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SubjectRe: [PATCH v5 3.1.0-rc4-tip 5/26] Uprobes: copy of the original instruction.
* Oleg Nesterov <oleg@redhat.com> [2011-10-03 18:29:05]:

> On 09/20, Srikar Dronamraju wrote:
> >
> > +static int __copy_insn(struct address_space *mapping,
> > + struct vm_area_struct *vma, char *insn,
> > + unsigned long nbytes, unsigned long offset)
> > +{
> > + struct file *filp = vma->vm_file;
> > + struct page *page;
> > + void *vaddr;
> > + unsigned long off1;
> > + unsigned long idx;
> > +
> > + if (!filp)
> > + return -EINVAL;
> > +
> > + idx = (unsigned long) (offset >> PAGE_CACHE_SHIFT);
> > + off1 = offset &= ~PAGE_MASK;
> > +
> > + /*
> > + * Ensure that the page that has the original instruction is
> > + * populated and in page-cache.
> > + */
>
> Hmm. But how we can ensure?


>
> > + page_cache_sync_readahead(mapping, &filp->f_ra, filp, idx, 1);
>
> This schedules the i/o,
>
> > + page = grab_cache_page(mapping, idx);
>
> This finds/locks the page in the page-cache,
>
> > + if (!page)
> > + return -ENOMEM;
> > +
> > + vaddr = kmap_atomic(page);
> > + memcpy(insn, vaddr + off1, nbytes);
>
> What if this page is not PageUptodate() ?

Since we do a synchronous read ahead, I thought the page would be
populated and upto date.

would these two lines after grab_cache_page help?

if (!PageUptodate(page))
mapping->a_ops->readpage(filp, page);

>
> Somehow this assumes that the i/o was already completed, I don't
> understand this.
>
> But I am starting to think I simply do not understand this change.
> To the point, I do not underestand why do we need copy_insn() at all.
> We are going to replace this page, can't we save/analyze ->insn later
> when we copy the content of the old page? Most probably I missed
> something simple...
>
>
> > +static struct task_struct *get_mm_owner(struct mm_struct *mm)
> > +{
> > + struct task_struct *tsk;
> > +
> > + rcu_read_lock();
> > + tsk = rcu_dereference(mm->owner);
> > + if (tsk)
> > + get_task_struct(tsk);
> > + rcu_read_unlock();
> > + return tsk;
> > +}
>
> Hmm. Do we really need task_struct?
>
> > -static int install_breakpoint(struct mm_struct *mm, struct uprobe *uprobe)
> > +static int install_breakpoint(struct mm_struct *mm, struct uprobe *uprobe,
> > + struct vm_area_struct *vma, loff_t vaddr)
> > {
> > - /* Placeholder: Yet to be implemented */
> > + struct task_struct *tsk;
> > + unsigned long addr;
> > + int ret = -EINVAL;
> > +
> > if (!uprobe->consumers)
> > return 0;
> >
> > - atomic_inc(&mm->mm_uprobes_count);
> > - return 0;
> > + tsk = get_mm_owner(mm);
> > + if (!tsk) /* task is probably exiting; bail-out */
> > + return -ESRCH;
> > +
> > + if (vaddr > TASK_SIZE_OF(tsk))
> > + goto put_return;
>
> But this should not be possible, no? How it can map this vaddr above
> TASK_SIZE ?
>
> get_user_pages(tsk => NULL) is fine. Why else do we need mm->owner ?

>
> Probably used by the next patches... Say, is_32bit_app(tsk). This
> can use mm->context.ia32_compat (hopefully will be replaced with
> MMF_COMPAT).
>

We used the tsk struct for checking if the application was 32 bit and
for calling get_user_pages. Since we can pass NULL to get_user_pages and
since we can use mm->context.ia32_compat or MMF_COMPAT, I will remove
get_mm_owner, that way we dont need to be dependent on CONFIG_MM_OWNER.

--
Thanks and Regards
Srikar


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