Re: [BUG][kprobes][vunmap?]: kprobes may cause memory corruption

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Masami Hiramatsu (mhiramat@redhat.com) wrote:
> Mathieu Desnoyers wrote:
> > * Masami Hiramatsu (mhiramat@redhat.com) wrote:
> >> Masami Hiramatsu wrote:
> > Hi Masami,
> > 
> > This would not surprise me if it came from bug in the new vmap()
> > implementation done in this commit :
> > 
> > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=db64fe02258f1507e13fe5212a989922323685ce
> > 
> > Especially because going from vmap -> vm_map_ram makes this behavior
> > disappear.
> > 
> > Looking at the commit, I notice that it delays vunmap so it's done in
> > batch to minimize locking effect. I think it would be good to create a
> > test case to try to isolate this, without any kprobes/text_poke
> > involved, which does something like this :
> > 
> > load module (this is also doing vmalloc, so it might be part of the
> >              problem)
> >   for i (i=0; i < 400; i++) {
> >     vmap()
> >     vfree()
>       ^^^^^ vunmap?

yep.

> >   }
> > unload module
> > 
> > Another interesting test would be :
> > 
> >   for i (i=0; i < 400; i++) {
> >     vmalloc()
> >     vfree()
> >   }
> 
> Hi Mathieu,
> 
> Thank you for test ideas.
> I made both of above two tests and run it. Both test modules
> do NOT cause memory corruption...
> 

OK

> > All this called in a loop. This would help isolating the "vmap" part of
> > the issue. If this test is not enough, then we should maybe try
> > something like this in a kernel module (which does what text_poke does
> > with vmalloc, more or less) in a loop :
> > 
> > char somedata[PAGE_SIZE] __attribute__((aligned(PAGE_SIZE)));
> > char copydata[PAGE_SIZE] __attribute__((aligned(PAGE_SIZE)));
> 
> Should both of them have PAGE_SIZE*2?
> 

Yes.

> > 
> > void test_vmap(void)
> > }
> >   struct page *pages[2];
> >   char *vaddr;
> >   int i;
> > 
> >   for (i = 0; i < 2 * PAGE_SIZE; i++)
> >     copydata[i] = somedata[i];
> >   page[0] = virt_to_page(&somedata);
> >   BUG_ON(!page[0]);
> >   page[1] = virt_to_page(&somedata + PAGE_SIZE);
> >   BUG_ON(!page[1]);
> >   vaddr = vmap(pages, 2, VM_MAP, PAGE_KERNEL);
> >   BUG_ON(!vaddr);
> > 
> >   for (i = 0; i < 2 * PAGE_SIZE; i++)
> >     vaddr[i] = copydata[i] + 1;
> >   
> >   vunmap(vaddr);
> >   
> >   for (i = 0; i < 2 * PAGE_SIZE; i++)
> >     BUG_ON(somedata[i] != copydata[i] + 1);
> > }
> 
> Hmm, when I ran above code, it hit the last BUG_ON().
> I checked that somedata[i] didn't updated.
> 

Do you hit the BUG_ON after the first loop ?

> > Given you don't seem to have hit the
> >         for (i = 0; i < len; i++)
> >                 BUG_ON(((char *)addr)[i] != ((char *)opcode)[i]);
> > test at the end of text_poke,
> 
> However, when I ran kprobe-based test, it doesn't hit the BUG_ON()
> in text_poke().
> 

The variable declarations should have been 2*PAGE_SIZE, hopefully you
fixed them.

There is also a sync_core() in text_poke. It should not matter, but
maybe that could help ?

> > I suspect the write through the vmapped
> > area is correctly done, but that the problem may lay in the mm layer.
> > Maybe it's running out of pre-allocated vmap areas or something like
> > this ?
> 
> I haven't seen vmalloc failure message on 2.6.29-rc2.
> 

It could be because the available vmalloc space is slightly higher.
Looking into the lazy vunmap threshold would be useful.

You could also try with loop values higher than 400.

Mathieu

> Thank you again,
> 
> 
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@redhat.com
> 

-- 
Mathieu Desnoyers
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