linux-kernel - Re: [PATCH 4/6] exec: Run sync_mm_rss before taking exec_update

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Date:   Sat, 09 May 2020 09:17:11 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Rob Landley <rob@...dley.net>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/6] exec: Run sync_mm_rss before taking exec_update_mutex

Kees Cook <keescook@...omium.org> writes:

> $ git grep exec_mm_release
> fs/exec.c:      exec_mm_release(tsk, old_mm);
> include/linux/sched/mm.h:extern void exec_mm_release(struct task_struct *, struct mm_struct *);
> kernel/fork.c:void exec_mm_release(struct task_struct *tsk, struct mm_struct *mm)
>
> kernel/fork.c:
>
> void exit_mm_release(struct task_struct *tsk, struct mm_struct *mm)
> {
>         futex_exit_release(tsk);
>         mm_release(tsk, mm);
> }
>
> void exec_mm_release(struct task_struct *tsk, struct mm_struct *mm)
> {
>         futex_exec_release(tsk);
>         mm_release(tsk, mm);
> }
>
> $ git grep exit_mm_release
> include/linux/sched/mm.h:extern void exit_mm_release(struct task_struct *, struct mm_struct *);
> kernel/exit.c:  exit_mm_release(current, mm);
> kernel/fork.c:void exit_mm_release(struct task_struct *tsk, struct mm_struct *mm)
>
> kernel/exit.c:
>
>         exit_mm_release(current, mm);
>         if (!mm)
>                 return;
>         sync_mm_rss(mm);
>
> It looks to me like both exec_mm_release() and exit_mm_release() could
> easily have the sync_mm_rss(...) folded into their function bodies and
> removed from the callers. *shrug*

Well it would have to be all of:
	if (mm) 
		sync_mm_rss(mm);

I remember reading through exit_mm_release and seeing that nothing
actually depended upon a non-NULL mm. Unless you have clear_child_tid
set.

I am not up to speed on that part of the mm layer right now to know if
it is a good idea to put sync_mm_rss in exit_mm_release but at a quick
look it feels like it.

Eric