linux-kernel - Re: [PATCH v2] workqueue.c: Increase workqueue name length

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Date: Wed, 10 Jan 2024 23:52:38 +0100
From: Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To: Rafael Aquini <aquini@...hat.com>
Cc: Audra Mitchell <audra@...hat.com>, linux-kernel@...r.kernel.org,
 tj@...nel.org, jiangshanlai@...il.com, hirokazu.yamauchi.hk@...achi.com,
 ddouwsma@...hat.com, loberman@...hat.com, raquini@...hat.com
Subject: Re: [PATCH v2] workqueue.c: Increase workqueue name length

On 10/01/2024 23.31, Rafael Aquini wrote:
> On Wed, Jan 10, 2024 at 11:06:22PM +0100, Rasmus Villemoes wrote:
>> On 10/01/2024 22.52, Rafael Aquini wrote:

>>> The extra vsnprintf call is required because the return of the existing 
>>> vsnprintf() is going to be already capped by sizeof(wq->name).
>>
>> No, it is not. vsnprintf() returns the length of the would-be-created
>> string if the buffer was big enough. That is independent of whether one
>> does a dummy NULL,0 call or just calls it with a real, but possibly too
>> small, buffer.
>>
>> This is true for userspace (as required by posix) as well as the kernel
>> implementation of vsnprintf(). What makes you think otherwise?
>>
> 
> this snippet from PRINTF(3) man page
> 
> RETURN VALUE
>        Upon successful return, these functions return the number of characters 
>        printed (excluding the null byte used to end output to strings).
> 

Assuming we have the same man pages installed, try reading the very next
paragraph:

 The functions snprintf() and vsnprintf() do not write  more  than  size
 bytes  (including the terminating null byte ('\0')).  If the output was
 truncated due to this limit, then the return value  is  the  number  of
 characters  (excluding the terminating null byte) which would have been
 written to the final string if enough space had been available.   Thus,
 a  return  value  of  size or more means that the output was truncated.

How else would you even expect the vsnprintf(NULL, 0, ...) thing to work?