[Darwin] Fix a bug where the symbolizer would examine the wrong process.

[delcypher]

Summary

Previously AtosSymbolizer would set the PID to examine in the
constructor which is called early on during sanitizer init. This can
lead to incorrect behaviour in the case of a fork() because if the
symbolizer is launched in the child it will be told examine the parent
process rather than the child.

To fix this the PID is determined just before the symbolizer is
launched.

A test case is included that triggers the buggy behaviour that existed
prior to this patch. The test observes the PID that atos was called
on. It also examines the symbolized stacktrace. Prior to this patch
atos failed to symbolize the stacktrace giving output that looked
like...

#0 0x100fc3bb5 in __sanitizer_print_stack_trace asan_stack.cpp:86
#1 0x10490dd36 in PrintStack+0x56 (/path/to/print-stack-trace-in-code-loaded-after-fork.cpp.tmp_shared_lib:x86_64+0xd36)
#2 0x100f6f986 in main+0x4a6 (path/to/print-stack-trace-in-code-loaded-after-fork.cpp.tmp_loader:x86_64+0x100001986)
#3 0x7fff714f1cc8 in start+0x0 (/usr/lib/system/libdyld.dylib:x86_64+0x1acc8)

After this patch stackframes #1 and #2 are fully symbolized.

This patch is also a pre-requisite refactor for rdar://problem/58789439.

[yln]

compiler-rt/test/sanitizer_common/TestCases/Darwin/print-stack-trace-in-code-loaded-after-fork.cpp

46

Can you explain why the separately loaded lib/dlopen is required to trigger the error condition? From the commit message I thought that a fork should be enough? Other than that: LGTM

[delcypher]

compiler-rt/test/sanitizer_common/TestCases/Darwin/print-stack-trace-in-code-loaded-after-fork.cpp

46

I'm deliberately loading code into the child process that doesn't exist in the parent process. If atos is told examine the parent process then it will be told to examine a PC in PrintStack which isn't loaded in that process and so will fail.

I'll add a comment to the test case to explain the reasoning here.

[delcypher]

Add comment explaining loading of dylib.

[delcypher]

@yln Is the comment I added clear enough?

[yln]

compiler-rt/test/sanitizer_common/TestCases/Darwin/print-stack-trace-in-code-loaded-after-fork.cpp

46

So before, we were always calling atos with the wrong pid (parent pid), but it only matters in cases where we try to symbolize code that isn't actually present in the parent. Got it, thanks!

compiler-rt/lib/sanitizer_common/sanitizer_symbolizer_mac.cpp

bool DlAddrSymbolizer::SymbolizeData(uptr addr, DataInfo *datainfo) {
   const char *demangled = DemangleSwiftAndCXX(info.dli_sname);
   datainfo->name = internal_strdup(demangled);
   datainfo->start = (uptr)info.dli_saddr;
   return true;
 }
 
 class AtosSymbolizerProcess : public SymbolizerProcess {
  public:
-  explicit AtosSymbolizerProcess(const char *path, pid_t parent_pid)
+  explicit AtosSymbolizerProcess(const char *path)
       : SymbolizerProcess(path, /*use_posix_spawn*/ true) {
-    // Put the string command line argument in the object so that it outlives
-    // the call to GetArgV.
-    internal_snprintf(pid_str_, sizeof(pid_str_), "%d", parent_pid);
+    pid_str_[0] = '\0';
   }
 
  private:
   bool StartSymbolizerSubprocess() override {
     // Configure sandbox before starting atos process.
+
+    // Put the string command line argument in the object so that it outlives
+    // the call to GetArgV.
+    internal_snprintf(pid_str_, sizeof(pid_str_), "%d", internal_getpid());
+
     return SymbolizerProcess::StartSymbolizerSubprocess();
   }
 
   bool ReachedEndOfOutput(const char *buffer, uptr length) const override {
     return (length >= 1 && buffer[length - 1] == '\n');
   }
 
   void GetArgV(const char *path_to_binary,
if (rest[0] == '(') {
     if (start_address) *start_address = addr - offset;
   }
 
