[PATCH] mm: use down_read_killable for locking mmap_sem in access_remote_vm

[PATCH] mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Konstantin Khlebnikov]

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.

Mmap_sem could be locked for a long time or forever if something wrong.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 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);

Re: mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Michal Koutný]

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Hi,
making this holder of mmap_sem killable was for the reasons of /proc/...
diagnostics was an idea I was pondeering too. However, I think the
approach of pretending we read 0 bytes is not correct. The API would IMO
need to be extended to allow pass a result such as EINTR to the end
caller.
Why do you think it's safe to return just 0?

Michal


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Re: mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Konstantin Khlebnikov]

On 15.05.2019 11:38, Michal Koutný wrote:
> Hi,
> making this holder of mmap_sem killable was for the reasons of /proc/...
> diagnostics was an idea I was pondeering too. However, I think the
> approach of pretending we read 0 bytes is not correct. The API would IMO
> need to be extended to allow pass a result such as EINTR to the end
> caller.
> Why do you think it's safe to return just 0?

This function ignores any error like reading from unmapped area and
returns only size of successful transfer. It never returned any error codes.

> 
> Michal
> 

Re: mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Matthew Wilcox]

On Wed, May 15, 2019 at 10:38:26AM +0200, Michal Koutný wrote:
> Hi,
> making this holder of mmap_sem killable was for the reasons of /proc/...
> diagnostics was an idea I was pondeering too. However, I think the
> approach of pretending we read 0 bytes is not correct. The API would IMO
> need to be extended to allow pass a result such as EINTR to the end
> caller.
> Why do you think it's safe to return just 0?

_killable_, not _interruptible_.

The return value will never be seen by userspace because it's dead.

Re: [PATCH] mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Oleg Nesterov]

> @@ -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;
> +

I too think that "return 0" looks a bit strange even if correct, to me
"return -EINTR" would look better.

But I won't insist, this is cosmetic.

Acked-by: Oleg Nesterov <oleg@redhat.com>

Re: mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Michal Koutný]

On Wed, May 15, 2019 at 11:48:32AM +0300, Konstantin Khlebnikov <khlebnikov@yandex-team.ru> wrote:
> This function ignores any error like reading from unmapped area and
> returns only size of successful transfer. It never returned any error codes.
This is a point I missed. Hence no need to adjust consumers of
__access_remote_vm() (they won't actually handle -EINTR correctly w/out
further changes). This beats my original idea with simplicity.


Reviewed-by: Michal Koutný <mkoutny@suse.com>

Michal

Re: [PATCH] mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Michal Hocko]

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@yandex-team.ru>

Acked-by: Michal Hocko <mhocko@suse.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