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

[Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>]

* Masami Hiramatsu (mhiramat@redhat.com) wrote:
> Mathieu Desnoyers wrote:
> > * Masami Hiramatsu (mhiramat@redhat.com) wrote:
> >> Mathieu Desnoyers wrote:
> [...]
> >>> 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]);
> 
> Oops, these should be vmalloc_to_page(), shouldn't it?
> 

Yes, my bad. That should fix your oopses.

Mathieu


> >>>   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 ?
> 
> At the first loop, it hit the BUG_ON.
> 
> >>> 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.
> 
> Sure,
> 
> > There is also a sync_core() in text_poke. It should not matter, but
> > maybe that could help ?
> 
> Adding sync_core() could not help me... anyway, I'll try again
> with using vmalloc_to_page().
> 
> >>> 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.
> 
> OK, Thanks,
> 
> -- 
> Masami Hiramatsu
> 
> Software Engineer
> Hitachi Computer Products (America) Inc.
> Software Solutions Division
> 
> e-mail: mhiramat@redhat.com
> 

-- 
Mathieu Desnoyers
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