Re: [PATCH v2] proc: fix /proc/*/map_files lookup some more

[Alexey Dobriyan <adobriyan@gmail.com>]

On Wed, Feb 21, 2018 at 12:04:03PM -0800, Andrew Morton wrote:
> On Wed, 21 Feb 2018 22:53:40 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:
> 
> > I totally forgot that _parse_integer() accepts arbitrary amount of
> > leading zeroes leading to the following:
> > 
> > 		OK
> > 	# readlink /proc/1/map_files/56427ecba000-56427eddc000
> > 	/lib/systemd/systemd
> > 
> > 		bogus
> > 	# readlink /proc/1/map_files/00000000000056427ecba000-56427eddc000
> > 	/lib/systemd/systemd
> > 	# readlink /proc/1/map_files/56427ecba000-00000000000056427eddc000
> > 	/lib/systemd/systemd
> > 
> > ...
> > 
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1916,6 +1916,8 @@ static int dname_to_vma_addr(struct dentry *dentry,
> >  	unsigned long long sval, eval;
> >  	unsigned int len;
> >  
> > +	if (str[0] == '0' && str[1])
> > +		return -EINVAL;
> >  	len = _parse_integer(str, 16, &sval);
> >  	if (len & KSTRTOX_OVERFLOW)
> >  		return -EINVAL;
> > @@ -1927,6 +1929,8 @@ static int dname_to_vma_addr(struct dentry *dentry,
> >  		return -EINVAL;
> >  	str++;
> >  
> > +	if (str[0] == '0' && str[1])
> > +		return -EINVAL;
> >  	len = _parse_integer(str, 16, &eval);
> >  	if (len & KSTRTOX_OVERFLOW)
> >  		return -EINVAL;
> 
> I don't know this code and I'm all confused.
> 
> - why is the code designed to accept addresses of "0"?

Now I'm confused.
Code rejects, say ,'07ff...-...' because printing with %lx-%lx would never
produce leading zero.

> - how do we know that the first digit of a VMA address will never be 0?

Except when address is exactly 0 but this case is handled by looking at
the second character.