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

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Date:	Thu, 10 Sep 2015 00:38:04 -0700
From:	Joe Perches <joe@...ches.com>
To:	Maurizio Lombardi <mlombard@...hat.com>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	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 Thu, 2015-09-10 at 09:04 +0200, Maurizio Lombardi wrote:
> Hi Rasmus,
> 
> On 09/09/2015 08:51 PM, Rasmus Villemoes wrote:
> > 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).
> 
> In the scsi-debug case scnprintf is used, but it doesn't really matter
> because the change I made would influence printk and all its friends as
> well... everything that will parse "%*pb[l]".
> 
> > 
> >> %*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?
> > 
> 
> Yes. To be precise, in the example I mentioned in the commit message, a
> bitmap of size = 524288 bits is created.
> If you assign this number to a s16 variable the result will be zero and
> nothing will be printed.

Maurizio, did you try the patch I posted?
I think it'll work, but it doesn't fix the
fundamental issue of %*pbl with large bitmaps.

vsnprintf / printk as used in the kernel does have
limitations that user mode vsnprintf does not.

> Joe, you mentioned that *pbl is meant for small bitmaps, what should I
> use with big ones?

Something else?

> scsi-debug used the bitmap_scnlistprintf() function but since commit
> dbc760bcc150cc27160f0131b15db76350df4334 this function is just a wrapper
> around scnprintf("%*pbl");
> as a consequence, the scsi-debug map_show() function stopped working
> correctly.

Perhaps the thin wrapper conversions in lib/bitmap.c
in that commit for bitmap_scnprintf, bscnl_emit, and 
bitmap_scnlistprintf should be reverted.


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