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

[Michal Hocko]

On Thu 02-05-19 14:52:03, Michal Koutny 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>

Just a nit. S-o-b chain is not correct here. The first s-o-b should
match the author (From) of the patch.

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  kernel/sys.c | 10 ++++++++--
>  mm/util.c    |  4 ++--
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 5e0a5edf47f8..14be57840511 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2122,9 +2122,14 @@ static int prctl_set_mm(int opt, unsigned long addr,
>  
>  	error = -EINVAL;
>  
> -	down_write(&mm->mmap_sem);
> +	/*
> +	 * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
> +	 * a) concurrent sys_brk, b) finding VMA for addr validation.
> +	 */
> +	down_read(&mm->mmap_sem);
>  	vma = find_vma(mm, addr);
>  
> +	spin_lock(&mm->arg_lock);
>  	prctl_map.start_code	= mm->start_code;
>  	prctl_map.end_code	= mm->end_code;
>  	prctl_map.start_data	= mm->start_data;
> @@ -2212,7 +2217,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
>  
>  	error = 0;
>  out:
> -	up_write(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
> +	up_read(&mm->mmap_sem);
>  	return error;
>  }
>  
> diff --git a/mm/util.c b/mm/util.c
> index 43a2984bccaa..5cf0e84a0823 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -758,12 +758,12 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
>  	if (!mm->arg_end)
>  		goto out_mm;	/* Shh! No looking before we're done */
>  
> -	down_read(&mm->mmap_sem);
> +	spin_lock(&mm->arg_lock);
>  	arg_start = mm->arg_start;
>  	arg_end = mm->arg_end;
>  	env_start = mm->env_start;
>  	env_end = mm->env_end;
> -	up_read(&mm->mmap_sem);
> +	spin_unlock(&mm->arg_lock);
>  
>  	len = arg_end - arg_start;
>  
> -- 
> 2.16.4

-- 
Michal Hocko
SUSE Labs