linux-kernel - Re: [PATCH] lib/vsprintf.c: increase the size of the field

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Date:	Wed, 09 Sep 2015 20:51:33 +0200
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Joe Perches <joe@...ches.com>
Cc:	Maurizio Lombardi <mlombard@...hat.com>, akpm@...ux-foundation.org,
	tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lib/vsprintf.c: increase the size of the field_width variable

On Wed, Sep 09 2015, Joe Perches <joe@...ches.com> wrote:

> On Wed, 2015-09-09 at 12:13 +0200, Maurizio Lombardi wrote:
>> When printing a bitmap using the "%*pb[l]" printk format
>> a 16 bit variable (field_width) is used to store the size of the bitmap.
>> In some cases 16 bits are not sufficient, the variable overflows and
>> printk does not work as expected.
>
> If more than 16 bits are necessary, it couldn't work
> as a single printk is limited to 1024 bytes.

I'm also a little confused; I don't see what printk has to do with the
reported problem (I'd expect the /sys/... file to be generated by
something like seq_printf).

>> 3. Bitmap should be set, but still empty
>> # cat /sys/bus/pseudo/drivers/scsi_debug/map
> []
>> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> []
>> @@ -384,8 +384,8 @@ struct printf_spec {
>>  	u8	flags;		/* flags to number() */
>>  	u8	base;		/* number base, 8, 10 or 16 only */
>>  	u8	qualifier;	/* number qualifier, one of 'hHlLtzZ' */
>> -	s16	field_width;	/* width of output field */
>>  	s16	precision;	/* # of digits/chars */
>> +	s32	field_width;	/* width of output field */
>>  };
>>  
>>  static noinline_for_stack
>
> And this makes the sizeof struct printf_spec more than
> 8 bytes which isn't desireable on x86-32.

Or other architectures, I imagine. I'm pretty sure struct printf_spec
purposely has sizeof==8 to allow it to be (relatively cheaply) passed
around by value.

> %*pb is meant for smallish bitmaps, not big ones.

Yup. And that leads to my other confusion: Given that the expected
output is given as "0-15", does the bitmap really consist of > S16_MAX
bits with only the first 16 set?

Rasmus
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