Re: [PATCH v6 5/7] mm: support both pid and pidfd for process_madvise

[Minchan Kim <minchan@kernel.org>]

On Fri, Feb 28, 2020 at 02:41:07PM -0800, Suren Baghdasaryan wrote:
> On Tue, Feb 18, 2020 at 5:44 PM Minchan Kim <minchan@kernel.org> wrote:
> >
> > There is a demand[1] to support pid as well pidfd for process_madvise
> > to reduce unnecessary syscall to get pidfd if the user has control of
> > the target process(ie, they could guarantee the process is not gone
> > or pid is not reused. Or, it might be okay to give a hint to wrong
> > process).
> 
> nit: When would "give a hint to wrong process" be ok? I would just
> remove this part.

I wanted to say non destructive hints. It's already true for other
some hints because they are just best effort so it's not critical
to be failed. If you mind it, I will remove the phrase.

Thanks.

> 
> >
> > This patch aims for supporting both options like waitid(2). So, the
> > syscall is currently,
> >
> >         int process_madvise(int which, pid_t pid, void *addr,
> >                 size_t length, int advise, unsigned long flag);
> >
> > @which is actually idtype_t for userspace libray and currently,
> > it supports P_PID and P_PIDFD.
> >
> > [1]  https://lore.kernel.org/linux-mm/9d849087-3359-c4ab-fbec-859e8186c509@virtuozzo.com/
> >
> > Cc: Christian Brauner <christian@brauner.io>
> > Suggested-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> > Signed-off-by: Minchan Kim <minchan@kernel.org>
> > ---
> >  include/linux/syscalls.h |  3 ++-
> >  mm/madvise.c             | 34 ++++++++++++++++++++++------------
> >  2 files changed, 24 insertions(+), 13 deletions(-)
> >
> > diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> > index e4cd2c2f8bb4..f5ada20e2943 100644
> > --- a/include/linux/syscalls.h
> > +++ b/include/linux/syscalls.h
> > @@ -876,7 +876,8 @@ asmlinkage long sys_munlockall(void);
> >  asmlinkage long sys_mincore(unsigned long start, size_t len,
> >                                 unsigned char __user * vec);
> >  asmlinkage long sys_madvise(unsigned long start, size_t len, int behavior);
> > -asmlinkage long sys_process_madvise(int pidfd, unsigned long start,
> > +
> > +asmlinkage long sys_process_madvise(int which, pid_t pid, unsigned long start,
> >                         size_t len, int behavior, unsigned long flags);
> >  asmlinkage long sys_remap_file_pages(unsigned long start, unsigned long size,
> >                         unsigned long prot, unsigned long pgoff,
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index def1507c2030..f6d9b9e66243 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1182,11 +1182,10 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
> >         return do_madvise(current, current->mm, start, len_in, behavior);
> >  }
> >
> > -SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start,
> > +SYSCALL_DEFINE6(process_madvise, int, which, pid_t, upid, unsigned long, start,
> >                 size_t, len_in, int, behavior, unsigned long, flags)
> >  {
> >         int ret;
> > -       struct fd f;
> >         struct pid *pid;
> >         struct task_struct *task;
> >         struct mm_struct *mm;
> > @@ -1197,20 +1196,31 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start,
> >         if (!process_madvise_behavior_valid(behavior))
> >                 return -EINVAL;
> >
> > -       f = fdget(pidfd);
> > -       if (!f.file)
> > -               return -EBADF;
> > +       switch (which) {
> > +       case P_PID:
> > +               if (upid <= 0)
> > +                       return -EINVAL;
> > +
> > +               pid = find_get_pid(upid);
> > +               if (!pid)
> > +                       return -ESRCH;
> > +               break;
> > +       case P_PIDFD:
> > +               if (upid < 0)
> > +                       return -EINVAL;
> >
> > -       pid = pidfd_pid(f.file);
> > -       if (IS_ERR(pid)) {
> > -               ret = PTR_ERR(pid);
> > -               goto fdput;
> > +               pid = pidfd_get_pid(upid);
> > +               if (IS_ERR(pid))
> > +                       return PTR_ERR(pid);
> > +               break;
> > +       default:
> > +               return -EINVAL;
> >         }
> >
> >         task = get_pid_task(pid, PIDTYPE_PID);
> >         if (!task) {
> >                 ret = -ESRCH;
> > -               goto fdput;
> > +               goto put_pid;
> >         }
> >
> >         mm = mm_access(task, PTRACE_MODE_ATTACH_FSCREDS);
> > @@ -1223,7 +1233,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, unsigned long, start,
> >         mmput(mm);
> >  release_task:
> >         put_task_struct(task);
> > -fdput:
> > -       fdput(f);
> > +put_pid:
> > +       put_pid(pid);
> >         return ret;
> >  }
> > --
> > 2.25.0.265.gbab2e86ba0-goog
> >
> 
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>