Messages in this thread | | | From | Arnd Bergmann <> | Date | Tue, 9 Jun 2020 16:22:34 +0200 | Subject | Re: ARM: vmsplit 4g/4g |
| |
On Tue, Jun 9, 2020 at 2:15 PM afzal mohammed <afzal.mohd.ma@gmail.com> wrote: > On Mon, Jun 08, 2020 at 08:47:27PM +0530, afzal mohammed wrote: > > On Mon, Jun 08, 2020 at 04:43:57PM +0200, Arnd Bergmann wrote: > > > > There is another difference: get_user_pages_fast() does not return > > > a vm_area_struct pointer, which is where you would check the access > > > permissions. I suppose those pointers could not be returned to callers > > > that don't already hold the mmap_sem. > > > > Ok, thanks for the details, i need to familiarize better with mm. > > i was & now more confused w.r.t checking access permission using > vm_area_struct to deny write on a read only user page. > > i have been using get_user_pages_fast() w/ FOLL_WRITE in copy_to_user. > Isn't that sufficient ?, afaiu, get_user_pages_fast() will ensure that > w/ FOLL_WRITE, pte has write permission, else no struct page * is > handed back to the caller.
Ah, that does make a lot of sense, I had just never used that flag myself so I wasn't aware of this.
> Am i missing something or wrong in the analysis ?
As far as I can tell, you are absolutely right, and get_user_pages_fast() is the best way to handle this correctly and efficiently.
Arnd
| |