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Subject: Re: [PATCH RFCv3 2/3] lib/vsprintf.c: make %pD print full path for
 file
To:     Justin He <Justin.He@arm.com>, Petr Mladek <pmladek@suse.com>
Cc:     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>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Eric Biggers <ebiggers@google.com>,
        "Ahmed S. Darwish" <a.darwish@linutronix.de>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
References: <20210611155953.3010-1-justin.he@arm.com>
 <20210611155953.3010-3-justin.he@arm.com> <YMd4ixry8ztzlG/e@alley>
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From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <113d9380-8493-1bde-9c76-992f4ce675d9@rasmusvillemoes.dk>
Date:   Tue, 15 Jun 2021 09:14:37 +0200
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On 15/06/2021 08.48, Justin He wrote:
> Hi Petr
> 

>>> +   /* no filling space at all */
>>> +   if (buf >= end || !buf)
>>> +           return buf + reserved_size;
>>> +
>>> +   /* small space for long name */
>>> +   if (buf < end && prepend_len < 0)
>>> +           return string_truncate(buf, end, p, dpath_len, spec);
>>
>> We need this only because we allowed to write the path behind
>> spec.field_width. Do I get it right?
> 
> Both of field_width and precision:
> "%.14pD" or "%8.14pD"

Precision is never gonna be used with %p (or any of its kernel
extensions) because gcc would tell you

foo.c:5:13: warning: precision used with ?%p? gnu_printf format [-Wformat=]
    5 |  printf("%.5p\n", foo);

and there's no way -Wformat is going to be turned off to allow that usage.

IOW, there's no need to add complexity to make "%.3pD" of something that
would normally print "/foo/bar" merely print "/fo", similar to what a
precision with %s would mean.

As for field width, I don't know if it's worth honouring, but IIRC the
original %pd and %pD did that (that's where we have widen_string etc. from).

Other %p extensions put the field with to some other use (e.g. the
bitmap and hex string printing), so they obviously cannot simultaneously
use it in the traditional sense.

Rasmus