From ab99e589ddf2940bd3a4c603f2c228103aa6c20b Mon Sep 17 00:00:00 2001
From: John Baldwin <jhb@FreeBSD.org>
Date: Wed, 3 Mar 2010 16:18:04 +0000
Subject: [PATCH] Allow lseek(SEEK_END) to work on disk devices by using the
 DIOCGMEDIASIZE to determine the media size.

Submitted by:	nox
MFC after:	1 week
---
 sys/kern/vfs_syscalls.c | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/sys/kern/vfs_syscalls.c b/sys/kern/vfs_syscalls.c
index f1e6ca893950..7e700569d568 100644
--- a/sys/kern/vfs_syscalls.c
+++ b/sys/kern/vfs_syscalls.c
@@ -45,6 +45,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/systm.h>
 #include <sys/bio.h>
 #include <sys/buf.h>
+#include <sys/disk.h>
 #include <sys/sysent.h>
 #include <sys/malloc.h>
 #include <sys/mount.h>
@@ -1920,7 +1921,7 @@ lseek(td, uap)
 	struct file *fp;
 	struct vnode *vp;
 	struct vattr vattr;
-	off_t offset;
+	off_t offset, size;
 	int error, noneg;
 	int vfslocked;
 
@@ -1951,6 +1952,15 @@ lseek(td, uap)
 		VOP_UNLOCK(vp, 0);
 		if (error)
 			break;
+
+		/*
+		 * If the file references a disk device, then fetch
+		 * the media size and use that to determine the ending
+		 * offset.
+		 */
+		if (vattr.va_size == 0 && vp->v_type == VCHR &&
+		    fo_ioctl(fp, DIOCGMEDIASIZE, &size, cred, td) == 0)
+			vattr.va_size = size;
 		if (noneg &&
 		    (vattr.va_size > OFF_MAX ||
 		    (offset > 0 && vattr.va_size > OFF_MAX - offset))) {