linux-kernel - Re: [PATCH v2 2/4] lib/vsprintf.c: make '%pD' print the full path of file

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Message-ID: <YNRJ61m6duXjpGrp@alley>
Date:   Thu, 24 Jun 2021 11:01:31 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Jia He <justin.he@....com>
Cc:     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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@...radead.org>, nd@....com
Subject: Re: [PATCH v2 2/4] lib/vsprintf.c: make '%pD' print the full path of
 file

On Wed 2021-06-23 13:50:09, Jia He wrote:
> Previously, the specifier '%pD' is for printing dentry name of struct
> file. It may not be perfect (by default it only prints one component.)
> 
> As suggested by Linus [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 change the behavior of '%pD' to print the full path of that file.
> 
> If someone invokes snprintf() with small but positive space,
> prepend_name_with_len() moves or truncates the string partially.

Does this comment belong to the 1st patch?
prepend_name_with_len() is not called in this patch.

> More
> than that, kasprintf() will pass NULL @buf and @end as the parameters,
> and @end - @buf can be negative in some case. Hence make it return at
> the very beginning with false in these cases.

Same here. file_d_path_name() does not return bool.

Well, please mention in the commit message that %pD uses the entire
given buffer as a scratch space. It might write something behind
the trailing '\0'.

It would make sense to warn about this also in
Documentation/core-api/printk-formats.rst. It is a bit non-standard
behavior.

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index f0c35d9b65bf..f4494129081f 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -920,13 +921,44 @@ char *dentry_name(char *buf, char *end, const struct dentry *d, struct printf_sp
>  }
>  
>  static noinline_for_stack
> -char *file_dentry_name(char *buf, char *end, const struct file *f,
> +char *file_d_path_name(char *buf, char *end, const struct file *f,
>  			struct printf_spec spec, const char *fmt)
>  {
> +	char *p;
> +	const struct path *path;
> +	int prepend_len, widen_len, dpath_len;
> +
>  	if (check_pointer(&buf, end, f, spec))
>  		return buf;
>  
> -	return dentry_name(buf, end, f->f_path.dentry, spec, fmt);
> +	path = &f->f_path;
> +	if (check_pointer(&buf, end, path, spec))
> +		return buf;
> +
> +	p = d_path_unsafe(path, buf, end - buf, &prepend_len);
> +
> +	/* Calculate the full d_path length, ignoring the tail '\0' */
> +	dpath_len = end - buf - prepend_len - 1;
> +
> +	widen_len = max_t(int, dpath_len, spec.field_width);
> +
> +	/* Case 1: Already started past the buffer. Just forward @buf. */
> +	if (buf >= end)
> +		return buf + widen_len;
> +
> +	/*
> +	 * Case 2: The entire remaining space of the buffer filled by
> +	 * the truncated path. Still need to get moved right when
> +	 * the filled width is greather than the full path length.

s/filled/field/ ?

> +	 */
> +	if (prepend_len < 0)
> +		return widen_string(buf + dpath_len, dpath_len, end, spec);

Otherwise, it looks good to me.

Best Regards,
Petr