Re: [PATCH v3 2/2] prctl_set_mm: downgrade mmap_sem to read lock

[Cyrill Gorcunov]

On Thu, May 02, 2019 at 02:52:03PM +0200, Michal Koutný wrote:
> The commit a3b609ef9f8b ("proc read mm's {arg,env}_{start,end} with mmap
> semaphore taken.") added synchronization of reading argument/environment
> boundaries under mmap_sem. Later commit 88aa7cc688d4 ("mm: introduce
> arg_lock to protect arg_start|end and env_start|end in mm_struct")
> avoided the coarse use of mmap_sem in similar situations. But there
> still remained two places that (mis)use mmap_sem.
> 
> get_cmdline should also use arg_lock instead of mmap_sem when it reads the
> boundaries.
> 
> The second place that should use arg_lock is in prctl_set_mm. By
> protecting the boundaries fields with the arg_lock, we can downgrade
> mmap_sem to reader lock (analogous to what we already do in
> prctl_set_mm_map).
> 
> v2: call find_vma without arg_lock held
> v3: squashed get_cmdline arg_lock patch
> 
> Fixes: 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct")
> Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> Cc: Mateusz Guzik <mguzik@xxxxxxxxxx>
> CC: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
> Co-developed-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
> Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
Reviewed-by: Cyrill Gorcunov <gorcunov@xxxxxxxxx>