parisc: fix bug in compat_arch_ptrace

This commit in 2.6.28-rc:

	81e192d6ce303b6792aa38ff35f41a1a7357f23a
	parisc: convert to generic compat_sys_ptrace

introduced a bug which segfaults the parisc 64bit kernel when stracing 32bit applications:

Kernel Fault: Code=15 regs=00000000bafa42b0 (Addr=00000001baf5ab57)
YZrvWESTHLNXBCVMcbcbcbcbOGFRQPDI
PSW: 00001000000001101111111100001011 Tainted: G W

r00-03  000000ff0806ff0b 000000004068edc0 00000000401203f8 00000000fb3e2508
r04-07  0000000040686dc0 00000000baf5a800 fffffffffffffffc fffffffffb3e2508
r08-11  00000000baf5a800 000000000004b068 00000000000402b0 0000000000040d68
r12-15  0000000000042a9c 0000000000040a9c 0000000000040d60 0000000000042e9c
r16-19  000000000004b060 000000000004b058 0000000000042d9c ffffffffffffffff
r20-23  000000000800000b 0000000000000000 000000000800000b fffffffffb3e2508
r24-27  00000000fffffffc 0000000000000003 00000000fffffffc 0000000040686dc0
r28-31  00000001baf5a7ff 00000000bafa4280 00000000bafa42b0 00000000000001d7

sr00-03 0000000000fca000 0000000000000000 0000000000000000 0000000000fca000 sr04-07 0000000000000000 0000000000000000 0000000000000000 0000000000000000

IASQ: 0000000000000000 0000000000000000 IAOQ: 0000000040120400 0000000040120404
IIR: 4b9a06b0 ISR: 0000000000000000 IOR: 00000001baf5ab57
CPU: 0 CR30: 00000000bafa4000 CR31: 00000000d22344e0
ORIG_R28: 00000000fb3e2248
IAOQ[0]: compat_arch_ptrace+0xb8/0x160
IAOQ[1]: compat_arch_ptrace+0xbc/0x160
RP(r2): compat_arch_ptrace+0xb0/0x160

Backtrace:
 [<00000000401612ac>] compat_sys_ptrace+0x15c/0x180
 [<0000000040104ef8>] syscall_exit+0x0/0x14

Problem is, that compat_arch_ptrace() enters with an addr value of type compat_ulong_t and calls translate_usr_offset() to translate the address offset into a struct pt_regs offset like this:
addr = translate_usr_offset(addr)
This means, any return value of translate_usr_offset() is stored back as compat_ulong_t type into the addr variable.

But since translate_usr_offset() returns -1 for invalid offsets, addr can now get the value 0xffffffff which then fails the next return-value sanity check and thus the kernel tries to access invalid memory:

	if (addr < 0)
		break;

Fix this bug by modifying translate_usr_offset() to take and return values of type compat_ulong_t, and by returning the value "sizeof(struct pt_regs)" as an error indicator. Additionally change the sanity check to check for return values for >= sizeof(struct pt_regs).

This patch survived my compile and run-tests.

index 90904f9..927db36 100644
@@ -183,10 +183,10 @@ long arch_ptrace(struct task_struct *child, long request, long addr, long data)
  * being 64 bit in both cases.
  */
 
-static long translate_usr_offset(long offset)
+static compat_ulong_t translate_usr_offset(compat_ulong_t offset)
 {
 	if (offset < 0)
-		return -1;
+		return sizeof(struct pt_regs);
 	else if (offset <= 32*4)	/* gr[0..31] */
 		return offset * 2 + 4;
 	else if (offset <= 32*4+32*8)	/* gr[0..31] + fr[0..31] */

@@ -194,7 +194,7 @@ static long translate_usr_offset(long offset)
 	else if (offset < sizeof(struct pt_regs)/2 + 32*4)
 		return offset * 2 + 4 - 32*8;
 	else
-		return -1;
+		return sizeof(struct pt_regs);
 }
 
 long compat_arch_ptrace(struct task_struct *child, compat_long_t request,

@@ -209,7 +209,7 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,
 		if (addr & (sizeof(compat_uint_t)-1))
 			break;
 		addr = translate_usr_offset(addr);
-		if (addr < 0)
+		if (addr >= sizeof(struct pt_regs))
 			break;
 
 		tmp = *(compat_uint_t *) ((char *) task_regs(child) + addr);

@@ -236,7 +236,7 @@ long compat_arch_ptrace(struct task_struct *child, compat_long_t request,
 			if (addr & (sizeof(compat_uint_t)-1))
 				break;
 			addr = translate_usr_offset(addr);
-			if (addr < 0)
+			if (addr >= sizeof(struct pt_regs))
 				break;
 			if (addr >= PT_FR0 && addr <= PT_FR31 + 4) {
 				/* Special case, fp regs are 64 bits anyway */

Thanks, looks fine.