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Message-ID: <20090915025710.GA23881@localhost>
Date: Tue, 15 Sep 2009 10:57:10 +0800
From: Wu Fengguang <fengguang.wu@...el.com>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
"mtosatti@...hat.com" <mtosatti@...hat.com>,
"gregkh@...e.de" <gregkh@...e.de>,
"broonie@...nsource.wolfsonmicro.com"
<broonie@...nsource.wolfsonmicro.com>,
"johannes@...solutions.net" <johannes@...solutions.net>,
"avi@...ranet.com" <avi@...ranet.com>,
"andi@...stfloor.org" <andi@...stfloor.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
WANG Cong <xiyou.wangcong@...il.com>,
Mike Smith <scgtrp@...il.com>,
Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [PATCH] devmem: handle partial kmem write/read
On Tue, Sep 15, 2009 at 10:31:13AM +0800, KAMEZAWA Hiroyuki wrote:
> On Tue, 15 Sep 2009 10:02:08 +0800
> Wu Fengguang <fengguang.wu@...el.com> wrote:
>
> > On Tue, Sep 15, 2009 at 08:24:48AM +0800, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 14 Sep 2009 12:34:44 +0800
> > > Wu Fengguang <fengguang.wu@...el.com> wrote:
> > >
> > > > Hi Kame,
> > > >
> > > > This patch needs more work. I first intent to fix a bug:
> > > >
> > > > sz = vwrite(kbuf, (char *)p, sz);
> > > > p += sz;
> > > > }
> > > >
> > > > So if the returned len is 0, the kbuf/p pointers will mismatch.
> > > >
> > > > Then I realize it changed the write behavior. The current vwrite()
> > > > behavior is strange, it returns 0 if _whole range_ is hole, otherwise
> > > > ignores the hole silently. So holes can be treated differently even in
> > > > the original code.
> > > >
> > > Ah, ok...
> > >
> > > > I'm not really sure about the right behavior. KAME-san, do you have
> > > > any suggestions?
> > > >
> > > maybe following make sense.
> > > =
> > > written = vwrite(kbuf, (char *p), sz);
> > > if (!written) // whole vmem was a hole
> > > written = sz;
> >
> > Since the 0 return value won't be used at all, it would be simpler to
> > tell vwrite() return the untouched buflen/sz, like this. It will ignore
> > _all_ the holes silently. Need to update comments too.
> >
>
> Hmm. IIUC the original kmem code returns immediately if vread/vwrite returns 0.
Agreed for vread. It seems vwrite don't do that for both kmem and
vmalloc part.
> But it seems this depends on wrong assumption that vmap area is continuous,
> at the first look.
You mean it is possible that not all pages are mapped in a vmlist addr,size range?
Is vmalloc areas guaranteed to be page aligned? Sorry for the dumb questions.
> In man(4) mem,kmem
> ==
> Byte addresses in mem are interpreted as physical memory addresses.
> References to nonexistent locations cause errors to be returned.
> .....
> The file kmem is the same as mem, except that the kernel virtual memory
> rather than physical memory is accessed.
>
> ==
>
> Then, we have to return error for accesses to "nonexistent locations".
That's one important factor to consider. On the other hand, the
original kmem read/write implementation don't return error code for
holes. Instead kmem read returns early, while kmem write ignores holes
but is buggy..
> memory-hole in vmap area is ....."nonexistent" ?
> I think it's nonexistent if there are no overlaps between requested [pos, pos+len)
> and registerred vmalloc area.
> But, hmm, there are no way for users to know "existing vmalloc area".
> Then, my above idea may be wrong.
>
> Then, I'd like to modify as following,
>
> - If is_vmalloc_or_module_addr(requested address) is false, return -EFAULT.
> - If is_vmalloc_or_module_addr(requested address) is true, return no error.
> Even if specified range doesn't include no exsiting vmalloc area.
>
> How do you think ?
Looks reasonable to me. But it's good to hear more wide opinions..
> Thanks,
> -Kame
> p.s. I wonder current /dev/kmem cannot read text area of kernel if it's not
> directly mapped.
Attached is a small tool (from LKML) for reading 'modprobe_path' from kmem,
it's not text, but is close..
Thanks,
Fengguang
>
>
> > --- linux-mm.orig/mm/vmalloc.c 2009-09-15 09:40:08.000000000 +0800
> > +++ linux-mm/mm/vmalloc.c 2009-09-15 09:43:33.000000000 +0800
> > @@ -1834,7 +1834,6 @@ long vwrite(char *buf, char *addr, unsig
> > struct vm_struct *tmp;
> > char *vaddr;
> > unsigned long n, buflen;
> > - int copied = 0;
> >
> > /* Don't allow overflow */
> > if ((unsigned long) addr + count < count)
> > @@ -1858,7 +1857,6 @@ long vwrite(char *buf, char *addr, unsig
> > n = count;
> > if (!(tmp->flags & VM_IOREMAP)) {
> > aligned_vwrite(buf, addr, n);
> > - copied++;
> > }
> > buf += n;
> > addr += n;
> > @@ -1866,8 +1864,6 @@ long vwrite(char *buf, char *addr, unsig
> > }
> > finished:
> > read_unlock(&vmlist_lock);
> > - if (!copied)
> > - return 0;
> > return buflen;
> > }
> >
> > > ==
> > > needs fix.
> > >
> > > Anyway, I wonder kmem is broken. It's should be totally rewritten.
> > >
> > > For example, this doesn't check anything.
> > > ==
> > > if (p < (unsigned long) high_memory) {
> > >
> > > ==
> > >
> > > But....are there users ?
> > > If necessary, I'll write some...
> >
> > I'm trying to stop possible mem/kmem users to access hwpoison pages..
> > I'm not the user, but rather a tester ;)
> >
> > Thanks,
> > Fengguang
> >
> > > Thanks,
> > > -Kame
> > >
> > >
> > > > Thanks,
> > > > Fengguang
> > > >
> > > > On Mon, Sep 14, 2009 at 11:29:01AM +0800, Wu Fengguang 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@...stfloor.org>
> > > > > Signed-off-by: Wu Fengguang <fengguang.wu@...el.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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> > > > Please read the FAQ at http://www.tux.org/lkml/
> > > >
> >
View attachment "tmap.c" of type "text/x-csrc" (1386 bytes)
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