svn commit: r286371 - head/sys/fs/devfs

[John Baldwin]

Author: jhb
Date: Thu Aug  6 16:50:37 2015
New Revision: 286371
URL: https://svnweb.freebsd.org/changeset/base/286371

Log:
  The changes that introduced fo_mmap() treated all character device
  mappings as if MAP_SHARED was always present since in general MAP_PRIVATE
  is not permitted for character devices.  However, there is one exception
  in that MAP_PRIVATE mappings are permitted for /dev/zero.
  
  Only require a writable file descriptor (FWRITE) for shared, writable
  mappings of character devices.  vm_mmap_cdev() will reject any private
  mappings for other devices.
  
  Reviewed by:	kib
  Reported by:	sbruno (broke qemu cross-builds), peter
  Differential Revision:	https://reviews.freebsd.org/D3316

Modified:
  head/sys/fs/devfs/devfs_vnops.c

Modified: head/sys/fs/devfs/devfs_vnops.c
==============================================================================
--- head/sys/fs/devfs/devfs_vnops.c	Thu Aug  6 16:14:29 2015	(r286370)
+++ head/sys/fs/devfs/devfs_vnops.c	Thu Aug  6 16:50:37 2015	(r286371)
@@ -1774,13 +1774,24 @@ devfs_mmap_f(struct file *fp, vm_map_t m
 		return (EACCES);
 
 	/*
-	 * Character devices always share mappings, so
-	 * require a writable fd for writable mappings.
+	 * If we are sharing potential changes via MAP_SHARED and we
+	 * are trying to get write permission although we opened it
+	 * without asking for it, bail out.
+	 *
+	 * Note that most character devices always share mappings.
+	 * The one exception is that D_MMAP_ANON devices
+	 * (i.e. /dev/zero) permit private writable mappings.
+	 *
+	 * Rely on vm_mmap_cdev() to fail invalid MAP_PRIVATE requests
+	 * as well as updating maxprot to permit writing for
+	 * D_MMAP_ANON devices rather than doing that here.
 	 */
-	if ((fp->f_flag & FWRITE) != 0)
-		maxprot |= VM_PROT_WRITE;
-	else if ((prot & VM_PROT_WRITE) != 0)
-		return (EACCES);
+	if ((flags & MAP_SHARED) != 0) {
+		if ((fp->f_flag & FWRITE) != 0)
+			maxprot |= VM_PROT_WRITE;
+		else if ((prot & VM_PROT_WRITE) != 0)
+			return (EACCES);
+	}
 	maxprot &= cap_maxprot;
 
 	fpop = td->td_fpop;