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SubjectRe: Question: how to handle too big f_pos Re: [PATCH] devmem: handle partial kmem write/read
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@intel.com> 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@firstfloor.org>
> > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > ---
> > 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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> > More majordomo info at http://vger.kernel.org/majordomo-info.html
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> >


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