linux-kernel - Re: [PATCH] mm/mincore: allow for making sys

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Date:   Thu, 24 Jan 2019 00:12:47 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Josh Snyder <joshs@...flix.com>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

On Thu, 24 Jan 2019, Linus Torvalds wrote:

> Side note: the inode_permission() addition to can_do_mincore() in that
> patch 0002, seems to be questionable. We do
> 
> +static inline bool can_do_mincore(struct vm_area_struct *vma)
> +{
> +       return vma_is_anonymous(vma)
> +               || (vma->vm_file && (vma->vm_file->f_mode & FMODE_WRITE))
> +               || inode_permission(file_inode(vma->vm_file), MAY_WRITE) == 0;
> +}
> 
> note how it tests whether vma->vm_file is NULL for the FMODE_WRITE
> test, but not for the inode_permission() test.
> 
> So either we test unnecessarily in the second line, or we don't
> properly test it in the third one.
> 
> I think the "test vm_file" thing may be unnecessary, because a
> non-anonymous mapping should always have a file pointer and an inode.
> But I could  imagine some odd case (vdso mapping, anyone?) that
> doesn't have a vm_file, but also isn't anonymous.

Hmm, good point.

So dropping the 'vma->vm_file' test and checking whether given vma is 
special mapping should hopefully provide the desired semantics, shouldn't 
it?

-- 
Jiri Kosina
SUSE Labs