Re: [PATCH] Fix mmap_kmem (was: [question] What's the difference between /dev/kmem and /dev/mem)

["Martin J. Bligh" <mbligh@mbligh.org>]

--Arjan van de Ven <arjan@infradead.org> wrote (on Saturday, August 13, 2005 18:50:10 +0200):

> On Fri, 2005-08-12 at 09:35 -0700, Linus Torvalds wrote:
>> 
>> On Thu, 11 Aug 2005, Steven Rostedt wrote:
>> > 
>> > Found the problem.  It is a bug with mmap_kmem.  The order of checks is
>> > wrong, so here's the patch.  Attached is a little program that reads the
>> > System map looking for the variable modprobe_path.  If it finds it, then
>> > it opens /dev/kmem for read only and mmaping it to read the contents of
>> > modprobe_path.
>> 
>> I'm actually more inclined to try to deprecate /dev/kmem.. I don't think 
>> anybody has ever really used it except for some rootkits. It only exists 
>> in the first place because it's historical.
>> 
>> We do need to support /dev/mem for X, but even that might go away some 
>> day. 
>> 
>> So I'd be perfectly happy to fix this, but I'd be even happier if we made 
>> the whole kmem thing a config variable (maybe even default it to "off").
> 
> attached is a simple patch that does exactly this...

Whilst there's no normal legitimite usage for it, it is useful for debugging.
One thing I often do is create a circular log buffer, then fish it back 
out by mmaping /dev/mem or /dev/kmem, and going by system.map offsets.
No, nobody could claim it was clean or elegant, but it *is* useful.
M.