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SubjectRe: [PATCH] Add a text_poke syscall
Can't really comment this patch, just a couple of nits...

On 11/18, Andi Kleen wrote:
>
> int poke_int3_handler(struct pt_regs *regs)
> {
> @@ -602,6 +619,14 @@ int poke_int3_handler(struct pt_regs *regs)
> if (likely(!bp_patching_in_progress))
> return 0;
>
> + if (user_mode_vm(regs) &&
> + bp_target_mm &&
> + current->mm == bp_target_mm &&
> + regs->ip == (unsigned long)bp_int3_addr) {
> + regs->ip = (unsigned long) bp_int3_handler;
> + return 1;
> + }
> +
> if (user_mode_vm(regs) || regs->ip != (unsigned long)bp_int3_addr)
> return 0;

This looks a bit confusing, afaics we can avoid the duplications:

// also handles bp_target_mm == NULL case
if (user_mode_vm(regs) && current->mm != bp_target_mm)
return 0;

if (regs->ip != (unsigned long)bp_int3_addr)
return 0;

/* set up the specified breakpoint handler */
regs->ip = (unsigned long) bp_int3_handler;

return 1;

> +SYSCALL_DEFINE5(text_poke,
> + __user void *, addr,
> + const void *, opcode,
> + size_t, len,
> + void *, handler,
> + int, timeout)
> +{
> + struct page *pages[2];
> + char insn[MAX_INSN_LEN];
> + int err, npages;
> +
> + if ((unsigned long)len > MAX_INSN_LEN || len == 0)
> + return -EINVAL;
> + /* For future extension */
> + if (timeout != 0)
> + return -EINVAL;
> + if (copy_from_user(insn, opcode, len))
> + return -EFAULT;
> + pages[1] = NULL;
> + npages = ((unsigned long)addr & PAGE_MASK) ==
> + (((unsigned long)addr + len) & PAGE_MASK) ? 1 : 2;
> + err = get_user_pages_fast((unsigned long)addr, npages, 1, pages);
> + if (err < 0)
> + return err;
> + err = 0;
> + mutex_lock(&text_mutex);
> + bp_target_mm = current->mm;
> + bp_int3_addr = (u8 *)addr + 1;
> + __text_poke_bp(pages,
> + (unsigned long)addr & ~PAGE_MASK,
> + insn, len, handler);

This doesn't look right if npages == 2 but get_user_pages_fast() returns 1.

__text_poke() checks pages[1] != NULL, but in this case it assumes
that memcpy(vaddr, opcode, len) should fit into the 1st page.


Ingo, Andi, I do not think that it is good idea to implement this
via ptrace. If nothing else, you need to fork the tracer which can
do PTRACE_POKETEXT.

Please note that PTRACE_TRACEME does not mean self-tracing, it means
that current->real_parent becomes the tracer (imho this interface
should die but we obviously can't kill it). So I don't see how it
can help. And you can't ptrace yourself of a sub-thread, this is
explicitly forbidden by ptrace_attach()->same_thread_group() check.

And ptrace can't really help with CLONE_VM tasks.

Or I misunderstood the suggestion.

Oleg.



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