Re: [patch 2/3] vdso: improve print_fatal_signals support by adding memory maps

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Am Sonntag, 18. Juni 2006 19:42 schrieben Sie:
> On Sun, 18 Jun 2006 15:25:35 +0200 Simon Raffeiner wrote:
> > Am Sonntag, 18. Juni 2006 07:58 schrieben Sie:
> > > On Sat, 17 Jun 2006 21:58:18 -0700
> > >
> > > "Randy.Dunlap" <[email protected]> wrote:
> > > > On Sat, 17 Jun 2006 16:14:52 +0200 Simon Raffeiner wrote:
> > > > > When compiling 2.6.17-rc6-mm2 (which contains this patch) my gcc
> > > > > 4.0.3 (Ubuntu 4.0.3-1ubuntu5) complains about "int len;" being used
> > > > > uninitialized in print_vma(). AFAICS len is not initialized and
> > > > > then passed to
> > > > > pad_len_spaces(int len), which uses it for some calculations.
> > > > >
> > > > > I also noticed that similar code is used in fs/proc/task_mmu.c,
> > > > > where show_map_internal() passes an uninitialised int len; to
> > > > > pad_len_spaces(struct seq_file *m, int len).
> > > >
> > > > Ack both of those.  And both of them pass &len as a parameter to
> > > > printk/seq_printf where it looks as though they want just <len>
> > > > (after it has been initialized).
> > >
> > > printk("%n", &len) will initialise `len'.  gcc is being wrong again.
> >
> > pad_len_spaces() is called in the following way:
> >
> >
> > static int print_vma(struct vm_area_struct *vma)
> > {
> > 	int len;
> >
> > 	(...)
> >
> > 	pad_len_spaces(len);
> >
> > 	(...)
> >
> >
> > and is defined as:
> >
> >
> > static void pad_len_spaces(int len)
> > {
> >        len = 25 + sizeof(void*) * 6 - len;
> >
> >        if (len < 1)
> >                len = 1;
> >
> >        printk("%*c", len, ' ');
> > }
> >
> >
> > len is passed to pad_len_spaces() without initialization and is used for
> > calculations BEFORE printk() is called.
>
> Nope, len is used after printk(..., &len) is called.
> But I don't see how printk() inits len... ?  :(  Magic?

I finally got it: %n in the format string advises printk (have a look at 
lib/vsprintf.c, where vsnprintf() does the real work) to store the number of 
characters that have been written so far to a given memory location. So len 
is actually initialised.

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