ptrace with PTRACE_POKETEXT gives errno 5

Question

I was trying to use this project https://github.com/Chainfire/injectvm-binderjack to inject a shared library in the system server, using ptrace(). The problem is, that every attempt of writing memory (using PTRACE_POKETEXT) fails with errno 5 (Input/Output Error). What could be causing this? Every other ptrace operation works and the jni library doing the injection runs as root.

This is the function used to write remotely:

static bool remote_write(const char* debug, size_t dest, unsigned char *src, size_t blen) {
    remote_stop(); // suspend the target process

    size_t i = 0;
    long ret;

    // make sure the buffer is word aligned
    char *ptr = (char *) malloc(blen + blen % sizeof(size_t));
    memcpy(ptr, src, blen);

    for (i = 0; i < blen; i += sizeof(size_t)) {
        ret = trace( debug, PTRACE_POKETEXT, (void *) (dest + i), *(size_t *) &ptr[i] );
        if (ret == -1) {
            free(ptr);
            return false;
        }
    }

    free(ptr);

    return true;
}

I tested this in a device with Android 11 arm x64