On Sat, Oct 14, 2006 at 06:53:06AM +0200, Nick Piggin wrote:
> On Fri, Oct 13, 2006 at 03:33:42PM -0500, Robin Holt wrote:
> > Handle the case in get_user_pages() when a call to __handle_mm_fault()
> > inserts a writable pte, and a process doing dup_mmap converts it
> > to readable before get_user_pages() does the subsequent request to
> > follow_page().
> >
> >
> > Signed-off-by: Robin Holt <[email protected]>
> >
> > ---
> >
> > Hugh, Nick, and Linus,
> >
> > I think I have tripped over another flavor of a get_user_pages bug
> > we addressed back in 2005. I do not have a test case to prove it is
> > the issue I am trying to address, but I have done as thorough a code
> > walk-through as I can.
> >
> > Assume a pte is currently empty. A first pthread is in the kernel on
> > a call path which is leading to get_user_pages. A second pthread is
> > in the process of doing a fork. The process doing get_user_pages()
> > gets into __handle_mm_fault() and grabs ptl just before the process
> > doing a fork attempts to grab the ptl to convert the pages to COW.
> > __handle_mm_fault() will insert the writable pte and unlock ptl then
> > return with VM_FAULT_WRITE set. The process doing a fork then gets
> > the lock and starts converting the pte to RO/COW. The get_user_pages()
> > process then clears FOLL_WRITE from foll_flags and calls follow_page()
> > without write, adds to the map count for the page, but does not have a
> > writable mapping.
>
> Hi Robin,
>
> dup_mmap holds mmap_sem for write. get_user_pages caller must hold it
> for read.
>
> So it think it is OK? But if not, then you can't just get rid of this
> FOLL_WRITE bit, because then we get infinite loops when a 'force'
> write access (eg. ptrace setting a breakpoint in text).
What problem are you seeing, BTW?
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