RE: [PATCH 03/14] d_path: regularize handling of root dentry in __dentry_path()

Justin He
2021-07-07T04:50:55Z

> -----Original Message-----
> From: Al Viro <viro@ftp.linux.org.uk> On Behalf Of Al Viro
> Sent: Wednesday, May 19, 2021 8:49 AM
> To: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Justin He <Justin.He@arm.com>; Petr Mladek <pmladek@suse.com>; Steven
> Rostedt <rostedt@goodmis.org>; Sergey Senozhatsky
> <senozhatsky@chromium.org>; Andy Shevchenko
> <andriy.shevchenko@linux.intel.com>; Rasmus Villemoes
> <linux@rasmusvillemoes.dk>; Jonathan Corbet <corbet@lwn.net>; Heiko
> Carstens <hca@linux.ibm.com>; Vasily Gorbik <gor@linux.ibm.com>; Christian
> Borntraeger <borntraeger@de.ibm.com>; Eric W . Biederman
> <ebiederm@xmission.com>; Darrick J. Wong <darrick.wong@oracle.com>; Peter
> Zijlstra (Intel) <peterz@infradead.org>; Ira Weiny <ira.weiny@intel.com>;
> Eric Biggers <ebiggers@google.com>; Ahmed S. Darwish
> <a.darwish@linutronix.de>; open list:DOCUMENTATION <linux-
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> 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@zeniv.linux.org.uk>
Reviewed-by: Jia He <justin.he@arm.com>

--
Cheers,
Justin (Jia He)

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