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Fix use-after-free in externref example #8410

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merged 3 commits into from
Apr 19, 2024
Merged

Fix use-after-free in externref example #8410

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72fc660
Fix use-after-free in externref example
alexcrichton Apr 19, 2024
ced30d2
Run clang-format
alexcrichton Apr 19, 2024
958b72b
Fix compilation of C example
alexcrichton Apr 19, 2024
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121 changes: 64 additions & 57 deletions examples/externref.c
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Original file line number Diff line number Diff line change
Expand Up @@ -106,71 +106,78 @@ int main() {
wasmtime_extern_t item;

// Lookup the `table` export.
ok = wasmtime_instance_export_get(context, &instance, "table",
strlen("table"), &item);
assert(ok);
assert(item.kind == WASMTIME_EXTERN_TABLE);

// Set `table[3]` to our `externref`.
wasmtime_val_t externref_val;
externref_val.kind = WASMTIME_EXTERNREF;
externref_val.of.externref = externref;
error = wasmtime_table_set(context, &item.of.table, 3, &externref_val);
if (error != NULL)
exit_with_error("failed to set table", error, NULL);

// `table[3]` should now be our `externref`.
wasmtime_val_t elem;
ok = wasmtime_table_get(context, &item.of.table, 3, &elem);
assert(ok);
assert(elem.kind == WASMTIME_EXTERNREF);
assert(strcmp((char *)wasmtime_externref_data(context, elem.of.externref),
"Hello, World!") == 0);
wasmtime_val_delete(context, &elem);
{
ok = wasmtime_instance_export_get(context, &instance, "table",
strlen("table"), &item);
assert(ok);
assert(item.kind == WASMTIME_EXTERN_TABLE);

// Set `table[3]` to our `externref`.
wasmtime_val_t externref_val;
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This is now declared inside this block, but then reused in the other block and has wasmtime_val_delete called outside of the block. Can we move it up, out of this block then?

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Wait what how does this compile given this mistake...

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Ah the answer is that it doesn't...

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lol yeah I was wondering myself

externref_val.kind = WASMTIME_EXTERNREF;
externref_val.of.externref = externref;
error = wasmtime_table_set(context, &item.of.table, 3, &externref_val);
if (error != NULL)
exit_with_error("failed to set table", error, NULL);

// `table[3]` should now be our `externref`.
wasmtime_val_t elem;
ok = wasmtime_table_get(context, &item.of.table, 3, &elem);
assert(ok);
assert(elem.kind == WASMTIME_EXTERNREF);
assert(strcmp((char *)wasmtime_externref_data(context, elem.of.externref),
"Hello, World!") == 0);
wasmtime_val_delete(context, &elem);
}

printf("Touching `externref` global...\n");

// Lookup the `global` export.
ok = wasmtime_instance_export_get(context, &instance, "global",
strlen("global"), &item);
assert(ok);
assert(item.kind == WASMTIME_EXTERN_GLOBAL);

// Set the global to our `externref`.
error = wasmtime_global_set(context, &item.of.global, &externref_val);
if (error != NULL)
exit_with_error("failed to set global", error, NULL);

// Get the global, and it should return our `externref` again.
wasmtime_val_t global_val;
wasmtime_global_get(context, &item.of.global, &global_val);
assert(global_val.kind == WASMTIME_EXTERNREF);
assert(strcmp((char *)wasmtime_externref_data(context, elem.of.externref),
"Hello, World!") == 0);
wasmtime_val_delete(context, &global_val);
{
ok = wasmtime_instance_export_get(context, &instance, "global",
strlen("global"), &item);
assert(ok);
assert(item.kind == WASMTIME_EXTERN_GLOBAL);

// Set the global to our `externref`.
error = wasmtime_global_set(context, &item.of.global, &externref_val);
if (error != NULL)
exit_with_error("failed to set global", error, NULL);

// Get the global, and it should return our `externref` again.
wasmtime_val_t global_val;
wasmtime_global_get(context, &item.of.global, &global_val);
assert(global_val.kind == WASMTIME_EXTERNREF);
assert(strcmp((char *)wasmtime_externref_data(context,
global_val.of.externref),
"Hello, World!") == 0);
wasmtime_val_delete(context, &global_val);
}

printf("Calling `externref` func...\n");

// Lookup the `func` export.
ok = wasmtime_instance_export_get(context, &instance, "func", strlen("func"),
&item);
assert(ok);
assert(item.kind == WASMTIME_EXTERN_FUNC);

// And call it!
wasmtime_val_t results[1];
error = wasmtime_func_call(context, &item.of.func, &externref_val, 1, results,
1, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call function", error, trap);

// `func` returns the same reference we gave it, so `results[0]` should be our
// `externref`.
assert(results[0].kind == WASMTIME_EXTERNREF);
assert(
strcmp((char *)wasmtime_externref_data(context, results[0].of.externref),
"Hello, World!") == 0);
wasmtime_val_delete(context, &results[0]);
{
ok = wasmtime_instance_export_get(context, &instance, "func",
strlen("func"), &item);
assert(ok);
assert(item.kind == WASMTIME_EXTERN_FUNC);

// And call it!
wasmtime_val_t results[1];
error = wasmtime_func_call(context, &item.of.func, &externref_val, 1,
results, 1, &trap);
if (error != NULL || trap != NULL)
exit_with_error("failed to call function", error, trap);

// `func` returns the same reference we gave it, so `results[0]` should be
// our `externref`.
assert(results[0].kind == WASMTIME_EXTERNREF);
assert(strcmp((char *)wasmtime_externref_data(context,
results[0].of.externref),
"Hello, World!") == 0);
wasmtime_val_delete(context, &results[0]);
}
wasmtime_val_delete(context, &externref_val);

// We can GC any now-unused references to our externref that the store is
Expand Down