Re: [PATCH 1/1] mm: Remove the access_ok() call from gup_fast_fallback().

[David Laight]

On Sun, 9 Feb 2025 14:24:22 -0400
Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:

> On Sun, Feb 09, 2025 at 05:47:11PM +0000, David Laight wrote:
> > Historiaclly the code relied on access_ok() to validate the address range.
> > Commit 26f4c328079d7 added an explicit wrap check before access_ok().
> > Commit c28b1fc70390d then changed the wrap test to use check_add_overflow().
> > Commit 6014bc27561f2 relaxed the checks in x86-64's access_ok() and added
> >   an explicit check for TASK_SIZE here to make up for it.
> > That left a pointless access_ok() call with its associated 'lfence' that
> >   can never actually fail.
> > So just delete the test.
> > 
> > Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>
> > ---
> >  mm/gup.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)  
> 
> Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> 
> I often wonder about about access_ok() calls, if they still do
> anything..

They still do 'stuff' and end up containing a slow memory synchronising
instruction (to avoid speculative accesses controlled by the application).

But there are better ways to handle bad user pointers.
So, mostly access_ok() isn't needed outside the architecture code
that handles userspace accesses.

	David