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Date:   Thu, 30 Aug 2018 16:13:33 +0300
From:   Alexey Dobriyan <adobriyan@gmail.com>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     "akpm@linux-foundation.org" <akpm@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 06/13] proc: convert /proc/self to _print_integer()
Message-ID: <20180830131333.GA31422@avx2>
References: <20180827231503.26899-1-adobriyan@gmail.com>
 <20180827231503.26899-6-adobriyan@gmail.com>
 <54cb86c6d96b47bc8017a69be124c105@AcuMS.aculab.com>
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On Wed, Aug 29, 2018 at 09:50:50AM +0000, David Laight wrote:
> From: Alexey Dobriyan
> > Sent: 28 August 2018 00:15
> > ---
> >  fs/proc/self.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/proc/self.c b/fs/proc/self.c
> > index 127265e5c55f..b2279412237b 100644
> > --- a/fs/proc/self.c
> > +++ b/fs/proc/self.c
> > @@ -14,6 +14,7 @@ static const char *proc_self_get_link(struct dentry *dentry,
> >  {
> >  	struct pid_namespace *ns = proc_pid_ns(inode);
> >  	pid_t tgid = task_tgid_nr_ns(current, ns);
> > +	char buf[10], *p = buf + sizeof(buf);
> >  	char *name;
> > 
> >  	if (!tgid)
> > @@ -22,7 +23,11 @@ static const char *proc_self_get_link(struct dentry *dentry,
> >  	name = kmalloc(10 + 1, dentry ? GFP_KERNEL : GFP_ATOMIC);
> >  	if (unlikely(!name))
> >  		return dentry ? ERR_PTR(-ENOMEM) : ERR_PTR(-ECHILD);
> > -	sprintf(name, "%u", tgid);
> > +
> 
> Best not to 'hide' the initialisation of 'p' at the top of the function.
> Much easier to see what is going on if it is moved here.

It was written that way to have initialization near the buffer,
so that it is obvious that pointer and buffer are connected and
making

	p = _print_integer_uNN(p, x);

standard building block which can be copypasted left and right and
still be correct.

> > +	p = _print_integer_u32(p, tgid);
> 
> or just:
> 	p = _print_integer(buf + sizeof(buf), tgid);
> 
> (What a horrid interface ...)

It is. The problem is that adding bound checking will get you current
vsnprintf() and number() and num_to_str().

> > +	memcpy(name, p, buf + sizeof(buf) - p);
> > +	name[buf + sizeof(buf) - p] = '\0';
> > +
> >  	set_delayed_call(done, kfree_link, name);
> >  	return name;
> >  }