UBIFS: do not read unnecessary bytes when unpacking bits

Fixes the following Oops:

BUG: unable to handle kernel paging request at f8d24000
IP: [<f8ff0657>] :ubifs:ubifs_unpack_bits+0xcd/0x231
*pde = 34333067 *pte = 00000000
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: deflate zlib_deflate lzo lzo_decompress lzo_compress
ubifs ubi nandsim nand nand_ids nand_ecc mtd nfsd lockd sunrpc exportfs
[last unloaded: nand_ecc]

Pid: 7450, comm: sync Not tainted (2.6.27-rc8-ubifs-2.6 #27)
EIP: 0060:[<f8ff0657>] EFLAGS: 00010206 CPU: 0
EIP is at ubifs_unpack_bits+0xcd/0x231 [ubifs]
EAX: 00000000 EBX: 00000000 ECX: d7e43dc0 EDX: 0000ff00
ESI: 00000004 EDI: f8d23ffe EBP: d7e43db4 ESP: d7e43d8c
DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
Process sync (pid: 7450, ti=d7e42000 task=eb6f9530 task.ti=d7e42000)
Stack: 00000400 c0103db4 dc5e8090 d7e43dc0 d7e43dc0 d7e43dc4 0000001c 00000004
      f496d1e0 f8d23ffc d7e43dd4 f8ffac7e f8d23ffe 00000000 f8d23ffe f2b7af68
      f496d1e0 f8d23ffc d7e43e2c f8ffadc5 00000000 0001f000 00000000 c03b10a7
Call Trace:
[<c0103db4>] ? restore_nocheck_notrace+0x0/0xe
[<f8ffac7e>] ? is_a_node+0x43/0x92 [ubifs]
[<f8ffadc5>] ? dbg_check_ltab+0xf8/0x5c9 [ubifs]
[<c03b10a7>] ? mutex_lock_nested+0x1b2/0x2a0
[<f8ffc86e>] ? ubifs_lpt_start_commit+0x49/0xecb [ubifs]
[<c03b0ef3>] ? mutex_unlock+0xd/0xf
[<f8fef017>] ? ubifs_tnc_start_commit+0x1cf/0xef8 [ubifs]
[<f8fe65d8>] ? do_commit+0x18f/0x52d [ubifs]
[<f8fe69f6>] ? ubifs_run_commit+0x80/0xca [ubifs]
[<f8fd8d35>] ? ubifs_sync_fs+0xdb/0xf6 [ubifs]
[<c0181a07>] ? sync_filesystems+0xc6/0x10c
[<c019f279>] ? do_sync+0x3b/0x6a
[<c019f2ba>] ? sys_sync+0x12/0x18
[<c0103ced>] ? sysenter_do_call+0x12/0x35
=======================
Code: 4d ec 89 01 8b 45 e8 89 10 89 d8 89 f1 d3 e8 85 c0 74 07 29 d6 83 fe
20 75 2a 89 d8 83 c4 1c 5b 5e 5f 5d c3 0f b6 57 01 c1 e2 08 <0f> b6 47 02
c1 e0 10 09 c2 0f b6 07 09 c2 0f b
EIP: [<f8ff0657>] ubifs_unpack_bits+0xcd/0x231 [ubifs] SS:ESP 0068:d7e43d8c
---[ end trace 1bbb4c407a6dd816 ]---

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>

fs/ubifs/lpt.c

@@ -288,25 +288,56 @@ uint32_t ubifs_unpack_bits(uint8_t **addr, int *pos, int nrbits)
 	const int k = 32 - nrbits;
 	uint8_t *p = *addr;
 	int b = *pos;
-	uint32_t val;
+	uint32_t uninitialized_var(val);
+	const int bytes = (nrbits + b + 7) >> 3;
 
 	ubifs_assert(nrbits > 0);
 	ubifs_assert(nrbits <= 32);
 	ubifs_assert(*pos >= 0);
 	ubifs_assert(*pos < 8);
 	if (b) {
-		val = p[1] | ((uint32_t)p[2] << 8) | ((uint32_t)p[3] << 16) |
-		      ((uint32_t)p[4] << 24);
+		switch (bytes) {
+		case 2:
+			val = p[1];
+			break;
+		case 3:
+			val = p[1] | ((uint32_t)p[2] << 8);
+			break;
+		case 4:
+			val = p[1] | ((uint32_t)p[2] << 8) |
+				     ((uint32_t)p[3] << 16);
+			break;
+		case 5:
+			val = p[1] | ((uint32_t)p[2] << 8) |
+				     ((uint32_t)p[3] << 16) |
+				     ((uint32_t)p[4] << 24);
+		}
 		val <<= (8 - b);
 		val |= *p >> b;
 		nrbits += b;
-	} else
-		val = p[0] | ((uint32_t)p[1] << 8) | ((uint32_t)p[2] << 16) |
-		      ((uint32_t)p[3] << 24);
+	} else {
+		switch (bytes) {
+		case 1:
+			val = p[0];
+			break;
+		case 2:
+			val = p[0] | ((uint32_t)p[1] << 8);
+			break;
+		case 3:
+			val = p[0] | ((uint32_t)p[1] << 8) |
+				     ((uint32_t)p[2] << 16);
+			break;
+		case 4:
+			val = p[0] | ((uint32_t)p[1] << 8) |
+				     ((uint32_t)p[2] << 16) |
+				     ((uint32_t)p[3] << 24);
+			break;
+		}
+	}
 	val <<= k;
 	val >>= k;
 	b = nrbits & 7;
-	p += nrbits / 8;
+	p += nrbits >> 3;
 	*addr = p;
 	*pos = b;
 	ubifs_assert((val >> nrbits) == 0 || nrbits - b == 32);