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    SubjectRe: [BUG] fbdev: i740fb: Divide error when ‘var->p ixclock’ is zero
    Hi Helge,

    On Sun, Apr 3, 2022 at 5:41 PM Helge Deller <deller@gmx.de> wrote:
    > On 4/3/22 13:26, Zheyu Ma wrote:
    > > I found a bug in the function i740fb_set_par().
    >
    > Nice catch!
    >
    > > When the user calls the ioctl system call without setting the value to
    > > 'var->pixclock', the driver will throw a divide error.
    > >
    > > This bug occurs because the driver uses the value of 'var->pixclock'
    > > without checking it, as the following code snippet show:
    > >
    > > if ((1000000 / var->pixclock) > DACSPEED8) {
    > > dev_err(info->device, "requested pixclock %i MHz out of range
    > > (max. %i MHz at 8bpp)\n",
    > > 1000000 / var->pixclock, DACSPEED8);
    > > return -EINVAL;x
    > > }
    > >
    > > We can fix this by checking the value of 'var->pixclock' in the
    > > function i740fb_check_var() similar to commit
    > > b36b242d4b8ea178f7fd038965e3cac7f30c3f09, or we should set the lowest
    > > supported value when this field is zero.
    > > I have no idea about which solution is better.
    >
    > Me neither.
    > I think a solution like commit b36b242d4b8ea178f7fd038965e3cac7f30c3f09
    > is sufficient.
    >
    > Note that i740fb_set_par() is called in i740fb_resume() as well.
    > Since this doesn't comes form userspace I think adding a check for
    > the return value there isn't necessary.
    >
    > Would you mind sending a patch like b36b242d4b8ea178f7fd038965e3cac7f30c3f09 ?

    When passed an invalid value, .check_var() is supposed to
    round up the invalid to a valid value, if possible.

    Commit b36b242d4b8ea178 ("video: fbdev: asiliantfb: Error out if
    'pixclock' equals zero") does not do that.

    Gr{oetje,eeting}s,

    Geert

    --
    Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

    In personal conversations with technical people, I call myself a hacker. But
    when I'm talking to journalists I just say "programmer" or something like that.
    -- Linus Torvalds

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