Re: link(2) and EFAULT

["Simon 'corecode' Schubert" <corecode@xxxxxxxxxxxx>]

[TOFU fixed]

Jonas Trollvik wrote:
> On 7/27/07, Michael Galassi <nerd@xyz.com> wrote:
> > A few hours ago I wrote:
> > > copyin() is called by the system call code and returns
> > > efault after validating the address, before actually
> > > touching it.
> > This is the traditional way of doing things but is clearly expensive
> > as validating a user address takes forever and a bit.  Do we blindly
> > use whatever address is passed in and then add checking to the page
> > fault code to figure out if we should just return efault?  This would
> > be a blatant example of optimizing the common case at small cost for
> > the error case which most people probably don't care about.
> The kernel needs to validate that the pointers it gets from userland
> are valid, otherwise this could lead to an exploit/panic among other
> things.

The kernel actually does what Michael suggests:  It sets a custom fault handler and just copies away. See /sys/platform/pc32/i386/support.s

cheers
 simon

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