linux-kernel - Re: [PATCH v4 43/45] namei: initialize parameters passed to step

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Date:   Mon, 4 Jul 2022 14:55:06 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Evgenii Stepanov <eugenis@...gle.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Vitaly Buka <vitalybuka@...gle.com>,
        linux-toolchains <linux-toolchains@...r.kernel.org>
Subject: Re: [PATCH v4 43/45] namei: initialize parameters passed to
 step_into()

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).