Re: [PATCH] prctl_set_mm: downgrade mmap_sem to read lock

[Michal Hocko]

On Thu 18-04-19 15:50:39, Michal Koutny wrote:
> I learnt, it's, alas, too late to drop the non PRCTL_SET_MM_MAP calls
> [1], so at least downgrade the write acquisition of mmap_sem as in the
> patch below (that should be stacked on the previous one or squashed).
> 
> Cyrill, you mentioned lock changes in [1] but the link seems empty. Is
> it supposed to be [2]? That could be an alternative to this patch after
> some refreshments and clarifications.
> 
> 
> [1] https://lore.kernel.org/lkml/20190417165632.GC3040@xxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20180507075606.870903028@xxxxxxxxx/
> 
> ========
> 
> Since commit 88aa7cc688d4 ("mm: introduce arg_lock to protect
> arg_start|end and env_start|end in mm_struct") we use arg_lock for
> boundaries modifications. Synchronize prctl_set_mm with this lock and
> keep mmap_sem for reading only (analogous to what we already do in
> prctl_set_mm_map).
> 
> Also, save few cycles by looking up VMA only after performing basic
> arguments validation.
> 
> Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>

Looks good to me. Please send both patches in one series once you get a
review feedback from other people.

> ---
>  kernel/sys.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 12df0e5434b8..bbce0f26d707 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2125,8 +2125,12 @@ static int prctl_set_mm(int opt, unsigned long addr,
>  
>  	error = -EINVAL;
>  
> -	down_write(&mm->mmap_sem);
> -	vma = find_vma(mm, addr);
> +	/*
> +	 * 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);
> +	spin_lock(&mm->arg_lock);
>  
>  	prctl_map.start_code	= mm->start_code;
>  	prctl_map.end_code	= mm->end_code;
> @@ -2185,6 +2189,7 @@ static int prctl_set_mm(int opt, unsigned long addr,
>  	if (error)
>  		goto out;
>  
> +	vma = find_vma(mm, addr);
>  	switch (opt) {
>  	/*
>  	 * If command line arguments and environment
> @@ -2218,7 +2223,8 @@ 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;
>  }
>  
> -- 
> 2.16.4

-- 
Michal Hocko
SUSE Labs