cranelift/docs/wmemcheck.md

Wasm memcheck (wmemcheck)

wmemcheck provides the ability to check for invalid mallocs, reads, and writes inside a Wasm module, as long as Wasmtime is able to make certain assumptions (malloc and free functions are visible and your program uses only the default allocator). This is analogous to the Valgrind tool's memory checker (memcheck) tool for native programs.

How to use:

1. When building Wasmtime, add the CLI flag "--features wmemcheck" to compile with wmemcheck configured.

   cargo build --features wmemcheck

2. When running your wasm module, add the CLI flag "-W wmemcheck".

   wasmtime run -W wmemcheck test.wasm

If your program executes an invalid operation (load or store to non-allocated address, double-free, or an internal error in malloc that allocates the same memory twice) you will see an error that looks like a Wasm trap. For example, given the program

#include <stdlib.h>

int main() {
    char* p = malloc(1024);
    *p = 0;
    free(p);
    *p = 0;
}

compiled with WASI-SDK via

$ /opt/wasi-sdk/bin/clang -o test.wasm test.c

you can observe the memory checker working like so:

$ wasmtime run -W wmemcheck ./test.wasm
Error: failed to run main module `./test.wasm`

Caused by:
    0: failed to invoke command default
    1: error while executing at wasm backtrace:
           0:  0x103 - <unknown>!__original_main
           1:   0x87 - <unknown>!_start
           2: 0x2449 - <unknown>!_start.command_export
    2: Invalid store at addr 0x10610 of size 1