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

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Message-ID: <7b5517e6-41a9-cc7f-f42f-8ef449f3898e@redhat.com>
Date:   Fri, 25 Dec 2020 06:56:41 -0800
From:   Tom Rix <trix@...hat.com>
To:     Joe Perches <joe@...ches.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 12/24/20 2:39 PM, Joe Perches wrote:
> 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].
Yes, it uses the format attribute to find the logging functions.
>
> 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, ...);

Yes, this is found.

But since snprintf is not really a logging function, I ignore these.

If someone asks for them not to be ignored in a specific change, I will do that.

>
> 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:

Yes, this would also effect checkpatch.

Tom

>
> 	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
>
> $
>
>