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Date:   Wed, 7 Jul 2021 04:50:30 +0000
From:   Justin He <Justin.He@....com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: RE: [PATCH 03/14] d_path: regularize handling of root dentry in
 __dentry_path()



> -----Original Message-----
> From: Al Viro <viro@....linux.org.uk> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Justin He <Justin.He@....com>; Petr Mladek <pmladek@...e.com>; Steven
> Rostedt <rostedt@...dmis.org>; Sergey Senozhatsky
> <senozhatsky@...omium.org>; Andy Shevchenko
> <andriy.shevchenko@...ux.intel.com>; Rasmus Villemoes
> <linux@...musvillemoes.dk>; Jonathan Corbet <corbet@....net>; Heiko
> Carstens <hca@...ux.ibm.com>; Vasily Gorbik <gor@...ux.ibm.com>; Christian
> Borntraeger <borntraeger@...ibm.com>; Eric W . Biederman
> <ebiederm@...ssion.com>; Darrick J. Wong <darrick.wong@...cle.com>; Peter
> Zijlstra (Intel) <peterz@...radead.org>; Ira Weiny <ira.weiny@...el.com>;
> Eric Biggers <ebiggers@...gle.com>; Ahmed S. Darwish
> <a.darwish@...utronix.de>; open list:DOCUMENTATION <linux-
> doc@...r.kernel.org>; Linux Kernel Mailing List <linux-
> kernel@...r.kernel.org>; linux-s390 <linux-s390@...r.kernel.org>; linux-
> fsdevel <linux-fsdevel@...r.kernel.org>
> Subject: [PATCH 03/14] d_path: regularize handling of root dentry in
> __dentry_path()
>
> All path-forming primitives boil down to sequence of prepend_name()
> on dentries encountered along the way toward root.  Each time we prepend
> / + dentry name to the buffer.  Normally that does exactly what we want,
> but there's a corner case when we don't call prepend_name() at all (in case
> of __dentry_path() that happens if we are given root dentry).  We obviously
> want to end up with "/", rather than "", so this corner case needs to be
> handled.
>
> __dentry_path() used to manually put '/' in the end of buffer before
> doing anything else, to be overwritten by the first call of prepend_name()
> if one happens and to be left in place if we don't call prepend_name() at
> all.  That required manually checking that we had space in the buffer
> (prepend_name() and prepend() take care of such checks themselves) and lead
> to clumsy keeping track of return value.
>
> A better approach is to check if the main loop has added anything
> into the buffer and prepend "/" if it hasn't.  A side benefit of using
> prepend()
> is that it does the right thing if we'd already run out of buffer, making
> the overflow-handling logics simpler.
>
> Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Reviewed-by: Jia He <justin.he@....com>


--
Cheers,
Justin (Jia He)


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