linux-kernel - Re: Inverted mount options completely broken (iversion,relatime)

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Date:   Wed, 29 Jul 2020 21:58:41 +0100
From:   David Howells <dhowells@...hat.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     dhowells@...hat.com, Josef Bacik <josef@...icpanda.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Eric Sandeen <sandeen@...hat.com>,
        Christoph Hellwig <hch@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        David Sterba <dsterba@...e.com>
Subject: Re: Inverted mount options completely broken (iversion,relatime)

David Howells <dhowells@...hat.com> wrote:

> > So my question is, what do we do here?
> 
> Hmmm...  As the code stands, MS_RDONLY, MS_SYNCHRONOUS, MS_MANDLOCK,
> MS_I_VERSION and MS_LAZYTIME should all be masked off before the new flags are
> set if called from mount(2) rather than fsconfig(2).
> 
> do_remount() gives MS_RMT_MASK to fs_context_for_reconfigure() to load into
> fc->sb_flags_mask, which should achieve the desired effect in
> reconfigure_super() on this line:
> 
> 	WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
> 				 (fc->sb_flags & fc->sb_flags_mask)));

So applying the attached patch and then doing:

mount -t tmpfs none /mnt
mount -o remount,iversion /mnt
mount -o remount,noiversion /mnt
mount -o remount,norelatime /mnt
mount -o remount,relatime /mnt

prints:

sb=70010000 set=800000 mask=2800051
sb=70810000 set=0 mask=2800051
sb=70010000 set=0 mask=2800051
sb=70010000 set=0 mask=2800051

MS_RELATIME isn't included in MS_RMT_MASK, so remount shouldn't be able to
change it.

David
---
diff --git a/fs/super.c b/fs/super.c
index 904459b35119..540cb37c11e7 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -964,6 +964,7 @@ int reconfigure_super(struct fs_context *fc)
 		}
 	}
 
+	printk("sb=%lx set=%x mask=%x\n", sb->s_flags, fc->sb_flags, fc->sb_flags_mask);
 	WRITE_ONCE(sb->s_flags, ((sb->s_flags & ~fc->sb_flags_mask) |
 				 (fc->sb_flags & fc->sb_flags_mask)));
 	/* Needs to be ordered wrt mnt_is_readonly() */