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From:   Justin He <Justin.He@arm.com>
To:     Matthew Wilcox <willy@infradead.org>
CC:     Linus Torvalds <torvalds@linux-foundation.org>,
        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>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Luca Coelho <luciano.coelho@intel.com>,
        Kalle Valo <kvalo@codeaurora.org>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Johannes Berg <johannes.berg@intel.com>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-wireless@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>
Subject: RE: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for
 file
Thread-Topic: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for
 file
Thread-Index: AQHXU7ZL838wopFuOEK5GzAxNom8qqr420gAgAAQ9UA=
Date:   Fri, 28 May 2021 14:22:01 +0000
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Hi Matthew
> -----Original Message-----
> From: Matthew Wilcox <willy@infradead.org>
> Sent: Friday, May 28, 2021 9:00 PM
> To: Justin He <Justin.He@arm.com>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>; 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>; Alexander
> Viro <viro@zeniv.linux.org.uk>; Luca Coelho <luciano.coelho@intel.com>;
> Kalle Valo <kvalo@codeaurora.org>; David S. Miller <davem@davemloft.net>;
> Jakub Kicinski <kuba@kernel.org>; Heiko Carstens <hca@linux.ibm.com>;
> Vasily Gorbik <gor@linux.ibm.com>; Christian Borntraeger
> <borntraeger@de.ibm.com>; Johannes Berg <johannes.berg@intel.com>; linux-
> doc@vger.kernel.org; linux-kernel@vger.kernel.org; linux-
> wireless@vger.kernel.org; netdev@vger.kernel.org; linux-
> s390@vger.kernel.org
> Subject: Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path
> for file
>
> On Fri, May 28, 2021 at 07:39:50PM +0800, Jia He wrote:
> > We have '%pD' for printing a filename. It may not be perfect (by
> > default it only prints one component.)
> >
> > As suggested by Linus at [1]:
> > A dentry has a parent, but at the same time, a dentry really does
> > inherently have "one name" (and given just the dentry pointers, you
> > can't show mount-related parenthood, so in many ways the "show just
> > one name" makes sense for "%pd" in ways it doesn't necessarily for
> > "%pD"). But while a dentry arguably has that "one primary component",
> > a _file_ is certainly not exclusively about that last component.
> >
> > Hence "file_dentry_name()" simply shouldn't use "dentry_name()" at all.
> > Despite that shared code origin, and despite that similar letter
> > choice (lower-vs-upper case), a dentry and a file really are very
> > different from a name standpoint.
> >
> > Here stack space is preferred for file_d_path_name() because it is
> > much safer. The stack size 256 is a compromise between stack overflow
> > and too short full path.
>
> How is it "safer"?  You already have a buffer passed from the caller.
> Are you saying that d_path_fast() might overrun a really small buffer
> but won't overrun a 256 byte buffer?
No, it won't overrun a 256 byte buf. When the full path size is larger than=
 256, the p->len is < 0 in prepend_name, and this overrun will be
dectected in extract_string() with "-ENAMETOOLONG".

Each printk contains 2 vsnprintf. vsnprintf() returns the required size aft=
er formatting the string.
1. vprintk_store() will invoke 1st vsnprintf() will 8 bytes space to get th=
e reserve_size. In this case, the _buf_ could be less than _end_ by design.
2. Then it invokes 2nd printk_sprint()->vscnprintf()->vsnprintf() to really=
 fill the space.

If we choose the stack space, it can meet above 2 cases.

If we pass the parameter like:
p =3D d_path_fast(path, buf, end - buf);
We need to handle the complicated logic in prepend_name()
I have tried this way in local test, the code logic is very complicated
and not so graceful.
e.g. I need to firstly go through the loop and get the full path size of
that file. And then return reserved_size for that 1st vsnprintf

Thanks for any suggestion

--
Cheers,
Justin (Jia He)

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