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Date:   Mon, 4 Jul 2022 14:55:06 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Alexander Potapenko <glider@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to
 step_into()
Message-ID: <YsLxOkApqKPQ8Bep@ZenIV>
References: <20220701142310.2188015-1-glider@google.com>
 <20220701142310.2188015-44-glider@google.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>
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On Mon, Jul 04, 2022 at 02:44:00PM +0100, Al Viro wrote:
> On Mon, Jul 04, 2022 at 10:20:53AM +0200, Alexander Potapenko wrote:
> 
> > What makes you think they are false positives? Is the scenario I
> > described above:
> > 
> > """
> > In particular, if the call to lookup_fast() in walk_component()
> > returns NULL, and lookup_slow() returns a valid dentry, then the
> > `seq` and `inode` will remain uninitialized until the call to
> > step_into()
> > """
> > 
> > impossible?
> 
> Suppose step_into() has been called in non-RCU mode.  The first
> thing it does is
> 	int err = handle_mounts(nd, dentry, &path, &seq);
> 	if (err < 0) 
> 		return ERR_PTR(err);
> 
> And handle_mounts() in non-RCU mode is
> 	path->mnt = nd->path.mnt;
> 	path->dentry = dentry;
> 	if (nd->flags & LOOKUP_RCU) {
> 		[unreachable code]
> 	}
> 	[code not touching seqp]
> 	if (unlikely(ret)) {
> 		[code not touching seqp]
> 	} else {
> 		*seqp = 0; /* out of RCU mode, so the value doesn't matter */
> 	}
> 	return ret;

Make that
 	[code assigning ret a non-negative value and never using seqp]
 	if (unlikely(ret)) {
 		[code never using seqp or ret]
 	} else {
 		*seqp = 0; /* out of RCU mode, so the value doesn't matter */
 	}
 	return ret;

so if (err < 0) in the caller is equivalent to if (err).