   InternalFree(trim);
   return true;
 }
 
 AtosSymbolizer::AtosSymbolizer(const char *path, LowLevelAllocator *allocator)
-    : process_(new(*allocator) AtosSymbolizerProcess(path, getpid())) {}
+    : process_(new (*allocator) AtosSymbolizerProcess(path)) {}
 
 bool AtosSymbolizer::SymbolizePC(uptr addr, SymbolizedStack *stack) {
   if (!process_) return false;
   if (addr == 0) return false;
   char command[32];
   internal_snprintf(command, sizeof(command), "0x%zx\n", addr);
   const char *buf = process_->SendCommand(command);
   if (!buf) return false;

compiler-rt/test/sanitizer_common/TestCases/Darwin/print-stack-trace-in-code-loaded-after-fork.cpp

+// RUN: %clangxx %s -g -DSHARED_LIB -shared -o %t_shared_lib.dylib
+// RUN: %clangxx %s -g -USHARED_LIB -o %t_loader
+// RUN: %env_tool_opts=verbosity=3 %run %t_loader %t_shared_lib.dylib > %t_loader_output.txt 2>&1
+// RUN: FileCheck -input-file=%t_loader_output.txt %s
+// RUN: FileCheck -check-prefix=CHECK-STACKTRACE -input-file=%t_loader_output.txt %s
+
+#include <stdio.h>
+
+#ifdef SHARED_LIB
+#include <sanitizer/common_interface_defs.h>
+
+extern "C" void PrintStack() {
+  fprintf(stderr, "Calling __sanitizer_print_stack_trace\n");
+  // CHECK-STACKTRACE: #0{{( *0x.* *in *)?}}  __sanitizer_print_stack_trace
+  // CHECK-STACKTRACE: #1{{( *0x.* *in *)?}} PrintStack {{.*}}print-stack-trace-in-code-loaded-after-fork.cpp:[[@LINE+1]]
+  __sanitizer_print_stack_trace();
+}
+#else
+#include <assert.h>
+#include <dlfcn.h>
+#include <stdlib.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+typedef void (*PrintStackFnPtrTy)(void);
+
+int main(int argc, char **argv) {
+  assert(argc == 2);
+  pid_t pid = fork();
+  if (pid != 0) {
+    // Parent
+    pid_t parent_pid = getpid();
+    fprintf(stderr, "parent: %d\n", parent_pid);
+    int status = 0;
+    pid_t child = waitpid(pid, &status, /*options=*/0);
+    assert(pid == child);
+    bool clean_exit = WIFEXITED(status) && WEXITSTATUS(status) == 0;
+    return !clean_exit;
+  }
+  // Child.
+  pid = getpid();
+  // CHECK: child: [[CHILD_PID:[0-9]+]]
+  fprintf(stderr, "child: %d\n", pid);
+  // We load new code into the child process that isn't loaded into the parent.
+  // When we symbolize in `PrintStack` if the symbolizer is told to symbolize
+  // the parent (an old bug) rather than the child then symbolization will
+  // fail.
+  const char *library_to_load = argv[1];
+  void *handle = dlopen(library_to_load, RTLD_NOW | RTLD_LOCAL);
+  assert(handle);
+  PrintStackFnPtrTy PrintStackFnPtr = (PrintStackFnPtrTy)dlsym(handle, "PrintStack");
+  assert(PrintStackFnPtr);
+  // Check that the symbolizer is told examine the child process.
+  // CHECK: Launching Symbolizer process: {{.+}}atos -p [[CHILD_PID]]
+  // CHECK-STACKTRACE: #2{{( *0x.* *in *)?}} main {{.*}}print-stack-trace-in-code-loaded-after-fork.cpp:[[@LINE+1]]
+  PrintStackFnPtr();
+  return 0;
+}
+
+#endif