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    SubjectRe: [PATCH] mpi: Avoid using freed pointer in mpi_lshift_limbs()
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    On Thu, Mar 1, 2012 at 11:33 PM, Jesper Juhl <jj@chaosbits.net> wrote:
    >
    > Ping?
    >
    > On Mon, 6 Feb 2012, Jesper Juhl wrote:
    >
    >> At the start of the function we assign 'a->d' to 'ap'. Then we use the
    >> RESIZE_IF_NEEDED macro on 'a' - this may free 'a->d' and replace it
    >> with newly allocaetd storage. In that case, we'll be operating on
    >> freed memory further down in the function when we index into 'ap[]'.
    >> Since we don't actually need 'ap' until after the use of the
    >> RESIZE_IF_NEEDED macro we can just delay the assignment to it until
    >> after we've potentially resized, thus avoiding the issue.
    >>
    >> While I was there anyway I also changed the integer variable 'n' to be
    >> const. It might as well be since we only assign to it once and use it
    >> as a constant, and then the compiler will tell us if we ever assign to
    >> it in the future.
    >>
    >> Signed-off-by: Jesper Juhl <jj@chaosbits.net>
    >> ---
    >>  lib/mpi/mpi-bit.c |    5 +++--
    >>  1 files changed, 3 insertions(+), 2 deletions(-)
    >>
    >>  only compile tested.
    >>
    >> diff --git a/lib/mpi/mpi-bit.c b/lib/mpi/mpi-bit.c
    >> index 2f52662..0c50536 100644
    >> --- a/lib/mpi/mpi-bit.c
    >> +++ b/lib/mpi/mpi-bit.c
    >> @@ -177,8 +177,8 @@ int mpi_rshift(MPI x, MPI a, unsigned n)
    >>   */
    >>  int mpi_lshift_limbs(MPI a, unsigned int count)
    >>  {
    >> -     mpi_ptr_t ap = a->d;
    >> -     int n = a->nlimbs;
    >> +     const int n = a->nlimbs;
    >> +     mpi_ptr_t ap;
    >>       int i;
    >>
    >>       if (!count || !n)
    >> @@ -187,6 +187,7 @@ int mpi_lshift_limbs(MPI a, unsigned int count)
    >>       if (RESIZE_IF_NEEDED(a, n + count) < 0)
    >>               return -ENOMEM;
    >>
    >> +     ap = a->d;
    >>       for (i = n - 1; i >= 0; i--)
    >>               ap[i + count] = ap[i];
    >>       for (i = 0; i < count; i++)
    >>
    >
    > --
    > Jesper Juhl <jj@chaosbits.net>       http://www.chaosbits.net/
    > Don't top-post http://www.catb.org/jargon/html/T/top-post.html
    > Plain text mails only, please.
    >
    > --
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    > Please read the FAQ at  http://www.tux.org/lkml/

    Hello,

    I was travelling last week.
    I will review the patch now..

    Thanks,
    Dmitry
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