linux-kernel - Re: "fs/namei.c: keep track of nd->root refcount status" causes boot panic

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Message-ID: <20190905121755.GA23258@ip-172-31-14-16>
Date:   Thu, 5 Sep 2019 12:17:55 +0000
From:   Kevin Easton <kevin@...rana.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>, Qian Cai <cai@....pw>,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: "fs/namei.c: keep track of nd->root refcount status" causes boot
 panic

On Tue, Sep 03, 2019 at 06:56:10PM +0100, Al Viro wrote:
> On Tue, Sep 03, 2019 at 08:39:30AM -0700, Christoph Hellwig wrote:
> 
> > > There's much nastier situation than "new upstream kernel released,
> > > need to rebuild" - it's bisect in mainline trying to locate something...
> > 
> > I really don't get the point.  And it's not like we've card about
> > this anywhere else.  And jumping wildly around with the numeric values
> > for constants will lead to bugs like the one you added and fixed again
> > and again.
> 
> The thing is, there are several groups - it's not as if all additions
> were guaranteed to be at the end.  So either we play with renumbering
> again and again, or we are back to the square one...
> 
> Is there any common trick that would allow to verify the lack of duplicates
> at the build time?

What about:

static_assert(
 (LOOKUP_FOLLOW^LOOKUP_DIRECTORY^LOOKUP_AUTOMOUNT^LOOKUP_EMPTY^LOOKUP_DOWN^
  LOOKUP_REVAL^LOOKUP_RCU^
  LOOKUP_OPEN^LOOKUP_CREATE^LOOKUP_EXCL^LOOKUP_RENAME_TARGET^
  LOOKUP_PARENT^LOOKUP_NO_REVAL^LOOKUP_JUMPED^LOOKUP_ROOT^LOOKUP_ROOT_GRABBED)
 ==
 (LOOKUP_FOLLOW|LOOKUP_DIRECTORY|LOOKUP_AUTOMOUNT|LOOKUP_EMPTY|LOOKUP_DOWN|
  LOOKUP_REVAL|LOOKUP_RCU|
  LOOKUP_OPEN|LOOKUP_CREATE|LOOKUP_EXCL|LOOKUP_RENAME_TARGET|
  LOOKUP_PARENT|LOOKUP_NO_REVAL|LOOKUP_JUMPED|LOOKUP_ROOT|LOOKUP_ROOT_GRABBED)
 , "duplicated LOOKUP_* constant");

?

    - Kevin