Consider whether a
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> pre-JIT check (the module/program already knows whether a `main` decl exists —
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> e.g. emit_llvm.zig:631 already null-checks `LLVMGetNamedFunction(.., "main")`)
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> is the better choke point so the diagnostic carries a source span rather than a
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> bare message. Either is acceptable; the hard requirement is *no crash*.
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>
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> Verification: `printf 'foo :: (n: u64) -> u64 { return n + 1; }\n' > /tmp/x.sx
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> && sx run /tmp/x.sx` — expect a clean error message + non-zero exit, NOT a
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> segfault. Add a pinned repro under `issues/` (or an `examples/11xx-diagnostics-*`
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> once the message is settled) asserting the diagnostic on stderr + the exit code.
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