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    SubjectRe: [PATCH] poll/select: avoid arithmetic overflow in __estimate_accuracy()
    On Sun, Aug 16, 2009 at 10:29:21PM +0200, Guillaume Knispel wrote:
    >__estimate_accuracy() was prone to integer overflow, for example
    >if *tv == {2147, 483648000} on a 32 bit computer (or even for delays
    >as small as {429, 500000000} if the task is niced).
    >
    >Because the result was already forced between 0 and 100ms, the effect
    >of the overflow was not too problematic, but the use of the hrtimer
    >range feature was not optimal in overflow cases.
    >
    >This patch ensures that there can not be an integer overflow in this
    >function.
    >
    >Signed-off-by: Guillaume Knispel <gknispel@proformatique.com>
    >---
    > fs/select.c | 14 ++++++++++----
    > 1 files changed, 10 insertions(+), 4 deletions(-)
    >
    >diff --git a/fs/select.c b/fs/select.c
    >index 8084834..a201fc3 100644
    >--- a/fs/select.c
    >+++ b/fs/select.c
    >@@ -41,22 +41,28 @@
    > * better solutions..
    > */
    >
    >+#define MAX_SLACK (100 * NSEC_PER_MSEC)
    >+
    > static long __estimate_accuracy(struct timespec *tv)
    > {
    > long slack;
    > int divfactor = 1000;
    >
    >+ if (tv->tv_sec < 0)
    >+ return 0;
    >+
    > if (task_nice(current) > 0)
    > divfactor = divfactor / 5;
    >
    >+ if (tv->tv_sec > MAX_SLACK / (NSEC_PER_SEC/divfactor))
    >+ return MAX_SLACK;
    >+


    Yes? Isn't MAX_SLACK also too big for 'long' on 32bit machine?

    Try:

    % printf "%d\n" "100*1000000000 > 0x7fffffff"
    1

    Am I missing something here?


    Thanks.


    > slack = tv->tv_nsec / divfactor;
    > slack += tv->tv_sec * (NSEC_PER_SEC/divfactor);
    >
    >- if (slack > 100 * NSEC_PER_MSEC)
    >- slack = 100 * NSEC_PER_MSEC;
    >+ if (slack > MAX_SLACK)
    >+ return MAX_SLACK;
    >
    >- if (slack < 0)
    >- slack = 0;
    > return slack;
    > }
    >
    >--
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