Re: [PATCH] inotify: hide internal kernel bits from fdinfo

[Eric Paris]

Acked-by: Eric Paris <eparis@xxxxxxxxxx>

On Mon, 2015-09-21 at 11:45 -0700, Dave Hansen wrote:
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> 
> There was a report that my patch:
> 
> 	inotify: actually check for invalid bits in
> sys_inotify_add_watch()
> 
> broke CRIU.
> 
> The reason is that CRIU looks up raw flags in /proc/$pid/fdinfo/*
> to figure out how to rebuild inotify watches and then passes those
> flags directly back in to the inotify API.  One of those flags
> (FS_EVENT_ON_CHILD) is set in mark->mask, but is not part of the
> inotify API.  It is used inside the kernel to _implement_ inotify
> but it is not and has never been part of the API.
> 
> My patch above ensured that we only allow bits which are part of
> the API (IN_ALL_EVENTS).  This broke CRIU.
> 
> FS_EVENT_ON_CHILD is really internal to the kernel.  It is set
> _anyway_ on all inotify marks.  So, CRIU was really just trying
> to set a bit that was already set.
> 
> This patch hides that bit from fdinfo.  CRIU will not see the
> bit, not try to set it, and should work as before.  We should not
> have been exposing this bit in the first place, so this is a good
> patch independent of the CRIU problem.
> 
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Reported-by: Andrey Wagin <avagin@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
> Cc: xemul@xxxxxxxxxxxxx
> Cc: Eric Paris <eparis@xxxxxxxxxx>
> Cc: john@xxxxxxxxxxxxxxxxx
> Cc: rlove@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
> 
>  b/fs/notify/fdinfo.c |    9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff -puN fs/notify/fdinfo.c~fdinfo-mask fs/notify/fdinfo.c
> --- a/fs/notify/fdinfo.c~fdinfo-mask	2015-09-21
> 10:24:01.031864268 -0700
> +++ b/fs/notify/fdinfo.c	2015-09-21 10:25:04.335723826 -0700
> @@ -82,9 +82,16 @@ static void inotify_fdinfo(struct seq_fi
>  	inode_mark = container_of(mark, struct inotify_inode_mark,
> fsn_mark);
>  	inode = igrab(mark->inode);
>  	if (inode) {
> +		/*
> +		 * IN_ALL_EVENTS represents all of the mask bits
> +		 * that we expose to userspace.  There is at
> +		 * least one bit (FS_EVENT_ON_CHILD) which is
> +		 * used only internally to the kernel.
> +		 */
> +		u32 mask = mark->mask & IN_ALL_EVENTS;
>  		seq_printf(m, "inotify wd:%x ino:%lx sdev:%x mask:%x
> ignored_mask:%x ",
>  			   inode_mark->wd, inode->i_ino, inode->i_sb
> ->s_dev,
> -			   mark->mask, mark->ignored_mask);
> +			   mask, mark->ignored_mask);
>  		show_mark_fhandle(m, inode);
>  		seq_putc(m, '\n');
>  		iput(inode);
> _
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