linux-kernel - Re: [PATCH v3 1/2] vsprintf: Add %p[mM]U for uppercase MAC address

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Date:   Thu, 27 Apr 2023 14:35:19 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Konrad Gräfe <k.graefe@...eware.de>,
        Quentin Schulz <quentin.schulz@...obroma-systems.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Andrzej Pietrasiewicz <andrzej.p@...labora.com>,
        Felipe Balbi <balbi@...com>
Cc:     stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] vsprintf: Add %p[mM]U for uppercase MAC address

On 27/04/2023 13.51, Konrad Gräfe wrote:
> The CDC-ECM specification requires an USB gadget to send the host MAC
> address as uppercase hex string. This change adds the appropriate
> modifier.
> 
> Cc: stable@...r.kernel.org

Why cc stable?

> Signed-off-by: Konrad Gräfe <k.graefe@...eware.de>
> ---
> Added in v3
> 
>  lib/vsprintf.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)

The diffstat here, or for some other patch in the same series,
definitely ought to mention lib/test_printf.c.

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index be71a03c936a..8aee1caabd9e 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -1269,9 +1269,10 @@ char *mac_address_string(char *buf, char *end, u8 *addr,
>  {
>  	char mac_addr[sizeof("xx:xx:xx:xx:xx:xx")];
>  	char *p = mac_addr;
> -	int i;
> +	int i, pos;
>  	char separator;
>  	bool reversed = false;
> +	bool uppercase = false;
>  
>  	if (check_pointer(&buf, end, addr, spec))
>  		return buf;
> @@ -1281,6 +1282,10 @@ char *mac_address_string(char *buf, char *end, u8 *addr,
>  		separator = '-';
>  		break;
>  
> +	case 'U':
> +		uppercase = true;
> +		break;
> +
>  	case 'R':
>  		reversed = true;
>  		fallthrough;

This seems broken, and I'm surprised the compiler doesn't warn about
separator possibly being uninitialized further down. I'm also surprised
your testing hasn't caught this. For reference, the full switch
statement is currently

        switch (fmt[1]) {
        case 'F':
                separator = '-';
                break;

        case 'R':
                reversed = true;
                fallthrough;

        default:
                separator = ':';
                break;
        }

> @@ -1292,9 +1297,14 @@ char *mac_address_string(char *buf, char *end, u8 *addr,
>  
>  	for (i = 0; i < 6; i++) {
>  		if (reversed)
> -			p = hex_byte_pack(p, addr[5 - i]);
> +			pos = 5 - i;
> +		else
> +			pos = i;
> +
> +		if (uppercase)
> +			p = hex_byte_pack_upper(p, addr[pos]);
>  		else
> -			p = hex_byte_pack(p, addr[i]);
> +			p = hex_byte_pack(p, addr[pos]);

I think this becomes quite hard to follow. We have string_upper() in
linux/string_helpers.h, so I'd rather just leave this loop alone and do

  if (uppercase)
    string_upper(mac_addr, mac_addr);

after the nul-termination.

Rasmus