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

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Date:	Thu, 10 Sep 2015 01:17:36 -0700
From:	Joe Perches <joe@...ches.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
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 Thu, 2015-09-10 at 09:56 +0200, Rasmus Villemoes wrote:
> On Thu, Sep 10 2015, Joe Perches <joe@...ches.com> wrote:
> > On Thu, 2015-09-10 at 09:04 +0200, Maurizio Lombardi wrote:
> >> 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.
> 
> It also won't work for the case at hand if/when the actual bitmap ever
> gets a bit set beyond S16_MAX.

But at least it should work for the bitmap sized <= S16_MAX
which should be the majority of uses.

> A (somewhat ugly?) solution might be to teach %pb another flag, say h (for
> huge), meaning that the pointer is actually (struct printf_bitmap*),
> with 
> 
> struct printf_bitmap { unsigned long *bits; unsigned long nbits; }
> 
> Then callers with potentially huge bitmaps would do
> 
> struct printf_bitmap tmp = { my_bitmap, my_size };
> snprintf("%pbhl", &tmp)

Yes, but it still couldn't work without the ability to have
large output buffers and printk doesn't support that.

seq_printf might have some performance issue with it too as
it would repetitively try to emit, fail, and grow the buffer.


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