[PATCH] Added CONFIG_VFAT_FS_DUALNAMES option

[tridge]

This is a new patch for VFAT long filename support, replacing the one
that I posted last month. It retains a lot more functionality then the
previous patch.

A FAQ will be posted immediately after this patch to answer the
questions that were raised from the previous discussion.

Cheers, Tridge


------------

When VFAT_FS_DUALNAMES is disabled we avoid the creation of 8.3 short
filenames for files on VFAT filesystems that require a long name. The
patch uses a pattern of 11 bytes in the directory entry which contains
invalid characters such that it cannot be considered to be a valid short
filename.

Signed-off-by: Andrew Tridgell <tridge@xxxxxxxxx>
Acked-by: Dave Kleikamp <shaggy@xxxxxxxxxxxxxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 fs/fat/Kconfig      |   20 +++++++++++++++++
 fs/fat/dir.c        |   15 ++++++-------
 fs/fat/namei_vfat.c |   59 +++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 86 insertions(+), 8 deletions(-)

diff --git a/fs/fat/Kconfig b/fs/fat/Kconfig
index 182f9ff..907a5de 100644
--- a/fs/fat/Kconfig
+++ b/fs/fat/Kconfig
@@ -74,6 +74,26 @@ config VFAT_FS
 	  To compile this as a module, choose M here: the module will be called
 	  vfat.
 
+config VFAT_FS_DUALNAMES
+	bool "VFAT dual names support"
+	depends on VFAT_FS
+	help
+	  This option provides support for dual filenames on VFAT filesystems.
+	  If this option is disabled then file creation will either put
+	  a short (8.3) name or a long name on the file, but never both.
+	  The field where a shortname would normally go is filled with
+	  invalid characters such that it cannot be considered a valid
+	  short filename.
+
+	  That means that long filenames created with this option
+	  disabled will not be accessible at all to operating systems
+	  that do not understand the VFAT extensions.
+
+	  Users considering enabling this option should consider the implications
+	  of any patents that may exist on dual filenames in VFAT.
+
+	  If unsure, say N
+
 config FAT_DEFAULT_CODEPAGE
 	int "Default codepage for FAT"
 	depends on MSDOS_FS || VFAT_FS
diff --git a/fs/fat/dir.c b/fs/fat/dir.c
index 38ff75a..cd5d3ec 100644
--- a/fs/fat/dir.c
+++ b/fs/fat/dir.c
@@ -420,14 +420,13 @@ parse_record:
 			}
 			i += chl;
 		}
-		if (!last_u)
-			continue;
-
-		/* Compare shortname */
-		bufuname[last_u] = 0x0000;
-		len = fat_uni_to_x8(sbi, bufuname, bufname, sizeof(bufname));
-		if (fat_name_match(sbi, name, name_len, bufname, len))
-			goto found;
+		if (last_u) {
+			/* Compare shortname */
+			bufuname[last_u] = 0x0000;
+			len = fat_uni_to_x8(sbi, bufuname, bufname, sizeof(bufname));
+			if (fat_name_match(sbi, name, name_len, bufname, len))
+				goto found;
+		}
 
 		if (nr_slots) {
 			void *longname = unicode + FAT_MAX_UNI_CHARS;
diff --git a/fs/fat/namei_vfat.c b/fs/fat/namei_vfat.c
index 73471b7..894f44d 100644
--- a/fs/fat/namei_vfat.c
+++ b/fs/fat/namei_vfat.c
@@ -22,6 +22,7 @@
 #include <linux/smp_lock.h>
 #include <linux/buffer_head.h>
 #include <linux/namei.h>
+#include <linux/random.h>
 #include "fat.h"
 
 /*
@@ -586,6 +587,59 @@ xlate_to_uni(const unsigned char *name, int len, unsigned char *outname,
 	return 0;
 }
 
+#ifndef CONFIG_VFAT_FS_DUALNAMES
+/*
+ * build a 11 byte 8.3 buffer which is not a short filename. We want 11
+ * bytes which:
+ *    - will be seen as a constant string to all APIs on Linux and Windows
+ *    - cannot be matched with wildcard patterns
+ *    - cannot be used to access the file
+ *    - has a low probability of collision within a directory
+ *    - has an invalid 3 byte extension
+ *    - contains at least one non-space and non-nul byte
+ */
+static void vfat_build_dummy_83_buffer(struct inode *dir, char *msdos_name)
+{
+	u32 rand_num = random32() & 0x3FFFFFFF;
+	int i;
+
+	/* a value of zero would leave us with only nul and spaces,
+	 * which would not work with older linux systems
+	 */
+	if (rand_num == 0)
+		rand_num = 1;
+
+	/* we start with a space followed by nul as spaces at the
+	 * start of an entry are trimmed in FAT, which means that
+	 * starting the 11 bytes with 0x20 0x00 gives us a value which
+	 * cannot be used to access the file. It also means that the
+	 * value as seen from all Windows and Linux APIs is a constant
+	 */
+	msdos_name[0]  = ' ';
+	msdos_name[1]  = 0;
+
+	/* we use / and 2 nul bytes for the extension. These are
+	 * invalid in FAT and mean that utilities that show the
+	 * directory show no extension, but still work via the long
+	 * name for old Linux kernels
+	 */
+	msdos_name[8]  = '/';
+	msdos_name[9]  = 0;
+	msdos_name[10] = 0;
+
+	/*
+	 * fill the remaining 6 bytes with random invalid values
+	 * This gives us a low collision rate, which means a low
+	 * chance of problems with chkdsk.exe and WindowsXP
+	 */
+	for (i = 2; i < 8; i++) {
+		msdos_name[i] = rand_num & 0x1F;
+		rand_num >>= 5;
+	}
+}
+#endif
+
+
 static int vfat_build_slots(struct inode *dir, const unsigned char *name,
 			    int len, int is_dir, int cluster,
 			    struct timespec *ts,
@@ -628,6 +682,11 @@ static int vfat_build_slots(struct inode *dir, const unsigned char *name,
 		goto shortname;
 	}
 
+#ifndef CONFIG_VFAT_FS_DUALNAMES
+	vfat_build_dummy_83_buffer(dir, msdos_name);
+	lcase = 0;
+#endif
+
 	/* build the entry of long file name */
 	cksum = fat_checksum(msdos_name);
 
-- 
1.6.0.4

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