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SubjectRe: [PATCH v7 3/6] Uprobes: Support SDT markers having reference count (semaphore)
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Hi Oleg,

Sorry for bit late reply.


On 08/03/2018 04:54 PM, Oleg Nesterov wrote:
> Hi Ravi,
>
> I was going to give up and ack this series, but it seems I noticed
> a bug...
>
> On 07/31, Ravi Bangoria wrote:
>>
>> +static int delayed_uprobe_add(struct uprobe *uprobe, struct mm_struct *mm)
>> +{
>> + struct delayed_uprobe *du;
>> +
>> + if (delayed_uprobe_check(uprobe, mm))
>> + return 0;
>> +
>> + du = kzalloc(sizeof(*du), GFP_KERNEL);
>> + if (!du)
>> + return -ENOMEM;
>> +
>> + du->uprobe = uprobe;
>> + du->mm = mm;
>
> I am surprised I didn't notice this before...
>
> So
> du->mm = mm;
>
> is fine, mm can't go away, uprobe_clear_state() does delayed_uprobe_remove(NULL,mm).
>
> But
> du->uprobe = uprobe;
>
> doesn't look right, uprobe can go away and it can be freed, its memory can be reused.
> We can't rely on remove_breakpoint(),


I'm sorry. I didn't get this. How can uprobe go away without calling
uprobe_unregister()
-> rergister_for_each_vma()
-> remove_breakpoint()
And remove_breakpoint() will get called only when last uprobe consumer is going
away _for that mm_. So, we can rely on remove_breakpoint() to remove {uprobe,mm}
from delayed_uprobe_list. Am I missing anything? Or it would be even better if
you can tell me some example scenario.


> the application can unmap the probed page/vma.
> Yes we do not care about the application in this case, say, the next uprobe_mmap() can
> wrongly increment the counter, we do not care although this can lead to hard-to-debug
> problems. But, if nothing else, the kernel can crash if the freed memory is unmapped.
> So I think put_uprobe() should do delayed_uprobe_remove(uprobe, NULL) before kfree()
> and delayed_uprobe_remove() should be updated to handle the mm==NULL case.
>
> Also. delayed_uprobe_add() should check the list and avoid duplicates. Otherwise the
> trivial
>
> for (;;)
> munmap(mmap(uprobed_file));
>
> will eat the memory until uprobe is unregistered.


I'm already calling delayed_uprobe_check(uprobe, mm) from delayed_uprobe_add().
So, we don't add same {uprobe,mm} multiple time in delayed_uprobe_list.

Is it the same check you are asking me to add or something else.


>
>
>> +static bool valid_ref_ctr_vma(struct uprobe *uprobe,
>> + struct vm_area_struct *vma)
>> +{
>> + unsigned long vaddr = offset_to_vaddr(vma, uprobe->ref_ctr_offset);
>> +
>> + return uprobe->ref_ctr_offset &&
>> + vma->vm_file &&
>> + file_inode(vma->vm_file) == uprobe->inode &&
>> + vma->vm_flags & VM_WRITE &&
>> + !(vma->vm_flags & VM_SHARED) &&
>
> vma->vm_flags & (VM_WRITE|VM_SHARED) == VM_WRITE &&
>
> looks a bit better to me, but I won't insist.

Sure. I'll change it.

>
>> +static int delayed_uprobe_install(struct vm_area_struct *vma)
>> +{
>> + struct list_head *pos, *q;
>> + struct delayed_uprobe *du;
>> + unsigned long vaddr;
>> + int ret = 0, err = 0;
>> +
>> + mutex_lock(&delayed_uprobe_lock);
>> + list_for_each_safe(pos, q, &delayed_uprobe_list) {
>> + du = list_entry(pos, struct delayed_uprobe, list);
>> +
>> + if (!valid_ref_ctr_vma(du->uprobe, vma))
>> + continue;
>> +
>> + vaddr = offset_to_vaddr(vma, du->uprobe->ref_ctr_offset);
>> + ret = __update_ref_ctr(vma->vm_mm, vaddr, 1);
>> + /* Record an error and continue. */
>> + err = ret & !err ? ret : err;
>
> I try to avoid the cosmetic nits, but I simply can't look at this line ;)
>
> if (ret && !err)
> err = ret;

This is neat. Will replace it.

>
>> @@ -1072,7 +1281,14 @@ int uprobe_mmap(struct vm_area_struct *vma)
>> struct uprobe *uprobe, *u;
>> struct inode *inode;
>>
>> - if (no_uprobe_events() || !valid_vma(vma, true))
>> + if (no_uprobe_events())
>> + return 0;
>> +
>> + if (vma->vm_flags & VM_WRITE &&
>> + test_bit(MMF_HAS_UPROBES, &vma->vm_mm->flags))
>> + delayed_uprobe_install(vma);
>
> OK, so you also added the VM_WRITE check and I agree. But then I think we
> should also check VM_SHARED, just like valid_ref_ctr_vma() does?

Right. I'll add a check here.

Thanks for reviewing :)

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