linux-kernel - Re: [PATCH] nfp: remove h from printk format specifier

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Date:   Thu, 24 Dec 2020 14:39:16 -0800
From:   Joe Perches <joe@...ches.com>
To:     Tom Rix <trix@...hat.com>,
        Simon Horman <simon.horman@...ronome.com>
Cc:     kuba@...nel.org, davem@...emloft.net, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, gustavoars@...nel.org,
        louis.peens@...ronome.com, netdev@...r.kernel.org,
        bpf@...r.kernel.org, oss-drivers@...ronome.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfp: remove h from printk format specifier

On Thu, 2020-12-24 at 14:14 -0800, Tom Rix wrote:
> On 12/24/20 12:21 PM, Simon Horman wrote:
> > On Wed, Dec 23, 2020 at 12:20:53PM -0800, trix@...hat.com wrote:
> > > From: Tom Rix <trix@...hat.com>
> > > 
> > > This change fixes the checkpatch warning described in this commit
> > > commit cbacb5ab0aa0 ("docs: printk-formats: Stop encouraging use of unnecessary %h[xudi] and %hh[xudi]")
> > > 
> > > Standard integer promotion is already done and %hx and %hhx is useless
> > > so do not encourage the use of %hh[xudi] or %h[xudi].
> > > 
> > > Signed-off-by: Tom Rix <trix@...hat.com>
> > Hi Tom,
> > 
> > This patch looks appropriate for net-next, which is currently closed.
> > 
> > The changes look fine, but I'm curious to know if its intentionally that
> > the following was left alone in ethernet/netronome/nfp/nfp_net_ethtool.c:nfp_net_get_nspinfo()
> > 
> > 	snprintf(version, ETHTOOL_FWVERS_LEN, "%hu.%hu"
> 
> I am limiting changes to logging functions, what is roughly in checkpatch.
> 
> I can add this snprintf in if you want.

I'm a bit confused here Tom.

I thought your clang-tidy script was looking for anything marked with
__printf() that is using %h[idux] or %hh[idux].

Wouldn't snprintf qualify for this already?

include/linux/kernel.h-extern __printf(3, 4)
include/linux/kernel.h:int snprintf(char *buf, size_t size, const char *fmt, ...);

Kernel code doesn't use a signed char or short with %hx or %hu very often
but in case you didn't already know, any signed char/short emitted with
anything like %hx or %hu needs to be left alone as sign extension occurs so:

	signed char foo = -1;
	printk("%hx", foo);

emits ffff but

	printk("%x", foo);

emits ffffffff

An example:

$ gcc -x c -
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
	signed short i = -1;
	printf("hx: %hx\n", i);
	printf("x:  %x\n", i);
	printf("hu: %hu\n", i);
	printf("u:  %u\n", i);
	return 0;
}

$ ./a.out
hx: ffff
x:  ffffffff
hu: 65535
u:  4294967295

$