Re: [PATCH] mm: rcu-protected get_mm_exe_file()

[Oleg Nesterov <oleg@redhat.com>]

On 03/16, Konstantin Khlebnikov wrote:
>
> +/**
> + * set_mm_exe_file - change a reference to the mm's executable file
> + *
> + * This changes mm's executale file (shown as symlink /proc/[pid]/exe).
> + *
> + * Main users are mmput(), sys_execve() and sys_prctl(PR_SET_MM_MAP/EXE_FILE).
> + * Callers prevent concurrent invocations: in mmput() nobody alive left,
> + * in execve task is single-threaded, prctl holds mmap_sem exclusively.
> + */
>  void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
>  {
> +	struct file *old_exe_file = rcu_dereference_protected(mm->exe_file,
> +			!atomic_read(&mm->mm_users) || current->in_execve ||
> +			lock_is_held(&mm->mmap_sem));
> +
>  	if (new_exe_file)
>  		get_file(new_exe_file);
> -	if (mm->exe_file)
> -		fput(mm->exe_file);
> -	mm->exe_file = new_exe_file;
> +	rcu_assign_pointer(mm->exe_file, new_exe_file);
> +	if (old_exe_file)
> +		fput(old_exe_file);
>  }

Yes, I think this is correct, __fput() does call_rcu(file_free_rcu). And
much better than the new lock ;)

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



So I think the patch is fine, but personally I dislike the "prctl holds
mmap_sem exclusively" and rcu_dereference_protected().

I mean, I think we can do another cleanup on top of this change.

	1. set_mm_exe_file() should be called by exit/exec only, so
	   it should use

		rcu_dereference_protected(mm->exe_file,
					atomic_read(&mm->mm_users) <= 1);

	2. prctl() should not use it, it can do

	   get_file(new_exe);
	   old_exe = xchg(&mm->exe_file);
	   if (old_exe)
	   	fput(old_exe);

	3. and we can remove down_write(mmap_sem) from prctl paths.

	   Actually we can do this even without xchg() above, but we might
	   want to kill MMF_EXE_FILE_CHANGED and test_and_set_bit() check.

What do you think?

Oleg.

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