add formats for dentry/file pathnames

New formats: %p[dD][234]?.  The next pointer is interpreted as struct dentry *
or struct file * resp. ('d' => dentry, 'D' => file) and the last component(s)
of pathname are printed (%pd => just the last one, %pd2 => the last two, etc.)
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

Documentation/printk-formats.txt

@@ -168,6 +168,15 @@ UUID/GUID addresses:
 	Where no additional specifiers are used the default little endian
 	order with lower case hex characters will be printed.
 
+dentry names:
+	%pd{,2,3,4}
+	%pD{,2,3,4}
+
+	For printing dentry name; if we race with 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.
+
 struct va_format:
 
 	%pV

lib/vsprintf.c

@@ -26,6 +26,7 @@
 #include <linux/math64.h>
 #include <linux/uaccess.h>
 #include <linux/ioport.h>
+#include <linux/dcache.h>
 #include <net/addrconf.h>
 
 #include <asm/page.h>		/* for PAGE_SIZE */
@@ -532,6 +533,81 @@ char *string(char *buf, char *end, const char *s, struct printf_spec spec)
 	return buf;
 }
 
+static void widen(char *buf, char *end, unsigned len, unsigned spaces)
+{
+	size_t size;
+	if (buf >= end)	/* nowhere to put anything */
+		return;
+	size = end - buf;
+	if (size <= spaces) {
+		memset(buf, ' ', size);
+		return;
+	}
+	if (len) {
+		if (len > size - spaces)
+			len = size - spaces;
+		memmove(buf + spaces, buf, len);
+	}
+	memset(buf, ' ', spaces);
+}
+
+static noinline_for_stack
+char *dentry_name(char *buf, char *end, const struct dentry *d, struct printf_spec spec,
+		  const char *fmt)
+{
+	const char *array[4], *s;
+	const struct dentry *p;
+	int depth;
+	int i, n;
+
+	switch (fmt[1]) {
+		case '2': case '3': case '4':
+			depth = fmt[1] - '0';
+			break;
+		default:
+			depth = 1;
+	}
+
+	rcu_read_lock();
+	for (i = 0; i < depth; i++, d = p) {
+		p = ACCESS_ONCE(d->d_parent);
+		array[i] = ACCESS_ONCE(d->d_name.name);
+		if (p == d) {
+			if (i)
+				array[i] = "";
+			i++;
+			break;
+		}
+	}
+	s = array[--i];
+	for (n = 0; n != spec.precision; n++, buf++) {
+		char c = *s++;
+		if (!c) {
+			if (!i)
+				break;
+			c = '/';
+			s = array[--i];
+		}
+		if (buf < end)
+			*buf = c;
+	}
+	rcu_read_unlock();
+	if (n < spec.field_width) {
+		/* we want to pad the sucker */
+		unsigned spaces = spec.field_width - n;
+		if (!(spec.flags & LEFT)) {
+			widen(buf - n, end, n, spaces);
+			return buf + spaces;
+		}
+		while (spaces--) {
+			if (buf < end)
+				*buf = ' ';
+			++buf;
+		}
+	}
+	return buf;
+}
+
 static noinline_for_stack
 char *symbol_string(char *buf, char *end, void *ptr,
 		    struct printf_spec spec, const char *fmt)
@@ -1253,6 +1329,12 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
 		spec.base = 16;
 		return number(buf, end,
 			      (unsigned long long) *((phys_addr_t *)ptr), spec);
+	case 'd':
+		return dentry_name(buf, end, ptr, spec, fmt);
+	case 'D':
+		return dentry_name(buf, end,
+				   ((const struct file *)ptr)->f_path.dentry,
+				   spec, fmt);
 	}
 	spec.flags |= SMALL;
 	if (spec.field_width == -1) {