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    SubjectRe: [PATCH] mpi: Avoid using freed pointer in mpi_lshift_limbs()

    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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