linux-kernel - Re: [PATCH] mm: use down_read_killable for locking mmap_sem in access_remote

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Date:   Fri, 17 May 2019 14:51:34 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] mm: use down_read_killable for locking mmap_sem in
 access_remote_vm

On Wed 15-05-19 11:21:18, Konstantin Khlebnikov wrote:
> This function is used by ptrace and proc files like /proc/pid/cmdline and
> /proc/pid/environ. Return 0 (bytes read) if current task is killed.

Please add an explanation about why this is OK (as explained in the
follow up email).

> Mmap_sem could be locked for a long time or forever if something wrong.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  mm/memory.c |    4 +++-
>  mm/nommu.c  |    3 ++-
>  2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 96f1d473c89a..2e6846d09023 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4348,7 +4348,9 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
>  	void *old_buf = buf;
>  	int write = gup_flags & FOLL_WRITE;
>  
> -	down_read(&mm->mmap_sem);
> +	if (down_read_killable(&mm->mmap_sem))
> +		return 0;
> +
>  	/* ignore errors, just check how much was successfully transferred */
>  	while (len) {
>  		int bytes, ret, offset;
> diff --git a/mm/nommu.c b/mm/nommu.c
> index b492fd1fcf9f..cad8fb34088f 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -1791,7 +1791,8 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
>  	struct vm_area_struct *vma;
>  	int write = gup_flags & FOLL_WRITE;
>  
> -	down_read(&mm->mmap_sem);
> +	if (down_read_killable(&mm->mmap_sem))
> +		return 0;
>  
>  	/* the access must start within one of the target process's mappings */
>  	vma = find_vma(mm, addr);

-- 
Michal Hocko
SUSE Labs