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From:   Ondrej Zary <linux@zary.sk>
To:     Zheyu Ma <zheyuma97@gmail.com>
Subject: Re: [PATCH 1/7] video: fbdev: i740fb: Error out if 'pixclock' equals zero
Date:   Sun, 10 Apr 2022 11:02:13 +0200
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Cc:     Helge Deller <deller@gmx.de>,
        Linux Fbdev development list <linux-fbdev@vger.kernel.org>,
        DRI Development <dri-devel@lists.freedesktop.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
References: <20220404084723.79089-1-zheyuma97@gmail.com> <eb2edc5a-afad-f0c9-012f-9b9f226d2e5a@gmx.de> <CAMhUBjmm6ADp2Fr89CCQNX5FnhmBBrwFE0EQ3sq7CLER0J3ZEg@mail.gmail.com>
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On Friday 08 April 2022 03:58:10 Zheyu Ma wrote:
> On Fri, Apr 8, 2022 at 3:50 AM Helge Deller <deller@gmx.de> wrote:
> >
> > On 4/4/22 10:47, Zheyu Ma wrote:
> > > The userspace program could pass any values to the driver through
> > > ioctl() interface. If the driver doesn't check the value of 'pixclock',
> > > it may cause divide error.
> > >
> > > Fix this by checking whether 'pixclock' is zero in the function
> > > i740fb_check_var().
> > >
> > > The following log reveals it:
> > >
> > > divide error: 0000 [#1] PREEMPT SMP KASAN PTI
> > > RIP: 0010:i740fb_decode_var drivers/video/fbdev/i740fb.c:444 [inline]
> > > RIP: 0010:i740fb_set_par+0x272f/0x3bb0 drivers/video/fbdev/i740fb.c:739
> > > Call Trace:
> > >     fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1036
> > >     do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1112
> > >     fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1191
> > >     vfs_ioctl fs/ioctl.c:51 [inline]
> > >     __do_sys_ioctl fs/ioctl.c:874 [inline]
> > >
> > > Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
> >
> > Hello Zheyu,
> >
> > I've applied the patches #2-#7 of this series, but left
> > out this specific patch (for now).
> > As discussed on the mailing list we can try to come up with a
> > better fix (to round up the pixclock when it's invalid).
> > If not, I will apply this one later.
> 
> I'm also looking forward to a more appropriate patch for this driver!

I was not able to reproduce it at first but finally found it: the monitor must be unplugged. If a valid EDID is present, fb_validate_mode() call in i740fb_check_var() will refuse zero pixclock.

Haven't found any obvious way to correct zero pixclock value. Most other drivers simply return -EINVAL.

> Thanks,
> Zheyu Ma
> 


-- 
Ondrej Zary