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SubjectRe: CONFIG_DEBUG_PAGEALLOC and virt_addr_valid()
On Thu, 1 Apr 2004, Sridhar Samudrala wrote:

> On Thu, 1 Apr 2004, Christoph Hellwig wrote:
>
> > On Thu, Apr 01, 2004 at 11:11:39AM -0800, Sridhar Samudrala wrote:
> > > When CONFIG_DEBUG_PAGEALLOC is enabled, i am noticing that virt_addr_valid()
> > > (called from sctp_is_valid_kaddr()) is returning true even for freed objects.
> > > Is this a bug or expected behavior?
> >
> > Generally every use of virt_addr_valid() is a bug. What are you trying to
> > do?
>
> We are trying to validate a kernel address that is passed by the user. Is
> there a better way to do that?
>
> When an SCTP association is established, the pointer to the association
> structure is passed to the user as an identifier of the association. This
> identifier is used in the later calls by the user.
>
> -Sridhar

Are you now moving the protocol out of user-space and into the
kernel? If so, you should use the Unix/Linux methods of reading/
writing user memory.

Any protection is on a per-page basis. If you give a user some
pointer into the kernel, he can destroy a whole page before it
is detected! So, even if virt_addr_valid() did the "right" thing
you'd be in a lot of trouble using it.


Cheers,
Dick Johnson
Penguin : Linux version 2.4.24 on an i686 machine (797.90 BogoMips).
Note 96.31% of all statistics are fiction.


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