Re: [PATCH] i386 uaccess to fixmap pages

[Jamie Lokier]

Roland McGrath wrote:
> The address is valid, but above TASK_SIZE.  The purpose of access_ok
> is to say that it's ok to try it and let it fault, because it's a
> user-visible address and not the kernel memory mapped into the high
> part of every process's address space.  The accesses that follow are
> done in kernel mode, so there is no fault for pages marked as not
> user-visible.  The fixmap addresses are > TASK_SIZE and so fail the
> __range_ok test, heretofore making access_ok return false.  Those
> are the code paths leading to EFAULT that I mentioned.
> 
> So far I can't think of a better way to do it.

Why don't you change TASK_SIZE to 0xc0001000 (or so) and place the
user-visible fixmaps at 0xc0000000?

That would have no cost at all.

-- Jamie