Add custom getcontext() implementation for Android.

This adds a minimalistic implementation of getcontext()
for Android/ARM and Android/x86. The provided code is
in assembly and only implements the bare minimum required
by Breakpad to get the current processor state.

Note that:

- The FPU state is not saved to the ucontext_t on ARM.
  (that's actually the main difference with a normal
   getcontext() implementation).

  This is normal. On Linux/ARM, such state must be
  obtained with PTRACE_GETVFPREGS instead. This will
  be implemented in a future patch.

- On x86, only the 'regular' FPU state is saved, to
  mimic the GLibc/i386 implementation. The state of
  SSE/SSE2/etc registers is not part of the upstream
  getcontext() implementation.
Review URL: https://breakpad.appspot.com/444002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1024 4c0a9323-5329-0410-9bdc-e9ce6186880e

src/common/android/breakpad_getcontext.S

@@ -0,0 +1,145 @@
+// Copyright (c) 2012, Google Inc.
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+// A minimalistic implementation of getcontext() to be used by
+// Google Breakpad on Android.
+
+#include "common/android/ucontext_constants.h"
+
+/* int getcontext (ucontext_t *ucp) */
+
+#ifdef __arm__
+
+  .text
+  .global breakpad_getcontext
+  .hidden breakpad_getcontext
+  .type breakpad_getcontext, #function
+  .align 0
+  .fnstart
+breakpad_getcontext:
+
+  /* First, save r4-r11 */
+  add   r1, r0, #(MCONTEXT_GREGS_OFFSET + 4*4)
+  stm   r1, {r4-r11}
+
+  /* r12 is a scratch register, don't save it */
+
+  /* Save sp and lr explicitely. */
+  /* - sp can't be stored with stmia in Thumb-2 */
+  /* - STM instructions that store sp and pc are deprecated in ARM */
+  str   sp, [r0, #(MCONTEXT_GREGS_OFFSET + 13*4)]
+  str   lr, [r0, #(MCONTEXT_GREGS_OFFSET + 14*4)]
+
+  /* Save the caller's address in 'pc' */
+  str   lr, [r0, #(MCONTEXT_GREGS_OFFSET + 15*4)]
+
+  /* Save ucontext_t* pointer accross next call */
+  mov   r4, r0
+
+  /* Call sigprocmask(SIG_BLOCK, NULL, &(ucontext->uc_sigmask)) */
+  mov   r0, #0  /* SIG_BLOCK */
+  mov   r1, #0  /* NULL */
+  add   r2, r4, #UCONTEXT_SIGMASK_OFFSET
+  bl    sigprocmask(PLT)
+
+  /* Intentionally do not save the FPU state here. This is because on
+   * Linux/ARM, one should instead use ptrace(PTRACE_GETFPREGS) or
+   * ptrace(PTRACE_GETVFPREGS) to get it.
+   *
+   * Note that a real implementation of getcontext() would need to save
+   * this here to allow setcontext()/swapcontext() to work correctly.
+   */
+
+  /* Restore the values of r4 and lr */
+  mov   r0, r4
+  ldr   lr, [r0, #(MCONTEXT_GREGS_OFFSET + 14*4)]
+  ldr   r4, [r0, #(MCONTEXT_GREGS_OFFSET +  4*4)]
+
+  /* Return 0 */
+  mov   r0, #0
+  bx    lr
+
+  .fnend
+  .size breakpad_getcontext, . - breakpad_getcontext
+
+#elif defined(__i386__)
+
+  .text
+  .global breakpad_getcontext
+  .hidden breakpad_getcontext
+  .align 4
+  .type breakpad_getcontext, @function
+
+breakpad_getcontext:
+
+  movl 4(%esp), %eax   /* eax = uc */
+
+  /* Save register values */
+  movl %ecx, MCONTEXT_ECX_OFFSET(%eax)
+  movl %edx, MCONTEXT_EDX_OFFSET(%eax)
+  movl %ebx, MCONTEXT_EBX_OFFSET(%eax)
+  movl %edi, MCONTEXT_EDI_OFFSET(%eax)
+  movl %esi, MCONTEXT_ESI_OFFSET(%eax)
+  movl %ebp, MCONTEXT_EBP_OFFSET(%eax)
+
+  movl (%esp), %edx   /* return address */
+  lea  4(%esp), %ecx  /* exclude return address from stack */
+  mov  %edx, MCONTEXT_EIP_OFFSET(%eax)
+  mov  %ecx, MCONTEXT_ESP_OFFSET(%eax)
+
+  xorl %ecx, %ecx
+  movw %fs, %cx
+  mov  %ecx, MCONTEXT_FS_OFFSET(%eax)
+
+  movl $0, MCONTEXT_EAX_OFFSET(%eax)
+
+  /* Save floating point state to fpregstate, then update
+   * the fpregs pointer to point to it */
+  leal UCONTEXT_FPREGS_MEM_OFFSET(%eax), %ecx
+  fnstenv (%ecx)
+  fldenv  (%ecx)
+  mov %ecx, UCONTEXT_FPREGS_OFFSET(%eax)
+
+  /* Save signal mask: sigprocmask(SIGBLOCK, NULL, &uc->uc_sigmask) */
+  leal UCONTEXT_SIGMASK_OFFSET(%eax), %edx
+  xorl %ecx, %ecx
+  push %edx   /* &uc->uc_sigmask */
+  push %ecx   /* NULL */
+  push %ecx   /* SIGBLOCK == 0 on i386 */
+  call sigprocmask@PLT
+  addl $12, %esp
+
+  movl $0, %eax
+  ret
+
+  .size breakpad_getcontext, . - breakpad_getcontext
+
+#else
+#error "This file has not been ported for your CPU!"
+#endif