Fix bounds check for error-context.debug-message (#12879)

* Fix bounds check for `error-context.debug-message`

This fixes a possible subsequent panic when lowering the message itself.

* Review comments

Author: alexcrichton

crates/wasmtime/src/runtime/component/concurrent/futures_and_streams.rs

@@ -4205,11 +4205,15 @@ impl Instance {
 
         let lower_cx = &mut LowerContext::new(store, options, self);
         let debug_msg_address = usize::try_from(debug_msg_address)?;
-        // Lower the string into the component's memory
+        // Lower the string into the component's memory.
+        //
+        // Note that the "8" here is the size of a WIT `string` in linear
+        // memory, the ptr+length. This'll need to be updated when `memory64`
+        // comes along. (FIXME(#4311))
         let offset = lower_cx
             .as_slice_mut()
             .get(debug_msg_address..)
-            .and_then(|b| b.get(..debug_msg.bytes().len()))
+            .and_then(|b| b.get(..8))
             .map(|_| debug_msg_address)
             .ok_or_else(|| crate::format_err!("invalid debug message pointer: out of bounds"))?;
         debug_msg

tests/misc_testsuite/component-model/async/error-context.wast

@@ -34,3 +34,53 @@
   (core func $error-context-drop (canon error-context.drop))
   (core instance $i (instantiate $m (with "" (instance (export "error-context.drop" (func $error-context-drop))))))
 )
+
+;; Test edge-case behavior of `error-context.debug-message`.
+(component definition $A
+  (core module $libc
+    (memory (export "memory") 1)
+    (global $bump (mut i32) (i32.const 100))
+    (func (export "realloc") (param i32 i32 i32 i32) (result i32)
+      (local $ret i32)
+      (if (local.get 0) (then unreachable))
+      (if (local.get 1) (then unreachable))
+      (if (i32.ne (local.get 2) (i32.const 1)) (then unreachable))
+      (local.set $ret (global.get $bump))
+      (global.set $bump (i32.add (global.get $bump) (local.get 3)))
+      (local.get $ret)
+    )
+  )
+  (core instance $libc (instantiate $libc))
+
+  (core module $Core
+    (import "" "mem" (memory 1))
+    (import "" "error-context.new" (func $error-context.new (param i32 i32) (result i32)))
+    (import "" "error-context.debug-message" (func $error-context.debug-message (param i32 i32)))
+    (import "" "error-context.drop" (func $error-context.drop (param i32)))
+
+    (func (export "run") (param $dst i32)
+      (local $handle i32)
+      (i32.store8 (i32.const 0) (i32.const 0x61)) ;; 'a'
+      (local.set $handle (call $error-context.new (i32.const 0) (i32.const 1)))
+      (call $error-context.debug-message (local.get $handle) (local.get $dst))
+      (call $error-context.drop (local.get $handle))
+    )
+  )
+
+  (core func $error-context.new (canon error-context.new (memory $libc "memory")))
+  (core func $error-context.debug-message (canon error-context.debug-message (memory $libc "memory") (realloc (func $libc "realloc"))))
+  (core func $error-context.drop (canon error-context.drop))
+
+  (core instance $core (instantiate $Core (with "" (instance
+    (export "mem" (memory $libc "memory"))
+    (export "error-context.new" (func $error-context.new))
+    (export "error-context.debug-message" (func $error-context.debug-message))
+    (export "error-context.drop" (func $error-context.drop))
+  ))))
+
+  (func (export "run") (param "p" u32) (canon lift (core func $core "run")))
+)
+
+(component instance $A $A)
+(assert_return (invoke "run" (u32.const 65528)))
+(assert_trap (invoke "run" (u32.const 65532)) "invalid debug message pointer")