linux-kernel - Re: [PATCH v2 1/1] tty: n_gsm: fix SW flow control encoding/handling

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Message-ID: <YfFSD8WZiXs3yMmo@kroah.com>
Date:   Wed, 26 Jan 2022 14:52:15 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     daniel.starke@...mens.com
Cc:     linux-serial@...r.kernel.org, jirislaby@...nel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/1] tty: n_gsm: fix SW flow control encoding/handling

On Thu, Jan 20, 2022 at 02:18:57AM -0800, daniel.starke@...mens.com wrote:
> n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
> See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
> The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
> the newer 27.010 here. Chapter 5.2.7.3 states that DC1 (XON) and DC3 (XOFF)
> are the control characters defined in ISO/IEC 646. These shall be quoted if
> seen in the data stream to avoid interpretation as flow control characters.
> 
> ISO/IEC 646 refers to the set of ISO standards described as the ISO
> 7-bit coded character set for information interchange. Its final version
> is also known as ITU T.50.
> See https://www.itu.int/rec/T-REC-T.50-199209-I/en
> 
> To abide the standard it is needed to quote DC1 and DC3 correctly if these
> are seen as data bytes and not as control characters. The current
> implementation already tries to enforce this but fails to catch all
> defined cases. 3GPP 27.010 chapter 5.2.7.3 clearly states that the most
> significant bit shall be ignored for DC1 and DC3 handling. The current
> implementation handles only the case with the most significant bit set 0.
> Cases in which DC1 and DC3 have the most significant bit set 1 are left
> unhandled.
> 
> This patch fixes this by masking the data bytes with ISO_IEC_646_MASK (only
> the 7 least significant bits set 1) before comparing them with XON
> (a.k.a. DC1) and XOFF (a.k.a. DC3) when testing which byte values need
> quotation via byte stuffing.
> 
> Fixes: e1eaea46bb40 (tty: n_gsm line discipline, 2010-03-26)

Nit, no need for a date here, our tools get mad about stuff like this.
Look at the proper format for "Fixes:" line in the documentation.

thanks,

greg k-h