lib/vsprintf.c: make '%pD' print the full path of file

Previously, the specifier '%pD' was for printing dentry name of struct
file. It may not be perfect since 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.
It is worthy of noting that %pD uses the entire given buffer as a scratch
space. It might write something behind the trailing '\0' but never write
beyond the scratch space.

Precision specifier is never going to be used with %p (or any of its
kernel extensions) if -Wformat is turned on.

Link: https://lore.kernel.org/lkml/CAHk-=wimsMqGdzik187YWLb-ru+iktb4MYbMQG1rnZ81dXYFVg@mail.gmail.com/ [1]
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jia He <justin.he@arm.com>

Documentation/core-api/printk-formats.rst

@@ -418,12 +418,15 @@ dentry names
 ::
 
 	%pd{,2,3,4}
-	%pD{,2,3,4}
+	%pD
 
 For printing dentry name; if we race with :c:func:`d_move`, the name might
 be a mix of old and new ones, but it won't oops.  %pd dentry is a safer
 equivalent of %s dentry->d_name.name we used to use, %pd<n> prints ``n``
-last components.  %pD does the same thing for struct file.
+last components. %pD prints full file path together with mount-related
+parenthood. %pD uses the entire given buffer as a scratch space. It might
+write something behind the trailing '\0' but never write beyond the
+scratch space.
 
 Passed by reference.
 

lib/vsprintf.c

@@ -26,6 +26,7 @@
 #include <linux/types.h>
 #include <linux/string.h>
 #include <linux/ctype.h>
+#include <linux/dcache.h>
 #include <linux/kernel.h>
 #include <linux/kallsyms.h>
 #include <linux/math64.h>
@@ -947,13 +948,44 @@ 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)
 {
+	int prepend_len, widen_len, dpath_len;
+	const struct path *path;
+	char *p;
+
 	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 field width is greater than the full path length.
+	 */
+	if (prepend_len < 0)
+		return widen_string(buf + dpath_len, dpath_len, end, spec);
+
+	/*
+	 * Case 3: The full path is printed at the end of the buffer.
+	 * Print it at the right location in the same buffer.
+	 */
+	return string_nocheck(buf, end, p, spec);
 }
 #ifdef CONFIG_BLOCK
 static noinline_for_stack
@@ -2341,7 +2373,7 @@ early_param("no_hash_pointers", no_hash_pointers_enable);
  * - 'a[pd]' For address types [p] phys_addr_t, [d] dma_addr_t and derivatives
  *           (default assumed to be phys_addr_t, passed by reference)
  * - 'd[234]' For a dentry name (optionally 2-4 last components)
- * - 'D[234]' Same as 'd' but for a struct file
+ * - 'D' For the full path name of a struct file
  * - 'g' For block_device name (gendisk + partition number)
  * - 't[RT][dt][r][s]' For time and date as represented by:
  *      R    struct rtc_time
@@ -2440,7 +2472,7 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 	case 'C':
 		return clock(buf, end, ptr, spec, fmt);
 	case 'D':
-		return file_dentry_name(buf, end, ptr, spec, fmt);
+		return file_d_path_name(buf, end, ptr, spec, fmt);
 #ifdef CONFIG_BLOCK
 	case 'g':
 		return bdev_name(buf, end, ptr, spec, fmt);