Merge kern_descrip.c:1.286, 1.287, 1.288 from HEAD to RELENG_6:

Add a DDB "show files" command to list the current open file list, some
  state about each open file, and identify the first process in the process
  table that references the file.  This is helpful in debugging leaks of
  file descriptors.

  Expand the set of details printed for each file descriptor to include
  it's garbage collection flags.  Reformat generally to make this fit and
  leave some room for future expansion.

  Add the f_msgcount field to the set of struct file fields printed in show
  files.

sys/kern/kern_descrip.c

@@ -38,6 +38,7 @@
 __FBSDID("$FreeBSD$");
 
 #include "opt_compat.h"
+#include "opt_ddb.h"
 
 #include <sys/param.h>
 #include <sys/systm.h>
@@ -69,6 +70,8 @@ __FBSDID("$FreeBSD$");
 
 #include <vm/uma.h>
 
+#include <ddb/ddb.h>
+
 static MALLOC_DEFINE(M_FILEDESC, "file desc", "Open file descriptor table");
 static MALLOC_DEFINE(M_FILEDESC_TO_LEADER, "file desc to leader",
 		     "file desc to leader structures");
@@ -2466,6 +2469,77 @@ sysctl_kern_file(SYSCTL_HANDLER_ARGS)
 SYSCTL_PROC(_kern, KERN_FILE, file, CTLTYPE_OPAQUE|CTLFLAG_RD,
     0, 0, sysctl_kern_file, "S,xfile", "Entire file table");
 
+#ifdef DDB
+/*
+ * For the purposes of debugging, generate a human-readable string for the
+ * file type.
+ */
+static const char *
+file_type_to_name(short type)
+{
+
+	switch (type) {
+	case 0:
+		return ("zero");
+	case DTYPE_VNODE:
+		return ("vnod");
+	case DTYPE_SOCKET:
+		return ("sock");
+	case DTYPE_PIPE:
+		return ("pipe");
+	case DTYPE_FIFO:
+		return ("fifo");
+	case DTYPE_CRYPTO:
+		return ("crpt");
+	default:
+		return ("unkn");
+	}
+}
+
+/*
+ * For the purposes of debugging, identify a process (if any, perhaps one of
+ * many) that references the passed file in its file descriptor array. Return
+ * NULL if none.
+ */
+static struct proc *
+file_to_first_proc(struct file *fp)
+{
+	struct filedesc *fdp;
+	struct proc *p;
+	int n;
+
+	LIST_FOREACH(p, &allproc, p_list) {
+		if (p->p_state == PRS_NEW)
+			continue;
+		fdp = p->p_fd;
+		if (fdp == NULL)
+			continue;
+		for (n = 0; n < fdp->fd_nfiles; n++) {
+			if (fp == fdp->fd_ofiles[n])
+				return (p);
+		}
+	}
+	return (NULL);
+}
+
+DB_SHOW_COMMAND(files, db_show_files)
+{
+	struct file *fp;
+	struct proc *p;
+
+	db_printf("%8s %4s %8s %8s %4s %5s %6s %8s %5s %12s\n", "File",
+	    "Type", "Data", "Flag", "GCFl", "Count", "MCount", "Vnode",
+	    "FPID", "FCmd");
+	LIST_FOREACH(fp, &filehead, f_list) {
+		p = file_to_first_proc(fp);
+		db_printf("%8p %4s %8p %08x %04x %5d %6d %8p %5d %12s\n", fp,
+		    file_type_to_name(fp->f_type), fp->f_data, fp->f_flag,
+		    fp->f_gcflag, fp->f_count, fp->f_msgcount, fp->f_vnode,
+		    p != NULL ? p->p_pid : -1, p != NULL ? p->p_comm : "-");
+	}
+}
+#endif
+
 SYSCTL_INT(_kern, KERN_MAXFILESPERPROC, maxfilesperproc, CTLFLAG_RW,
     &maxfilesperproc, 0, "Maximum files allowed open per process");