Re: [PATCH 12/13] tracing/uprobes: Add more fetch functions

[Namhyung Kim]

Hi Steve,

On Mon, 4 Nov 2013 12:17:06 -0500, Steven Rostedt wrote:
> On Mon, 4 Nov 2013 17:44:31 +0100
> Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
>> On 11/04, Namhyung Kim wrote:
>> >
>> > On Thu, 31 Oct 2013 19:22:18 +0100, Oleg Nesterov wrote:
>> > > On 10/29, Namhyung Kim wrote:
>> > >>
>> > >> +static void __user *get_user_vaddr(unsigned long addr, struct trace_uprobe *tu)
>> > >> +{
>> > >> +	unsigned long pgoff = addr >> PAGE_SHIFT;
>> > >> +	struct vm_area_struct *vma;
>> > >> +	struct address_space *mapping;
>> > >> +	unsigned long vaddr = 0;
>> > >> +
>> > >> +	if (tu == NULL) {
>> > >> +		/* A NULL tu means that we already got the vaddr */
>> > >> +		return (void __force __user *) addr;
>> > >> +	}
>> > >> +
>> > >> +	mapping = tu->inode->i_mapping;
>> > >> +
>> > >> +	mutex_lock(&mapping->i_mmap_mutex);
>> > >> +	vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
>> > >> +		if (vma->vm_mm != current->mm)
>> > >> +			continue;
>> > >> +		if (!(vma->vm_flags & VM_READ))
>> > >> +			continue;
>> > >> +
>> > >> +		vaddr = offset_to_vaddr(vma, addr);
>> > >> +		break;
>> > >> +	}
>> > >> +	mutex_unlock(&mapping->i_mmap_mutex);
>> > >> +
>> > >> +	WARN_ON_ONCE(vaddr == 0);
>> > >
>> > > Hmm. But unless I missed something this "addr" passed as an argument can
>> > > be wrong? And if nothing else this or another thread can unmap the vma?
>> >
>> > You mean WARN_ON_ONCE here is superfluous?  I admit that it should
>> > protect concurrent vma [un]mappings.  Please see my reply in other
>> > thread for a new approach.
>> 
>> Whatever we do this address can be unmapped. For example, just because of
>> @invalid_address passed to trace_uprobe.c.
>> 
>> We do not really care, copy_from_user() should fail. But we should not
>> WARN() in this case.
>> 
>
> I agree, the WARN_ON_ONCE() above looks like it's uncalled for.
> WARN()ings should only be used when an anomaly in the kernel logic is
> detected. Can this trigger on bad input from user space, or something
> else that userspace does? (a race with unmapping memory?). If so, error
> out to the user process, but do not call any of the WARN() functions.

Will do.  Thanks for the explanation.

Thanks,
Namhyung
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