Re: [PATCH] Re: bad pmd ffff810000207238(9090909090909090).

[Willy Tarreau <w@1wt.eu>]

On Wed, May 28, 2008 at 10:14:31PM +0200, Jan Engelhardt wrote:
> 
> On Wednesday 2008-05-28 21:56, Willy Tarreau wrote:
> >On Wed, May 28, 2008 at 07:36:07PM +0100, Hugh Dickins wrote:
> >> On Tue, 27 May 2008, Fede wrote:
> >> > 
> >> > Today I tried to start a firewalling script and failed due to an unrelated
> >> > issue, but when I checked the log I saw this:
> >> > 
> >> > May 27 20:38:15 kaoz ip_tables: (C) 2000-2006 Netfilter Core Team
> >> > May 27 20:38:28 kaoz Netfilter messages via NETLINK v0.30.
> >> > May 27 20:38:28 kaoz nf_conntrack version 0.5.0 (16384 buckets, 65536 max)
> >> > May 27 20:38:28 kaoz ctnetlink v0.93: registering with nfnetlink.
> >> > May 27 20:38:28 kaoz ClusterIP Version 0.8 loaded successfully
> >> > May 27 20:38:28 kaoz mm/memory.c:127: bad pmd
> >> > ffff810000207238(9090909090909090).
> >> > 
> >> > I also found another post with a very similar issue. The other post had almost
> >> > the same message (*mm*/*memory*.*c*:*127*: *bad* *pmd*
> >> > ffff810000207808(9090909090909090).)
> >> > 
> >> > Does anyone know what is it?
> >> 
> >> Thanks a lot for re-reporting this: it was fun to work it out.
> >> It's not a rootkit, it's harmless, but we ought to fix the noise.
> >> Simple patch below, but let me explain more verbosely first.
> >> 
> >> What was really interesting in your report was that the address
> >> is so close to that in OGAWA-San's report.  I had a look at that
> >> page on my x86_64 boxes, and they have lots of 0x90s there too.
> >> It's just some page alignment filler that x86_64 kernel startup
> >> has missed cleaning up - patch below fixes that.  There's no
> >> security aspect to it: the entries were already not-present,
> >> they just generate this noise by triggering the pmd_bad test.
> >
> >Is there a particular reason we use 0x90 as an alignment filler ?
> 
> Alignment within functions. You could use a JMP to jump over
> the alignment, but that would be costly. So in order to
> "run through the wall", you need an opcode that does not
> do anything, something like 0x90.
> 0xAF would map to scasd on x86, and I'd hardly call that a
> no-op.

OK, I did not understand from Hugh's explanation that it was
all about alignment within functions. Of course, 0x90 is fine
there (though there are multi-byte NOPs available).

Cheers,
Willy