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SubjectRe: [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@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

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