Re: [PATCH v4 3.0-rc2-tip 13/22] 13: uprobes: Handing int3 and singlestep exception.

Peter Zijlstra
2011-06-16T11:53:12Z

On Tue, 2011-06-07 at 18:30 +0530, Srikar Dronamraju wrote:
> +void uprobe_notify_resume(struct pt_regs *regs)
> +{
> + struct vm_area_struct *vma;
> + struct uprobe_task *utask;
> + struct mm_struct *mm;
> + struct uprobe *u = NULL;
> + unsigned long probept;
> +
> + utask = current->utask;
> + mm = current->mm;
> + if (!utask || utask->state == UTASK_BP_HIT) {
> + probept = get_uprobe_bkpt_addr(regs);
> + down_read(&mm->mmap_sem);
> + vma = find_vma(mm, probept);
> + if (vma && valid_vma(vma))
> + u = find_uprobe(vma->vm_file->f_mapping->host,
> + probept - vma->vm_start +
> + (vma->vm_pgoff << PAGE_SHIFT));
> + up_read(&mm->mmap_sem);
> + if (!u)
> + goto cleanup_ret;
> + if (!utask) {
> + utask = add_utask();
> + if (!utask)
> + goto cleanup_ret;

So if we fail to allocate task state,..

> + }
> + /* TODO Start queueing signals. */
> + utask->active_uprobe = u;
> + handler_chain(u, regs);
> + utask->state = UTASK_SSTEP;
> + if (!pre_ssout(u, regs, probept))
> + user_enable_single_step(current);
> + else
> + goto cleanup_ret;
> + } else if (utask->state == UTASK_SSTEP) {
> + u = utask->active_uprobe;
> + if (sstep_complete(u, regs)) {
> + put_uprobe(u);
> + utask->active_uprobe = NULL;
> + utask->state = UTASK_RUNNING;
> + user_disable_single_step(current);
> + xol_free_insn_slot(current);
> +
> + /* TODO Stop queueing signals. */
> + }
> + }
> + return;
> +
> +cleanup_ret:
> + if (u) {
> + down_read(&mm->mmap_sem);
> + if (!set_orig_insn(current, u, probept, true))

we try to undo the probe? That doesn't make any sense. I thought you
meant to return to userspace, let it re-take the trap and try again
until you do manage to allocate the user resource.

This behaviour makes probes totally unreliable under memory pressure.

> + atomic_dec(&mm->uprobes_count);
> + up_read(&mm->mmap_sem);
> + put_uprobe(u);
> + } else {
> + /*TODO Return SIGTRAP signal */
> + }
> + if (utask) {
> + utask->active_uprobe = NULL;
> + utask->state = UTASK_RUNNING;
> + }
> + set_instruction_pointer(regs, probept);
> +}

Also, there's a scary amount of TODO in there...