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From:   =?UTF-8?B?7Jyg7Jqp7IiY?= <yongsuyoo0215@gmail.com>
Date:   Tue, 13 Sep 2022 17:36:13 +0900
Message-ID: <CANXPkT44_qCcddg9Ayvk=cRLsUDq8G06toMTBGCsJ3kf+g+h-g@mail.gmail.com>
Subject: Re: [PATCH] media: dvb_ringbuffer : Fix a bug in dvb_ringbuffer.c
To:     Hans Petter Selasky <hps@selasky.org>,
        Kieran Bingham <kieran.bingham@ideasonboard.com>,
        linux-media@vger.kernel.org, linux-kernel@vger.kernel.org,
        mchehab@kernel.org, 0215yys@hanmail.net,
        =?UTF-8?B?7Jyg7Jqp7IiY?= <yongsuyoo0215@gmail.com>
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Dear Hans Petter Selasky

I understood your points.
Thank you for your kind explanation
I found that the buffer size is 65535 like below source code.
The 65535 is not the power of two.
So it can still be a problem.
...
#define RX_BUFFER_SIZE 65535
...
rxbuf =3D vmalloc(RX_BUFFER_SIZE);
...
dvb_ringbuffer_init(&ca->slot_info[slot].rx_buffer, rxbuf, RX_BUFFER_SIZE);
}
...

2022=EB=85=84 9=EC=9B=94 12=EC=9D=BC (=EC=9B=94) =EC=98=A4=ED=9B=84 9:36, H=
ans Petter Selasky <hps@selasky.org>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:

>
> Hi Mauro and YongSu,
>
> Answering my own question: The reason nobody has triggered this yet, is
> because the buffer size used is power of two. Because unsigned modulus
> is used, the result becomes correct. See below. But if non-power of two
> ring-buffer is used, then the result becomes incorrect. There is no
> block for non-power of two sized buffers. See:
>
> https://github.com/search?q=3Ddvb_set_pesfilter&type=3Dcode
>
> cat << EOF > testX.c
> #include <stdio.h>
>
> int
> main()
> {
> int consumed_old;
> int consumed_fix;
> size_t idx =3D 3;
> ssize_t pread =3D 15;
> ssize_t size =3D 256;
>
> consumed_old =3D (idx - pread) % size;
>
> consumed_fix =3D (idx - pread);
> if (consumed_fix < 0)
> consumed_fix +=3D size;
>
> printf("old=3D%d new=3D%d size=3D%zd\n", consumed_old, consumed_fix, size=
);
>
> size =3D 254;
>
> consumed_old =3D (idx - pread) % size;
>
> consumed_fix =3D (idx - pread);
> if (consumed_fix < 0)
> consumed_fix +=3D size;
>
> printf("old=3D%d new=3D%d size=3D%zd\n", consumed_old, consumed_fix, size=
);
>
> return (0);
> }
> EOF
>
> cc testX.c && ./a.out
> old=3D244 new=3D244 size=3D256
> old=3D244 new=3D242 size=3D254
>
> So either push the suggested fix, or block non-power of two buffer sizes!
>
> Best regards,
> --HPS