Why this ptrace error?

Question

I'm just testing read and write memory via ptrace() call, but when I run the code shown below, I got an error like this.

Processkey() : 0x80481240
readmem pid 3950
Original opcode : 0x4641682b
writemem pid 3950
readmem pid 3950
PEEKDATA error: No such file or directory

As you can see, readmem() works fine, but afterwards, readmem() prints an error.

long readmem(int pid, unsigned long addr)
{
    long ret=0;
    printf("readmem pid %d\n", pid);
    ret = ptrace(PTRACE_PEEKDATA, pid, (void *)(addr), 0);
    if(ret<0)
    {
        error("PEEKDATA error");
    }
    return ret;
}

void writemem(int pid, unsigned long addr, long data)
{
    long ret=0;
    printf("writemem pid %d\n", pid);
    ret = ptrace(PTRACE_POKEDATA, pid, (void *)addr, (void *)data);
    if(ret<0)
    {
        error("POKEDATA error");
    }
}

void detach(int pid)
{
    long ret=0;
    ret = ptrace(PTRACE_DETACH, pid, NULL, NULL);
    if(ret<0)
    {
        error("detach() error");
    }
}

void attach(int pid)
{
    long ret=0;
    ret = ptrace(PTRACE_ATTACH, pid, NULL, NULL);
    if(ret<0)
    {
        error("ptrace() error");
    }

    ret = waitpid(pid, NULL, WUNTRACED);
    if(ret<0)
    {
        error("waitpid ()");
    }
}

int main(int argc, char **argv)
{
    long ret = 0;
    pid_t pid = 0;
    REGS *regs;
    unsigned long processkey_addr = 0;
    int stat_value = 0;
    long op = 0;

    pid = atoi(argv[1]);
    processkey_addr = 0x80481240;
    printf("Processkey() : 0x%lx\n", processkey_addr);

    attach(pid);
    op = readmem(pid, processkey_addr);
    printf("Original opcode : 0x%lx\n", op);
    writemem(pid, processkey_addr, 0x41424344);
    printf("Changed opcode : 0x%lx\n", readmem(pid, processkey_addr));
    detach(pid);
}

But — and this made me crazy — when I tested code without the functions (just wrote the code inline in main()), this whole code worked fine!

Does anyone know why this happens...?

Answer 1

It seems a bit odd, but there's at least one actual error in your code. From the manual page:

On error, all requests return -1, and errno is set appropriately. Since the value returned by a successful PTRACE_PEEK* request may be -1, the caller must check errno after such requests to determine whether or not an error occurred.

It seems your code does not check errno, it just assumes an error occured if the return value is -1.