Re: Question: how to handle too big f_pos Re: [PATCH] devmem:handle partial kmem write/read

[Wu Fengguang]

Good catch, Kame!

FYI mmap is OK. I succeeded in reading this address via kmem:

        ffffffff81723880 D modprobe_path

Thanks,
Fengguang

On Tue, Sep 15, 2009 at 03:58:52PM +0800, KAMEZAWA Hiroyuki wrote:
> I'm writing a patch against /dev/kmem...I found a problem.
> 
> /dev/kmem (and /proc/<pid>/mem) puts virtual addres to f->f_pos.
> 
> And, f->f_pos is loff_t .... signed value (not unsigned)
> 
> Then, this check
> rw_verify_area(READ, file, pos, count);
> =>
>         pos = *ppos;
>         if (unlikely((pos < 0) || (loff_t) (pos + count) < 0))
>                 return retval;
> 	
> always returns -EINVAL when I read /dev/kmem's valid address.
> 
> Then, how should I do for read /dev/kmem ? Any idea ?
> (Added Al Viro to CC:)
> 
> What I'm really afraid of is /proc/<pid>/mem. IIUC, it's used by gdb to snoop
> memory, (for example, at gcore).
> I think gdb uses ptrace if it fails to read /proc/<pid>/mem but...it's deadly
> slow.
> 
> Thanks,
> -Kame
> 
> p.s. no problems in /proc/kcore..its offset is not bare addresss.
> 
> 
> 
> 
> 
> On Mon, 14 Sep 2009 11:29:01 +0800
> Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote:
> 
> > Current vwrite silently ignores vwrite() to vmap holes.
> > Better behavior should be stop the write and return
> > on such holes.
> > 
> > Also return on partial read, which may happen in future
> > (eg. hit hwpoison pages).
> > 
> > CC: Andi Kleen <andi@xxxxxxxxxxxxxx>
> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > ---
> >  drivers/char/mem.c |   30 ++++++++++++++++++------------
> >  1 file changed, 18 insertions(+), 12 deletions(-)
> > 
> > --- linux-mm.orig/drivers/char/mem.c	2009-09-14 10:59:50.000000000 +0800
> > +++ linux-mm/drivers/char/mem.c	2009-09-14 11:06:25.000000000 +0800
> > @@ -444,18 +444,22 @@ static ssize_t read_kmem(struct file *fi
> >  		if (!kbuf)
> >  			return -ENOMEM;
> >  		while (count > 0) {
> > +			unsigned long n;
> > +
> >  			sz = size_inside_page(p, count);
> > -			sz = vread(kbuf, (char *)p, sz);
> > -			if (!sz)
> > +			n = vread(kbuf, (char *)p, sz);
> > +			if (!n)
> >  				break;
> > -			if (copy_to_user(buf, kbuf, sz)) {
> > +			if (copy_to_user(buf, kbuf, n)) {
> >  				free_page((unsigned long)kbuf);
> >  				return -EFAULT;
> >  			}
> > -			count -= sz;
> > -			buf += sz;
> > -			read += sz;
> > -			p += sz;
> > +			count -= n;
> > +			buf += n;
> > +			read += n;
> > +			p += n;
> > +			if (n < sz)
> > +				break;
> >  		}
> >  		free_page((unsigned long)kbuf);
> >  	}
> > @@ -551,11 +555,13 @@ static ssize_t write_kmem(struct file * 
> >  				free_page((unsigned long)kbuf);
> >  				return -EFAULT;
> >  			}
> > -			sz = vwrite(kbuf, (char *)p, sz);
> > -			count -= sz;
> > -			buf += sz;
> > -			virtr += sz;
> > -			p += sz;
> > +			n = vwrite(kbuf, (char *)p, sz);
> > +			count -= n;
> > +			buf += n;
> > +			virtr += n;
> > +			p += n;
> > +			if (n < sz)
> > +				break;
> >  		}
> >  		free_page((unsigned long)kbuf);
> >  	}
> > --
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