* Oleg Nesterov <oleg@xxxxxxxxxx> [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() ?
>
> 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...
>
Copying the instruction at the time we replace the original instruction
would have been ideal. However there are a few irritants to handle.
- While inserting the breakpoint, we might find that the original
instruction to be the breakpoint instruction itself. (This could
happen if mmap_uprobe were to race with register_uprobe() or somebody
else like gdb inserted a breakpoint). How do we distinguish if the
breakpoint instruction was around in the text or somebody inserted a
breakpoint in that address-space? Since we read from the page-cache,
we can easily resolve this.
- On archs like x86, with variable size instructions, the original
instruction can be across 2 pages. This is because we copy the
maximum instruction size from the given vaddr into a buffer for
subsequent analysis. So the copy_insn takes care of getting two pages
if and when required.
Currently the insert and remove breakpoint
assumes that the instruction size of a breakpoint is the smallest
size for that architecture. Hence reading/writing to one page in
write_opcode is good enough.
- Again on variable instruction size supporting archs, if two
subsequent instructions are probed, the original instruction if
copied using get_user_pages might already have a breakpoint included.
(This shouldnt have any effect on the uprobes though.)
--
Thanks and Regards
Srikar
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