Breakpad Linux client: Simplify VerifyStackReadWithMultipleThreads unit test.

As written, the VerifyStackReadWithMultipleThreads unit test makes
assumptions about the layout of thread_function's stack frame. As a result,
the test will fail when compiled with some compilers, or built with certain
optimization levels.

As an extension to C++, the GNU compilers allow you to request that a
variable be placed in a specific register. Using this, we can have
thread_function put the thread id in place where the test can find it
reliably.

a=jimblandy, r=nealsid


git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@558 4c0a9323-5329-0410-9bdc-e9ce6186880e

2 files changed, 19 insertions, 10 deletions

diff --git a/src/client/linux/minidump_writer/linux_dumper_unittest.cc b/src/client/linux/minidump_writer/linux_dumper_unittest.cc
index 6f1043c3..da454742 100644
--- a/src/client/linux/minidump_writer/linux_dumper_unittest.cc
+++ b/src/client/linux/minidump_writer/linux_dumper_unittest.cc
@@ -108,16 +108,16 @@ TEST(LinuxDumperTest, VerifyStackReadWithMultipleThreads) {
   ThreadInfo one_thread;
   for(size_t i = 0; i < dumper.threads().size(); ++i) {
     EXPECT_TRUE(dumper.ThreadInfoGet(dumper.threads()[i], &one_thread));
-    // We know the threads are in a function which has allocated exactly
-    // one word off the stack to store its thread id.
+    // In the helper program, we stored a pointer to the thread id in a
+    // specific register. Check that we can recover its value.
 #if defined(__ARM_EABI__)
-    void* process_tid_location = (void *)(one_thread.regs.uregs[11] - 8);
+    pid_t *process_tid_location = (pid_t *)(one_thread.regs.uregs[3]);
 #elif defined(__i386)
-    void* process_tid_location = (void *)(one_thread.regs.ebp - 4);
+    pid_t *process_tid_location = (pid_t *)(one_thread.regs.ecx);
 #elif defined(__x86_64)
-    void* process_tid_location = (void *)(one_thread.regs.rbp - 4);
+    pid_t *process_tid_location = (pid_t *)(one_thread.regs.rcx);
 #else
-#error Platform not supported!
+#error This test has not been ported to this platform.
 #endif
     pid_t one_thread_id;
     dumper.CopyFromProcess(&one_thread_id,
diff --git a/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc b/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc
index f744d72c..388a6580 100644
--- a/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc
+++ b/src/client/linux/minidump_writer/linux_dumper_unittest_helper.cc
@@ -37,13 +37,22 @@
 #include <sys/syscall.h>
 #include <unistd.h>
 
-#pragma GCC optimize ("O0")
-void *thread_function(void *data) __attribute__((noinline, optimize("O2")));
+#if defined(__ARM_EABI__)
+#define TID_PTR_REGISTER "r3"
+#elif defined(__i386)
+#define TID_PTR_REGISTER "ecx"
+#elif defined(__x86_64)
+#define TID_PTR_REGISTER "rcx"
+#else
+#error This test has not been ported to this platform.
+#endif
 
 void *thread_function(void *data) {
   pid_t thread_id = syscall(SYS_gettid);
-  while (true) ;
-  asm("");
+  register pid_t *thread_id_ptr asm(TID_PTR_REGISTER) = &thread_id;
+  while (true)
+    asm("" : : "r" (thread_id_ptr));
+  return NULL;
 }
 
 int main(int argc, char *argv[]) {