linux-kernel - Re: [PATCH RFCv2 2/3] lib/vsprintf.c: make %pD print full path for file

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Date:   Fri, 28 May 2021 13:59:54 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jia He <justin.he@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Luca Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Johannes Berg <johannes.berg@...el.com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-s390@...r.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?

> @@ -920,13 +921,25 @@ 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)
>  {
> +	const struct path *path;
> +	char *p;
> +	char full_path[256];
> +
>  	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_fast(path, full_path, sizeof(full_path));
> +	if (IS_ERR(p))
> +		return err_ptr(buf, end, p, spec);
> +
> +	return string_nocheck(buf, end, p, spec);
>  }
>  #ifdef CONFIG_BLOCK
>  static noinline_for_stack