Re: [uml-devel] running UserModeLinux under Valgrind(memcheck)

[Jeff Dike <jdike@addtoit.com>]

On Thu, Dec 27, 2007 at 10:38:21AM -0800, John Reiser wrote:
> Patches have been developed which enable UserModeLinux for i686 to
> run under the memcheck tool of Valgrind on i686.  Thus it is possible
> to check dynamically the memory accesses made by a running Linux kernel
> against memcheck's model of allowed behavior.  This work was supported
> by Google Inc.
> 
> The goods:

Nice!

Did you find anything needing fixing besides the ubd and random
drivers reading uninitialized stuff and the random bytes that UML uses
to communicate with itself being uninitialized?

I did a quick scan of the UML patch, and didn't see any unexpected
fixes there.

> On the UML side, there is a significant technical issue: the semantics
> of kmalloc+kfree do not match the semantics of malloc+free.  The kernel
> slab allocator caches and re-issues identified objects, which accumulate
> state and retain it throughout execution, including from kfree to kmalloc.
> In contrast, a region that is passed to free() loses both its contents
> and its identity.  Also, size is an important parameter to malloc,
> but is implicit to kmalloc.  The initial patches finesse these issues
> (for instance: by supplying the size as trailing parameter to kmalloc,
> and by noticing that SLAB_POISON ==> free()), but there will be
> significant discussion and work in resolving the differences.

Any problem with supporting these sorts of allocation models in
valgrind?  Not all the world is malloc and free (although maybe 99%
is).

Can we pass the allocation size through a valgrind annotation rather
than add the parameter to kmem_cache_alloc?  

				Jeff

-- 
Work email - jdike at linux dot intel dot com

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