netdev - Re: [PATCH net-next v2] net: nfc: Propagate ISO14443 type A target ATS to userspace via netlink

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Message-ID: <a0d73e24863106f477abba75b996f4b9ff00d737.camel@sarinay.com>
Date: Wed, 06 Nov 2024 15:58:08 +0100
From: Juraj Šarinay <juraj@...inay.com>
To: Simon Horman <horms@...nel.org>
Cc: netdev@...r.kernel.org, krzk@...nel.org, kuba@...nel.org
Subject: Re: [PATCH net-next v2] net: nfc: Propagate ISO14443 type A target
 ATS to userspace via netlink

On Wed, 2024-11-06 at 10:18 +0000, Simon Horman wrote:
> > Add a 20-byte field ats to struct nfc_target and expose it as
> > NFC_ATTR_TARGET_ATS via the netlink interface. The payload contains
> > 'historical bytes' that help to distinguish cards from one another.
> > The information is commonly used to assemble an emulated ATR similar
> > to that reported by smart cards with contacts.
> 
> Perhaps I misunderstand things, and perhaps there is precedence in relation
> to ATR_RES. But I am slightly concerned that this leans towards exposing
> internal details rather then semantics via netlink.
> 

Hi Simon

Thanks for the feedback. NFC_ATTR_TARGET_ATS would serve a similar
purpose as the following attributes the kernel already exposes (see
nfc.h):

 * @NFC_ATTR_TARGET_SENS_RES: NFC-A targets extra information such as NFCID
 * @NFC_ATTR_TARGET_SEL_RES: NFC-A targets extra information (useful if the
 *	target is not NFC-Forum compliant)
 * @NFC_ATTR_TARGET_NFCID1: NFC-A targets identifier, max 10 bytes
 * @NFC_ATTR_TARGET_SENSB_RES: NFC-B targets extra information, max 12 bytes
 * @NFC_ATTR_TARGET_SENSF_RES: NFC-F targets extra information, max 18 bytes
 * @NFC_ATTR_TARGET_ISO15693_DSFID: ISO 15693 Data Storage Format Identifier
 * @NFC_ATTR_TARGET_ISO15693_UID: ISO 15693 Unique Identifier

The ATR I am after means "Answer To Reset" as defined in ISO 7816. It
has little to do with ATR_RES := "Attribute Request Response" defined
in the NFC Digital specification. I only mentioned ATR_RES as the
source of the information handled by nci_store_general_bytes_nfc_dep(),
the function that motivated some of the code I propose to add.

Part 3 of the PC/SC Specification, Section 3.1.3.2.3 on ATR may perhaps
serve as a more authoritative source of motivation for the patch.

https://pcscworkgroup.com/Download/Specifications/pcsc3_v2.01.09.pdf

The goal is to access contactless identification cards via PC/SC.

	Juraj