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

[Matthew Wilcox <willy@infradead.org>]

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