arm: implement safe reads by intercepting the data abort handler.

Implement functions to read from a memory location and indicate
failure in case this is not possible. Since we do not have a MMU,
intercept the data abort handler and simply return when the abort
comes from the safe read routines.

Change-Id: I08f2e59898dcac893319a8150d4cf626f3adabbd
Reviewed-on: http://gerrit.rockbox.org/207
Reviewed-by: Marcin Bukat <marcin.bukat@gmail.com>

lib/unwarminder/SOURCES

@@ -5,3 +5,4 @@ unwarm.c
 unwarminder.c
 unwarmmem.c
 unwarm_thumb.c
+safe_read.S

lib/unwarminder/backtrace.c

@@ -23,6 +23,7 @@
  ***************************************************************************/
 
 #include "backtrace.h"
+#include "safe_read.h"
 
 /***************************************************************************
  * Prototypes
@@ -86,20 +87,17 @@ static Boolean CliReport(void *data, Int32 address)
 
 static Boolean CliReadW(const Int32 a, Int32 *v)
 {
-    *v = *(Int32 *)a;
+    return safe_read32((uint32_t *)a, (uint32_t *)v);
-    return TRUE;
 }
 
 static Boolean CliReadH(const Int32 a, Int16 *v)
 {
-    *v = *(Int16 *)a;
+    return safe_read16((void *)a, (uint16_t *)v);
-    return TRUE;
 }
 
 static Boolean CliReadB(const Int32 a, Int8 *v)
 {
-    *v = *(Int8 *)a;
+    return safe_read8((void *)a, (uint8_t *)v);
-    return TRUE;
 }
 
 Boolean CliInvalidateW(const Int32 a)

lib/unwarminder/safe_read.S

@@ -0,0 +1,143 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2012 by Amaury Pouly
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#include "config.h"
+
+.data
+was_aborted:
+    .word   0
+
+.section    .text.safe_read8
+.type       safe_read8, %function
+.global     safe_read8
+@ bool safe_read8(uint8_t *addr, uint8_t *value)
+safe_read8:
+    @ was_aborted = 0
+    ldr     r2, =was_aborted
+    mov     r3, #0
+    str     r3, [r2]
+    @ r0=*addr
+safe_read8_faulty_addr:
+    ldrb    r0, [r0]
+    @ if(was_aborted)
+    ldr     r2, [r2]
+    cmp     r2, #1
+    @   return false;
+    moveq   r0, #0
+    bxeq    lr
+    @ if(value != NULL)
+    cmp     r1, #0
+    @   *value = r0
+    strneb  r0, [r1]
+    @ return true;
+    mov     r0, #1
+    bx      lr
+.size       safe_read8, . - safe_read8
+
+.section    .text.safe_read16
+.type       safe_read16, %function
+.global     safe_read16
+@ bool safe_read16(uint16_t *addr, uint16_t *value)
+safe_read16:
+    @ was_aborted = 0
+    ldr     r2, =was_aborted
+    mov     r3, #0
+    str     r3, [r2]
+    @ r0=*addr
+safe_read16_faulty_addr:
+    ldrh    r0, [r0]
+    @ if(was_aborted)
+    ldr     r2, [r2]
+    cmp     r2, #1
+    @   return false;
+    moveq   r0, #0
+    bxeq    lr
+    @ if(value != NULL)
+    cmp     r1, #0
+    @   *value = r0
+    strneh  r0, [r1]
+    @ return true;
+    mov     r0, #1
+    bx      lr
+.size       safe_read16, . - safe_read16
+
+.section    .text.safe_read32
+.type       safe_read32, %function
+.global     safe_read32
+@ bool safe_read32(uint32_t *addr, uint32_t *value)
+safe_read32:
+    @ was_aborted = 0
+    ldr     r2, =was_aborted
+    mov     r3, #0
+    str     r3, [r2]
+    @ r0=*addr
+safe_read32_faulty_addr:
+    ldr     r0, [r0]
+    @ if(was_aborted)
+    ldr     r2, [r2]
+    cmp     r2, #1
+    @   return false;
+    moveq   r0, #0
+    bxeq    lr
+    @ if(value != NULL)
+    cmp     r1, #0
+    @   *value = r0
+    strne   r0, [r1]
+    @ return true;
+    mov     r0, #1
+    bx      lr
+.size       safe_read32, . - safe_read32
+
+#if (CONFIG_PLATFORM & PLATFORM_NATIVE)
+.section    .text.data_abort_handler
+.type       data_abort_handler, %function
+.global     data_abort_handler
+data_abort_handler:
+    @ store minimal amount of registers
+    stmfd   sp!, {r0-r1}
+    @ compute faulty address
+    sub     r0, lr, #8
+    @ compare to safe_read8
+    ldr     r1, =safe_read8_faulty_addr
+    cmp     r0, r1
+    beq     1f
+    @ compare to safe_read16
+    ldr     r1, =safe_read16_faulty_addr
+    cmp     r0, r1
+    beq     1f
+    @ compare to safe_read32
+    ldr     r1, =safe_read32_faulty_addr
+    cmp     r0, r1
+    beq     1f
+    @ otherwise just normally to UIE
+    mov     r0, r1
+    mov     r1, #2
+    b       UIE
+1:
+    @ set was_aborted
+    ldr     r1, =was_aborted
+    mov     r0, #1
+    str     r0, [r1]
+    @ restore registers
+    ldmfd   sp!, {r0-r1}
+    @ restore mode and jump back to the *next* instruction
+    subs    pc, lr, #-4
+.size       data_abort_handler, . - data_abort_handler
+#endif

lib/unwarminder/safe_read.h

@@ -0,0 +1,33 @@
+/***************************************************************************
+ *             __________               __   ___.
+ *   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+ *   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+ *   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+ *   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+ *                     \/            \/     \/    \/            \/
+ * $Id$
+ *
+ * Copyright (C) 2012 by Amaury Pouly
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ ****************************************************************************/
+#ifndef __SAFE_READ__
+#define __SAFE_READ__
+
+#include "system.h"
+
+/* Try to read an X-bit unsigned integer. If the address is not readable,
+ * returns false. Otherwise returns true and store the result in *value
+ * if value is not NULL */
+bool safe_read8(uint8_t *addr, uint8_t *value);
+bool safe_read16(uint16_t *addr, uint16_t *value);
+bool safe_read32(uint32_t *addr, uint32_t *value);
+
+#endif /* __SAFE_READ__ */