Use ptrace to improve tracking of child processes on linux

renderdoc/os/posix/android/android_process.cpp

@@ -98,6 +98,19 @@ int GetIdentPort(pid_t childPid)
   return ret;
 }
 
+void StopAtMainInChild()
+{
+}
+
+bool StopChildAtMain(pid_t childPid)
+{
+  return false;
+}
+
+void ResumeProcess(pid_t childPid)
+{
+}
+
 // because OSUtility::DebuggerPresent is called often we want it to be
 // cheap. Opening and parsing a file would cause high overhead on each
 // call, so instead we just cache it at startup. This fails in the case

renderdoc/os/posix/apple/apple_process.cpp

@@ -148,6 +148,19 @@ int GetIdentPort(pid_t childPid)
   return 0;
 }
 
+void StopAtMainInChild()
+{
+}
+
+bool StopChildAtMain(pid_t childPid)
+{
+  return false;
+}
+
+void ResumeProcess(pid_t childPid)
+{
+}
+
 void CacheDebuggerPresent()
 {
 }

renderdoc/os/posix/ggp/ggp_process.cpp

@@ -136,6 +136,19 @@ int GetIdentPort(pid_t childPid)
   return ret;
 }
 
+void StopAtMainInChild()
+{
+}
+
+bool StopChildAtMain(pid_t childPid)
+{
+  return false;
+}
+
+void ResumeProcess(pid_t childPid)
+{
+}
+
 // because OSUtility::DebuggerPresent is called often we want it to be
 // cheap. Opening and parsing a file would cause high overhead on each
 // call, so instead we just cache it at startup. This fails in the case

renderdoc/os/posix/linux/linux_hook.cpp

@@ -83,6 +83,10 @@ __attribute__((visibility("default"))) void *dlopen(const char *filename, int fl
 
 int GetIdentPort(pid_t childPid);
 
+void StopAtMainInChild();
+bool StopChildAtMain(pid_t childPid);
+void ResumeProcess(pid_t childPid);
+
 __attribute__((visibility("default"))) pid_t fork()
 {
   if(!realfork)
@@ -96,25 +100,59 @@ __attribute__((visibility("default"))) pid_t fork()
 
   pid_t ret = realfork();
 
-  if(ret > 0)
+  if(ret == 0)
   {
-    // in parent process, kick off a thread to get the ident
-    Threading::ThreadHandle handle = Threading::CreateThread([ret]() {
-      // don't accept a return value of our own ident, that means we've checked too early and exec
-      // hasn't run yet
-      const uint32_t ownIdent = RenderDoc::Inst().GetTargetControlIdent();
-      uint32_t ident = ownIdent;
-      for(uint32_t i = 0; i < 10 && ident == ownIdent; i++)
-      {
-        ident = (uint32_t)GetIdentPort(ret);
-        if(ident == ownIdent)
-          usleep(1000);
-      }
+    StopAtMainInChild();
+  }
+  else if(ret > 0)
+  {
+    bool stopped = StopChildAtMain(ret);
 
-      RenderDoc::Inst().AddChildProcess((uint32_t)ret, (uint32_t)ident);
-      RenderDoc::Inst().CompleteChildThread((uint32_t)ret);
-    });
-    RenderDoc::Inst().AddChildThread((uint32_t)ret, handle);
+    if(stopped)
+    {
+      int ident = GetIdentPort(ret);
+
+      ResumeProcess(ret);
+
+      if(ident)
+      {
+        RDCLOG("Identified child process %u with ident %u", ret, ident);
+        RenderDoc::Inst().AddChildProcess((uint32_t)ret, (uint32_t)ident);
+      }
+      else
+      {
+        RDCERR("Couldn't get ident for PID %u after stopping at main", ret);
+      }
+    }
+    else
+    {
+      // resume the process just in case something went wrong. This should be harmless if we're not
+      // actually tracing
+      ResumeProcess(ret);
+
+      // ptrace_scope isn't amenable, or we hit an error. We'll have to spin up a thread to check
+      // the ident on the child process and add it as soon as it's available
+      Threading::ThreadHandle handle = Threading::CreateThread([ret]() {
+        RDCLOG("Starting thread to get ident for PID %u", ret);
+
+        // don't accept a return value of our own ident, that means we've checked too early and exec
+        // hasn't run yet
+        const uint32_t ownIdent = RenderDoc::Inst().GetTargetControlIdent();
+        uint32_t ident = ownIdent;
+        for(uint32_t i = 0; i < 10 && ident == ownIdent; i++)
+        {
+          ident = (uint32_t)GetIdentPort(ret);
+          if(ident == ownIdent)
+            usleep(1000);
+        }
+
+        RDCLOG("PID %u has ident %u", ret, ident);
+
+        RenderDoc::Inst().AddChildProcess((uint32_t)ret, (uint32_t)ident);
+        RenderDoc::Inst().CompleteChildThread((uint32_t)ret);
+      });
+      RenderDoc::Inst().AddChildThread((uint32_t)ret, handle);
+    }
   }
 
   return ret;

renderdoc/os/posix/linux/linux_process.cpp

@@ -22,13 +22,25 @@
  * THE SOFTWARE.
  ******************************************************************************/
 
+#include <elf.h>
+#include <sys/ptrace.h>
+#include <sys/types.h>
+#include <sys/user.h>
+#include <sys/wait.h>
+#include <time.h>
 #include <unistd.h>
 #include <algorithm>
 #include "api/replay/data_types.h"
 #include "common/common.h"
 #include "common/formatting.h"
+#include "core/core.h"
+#include "core/settings.h"
 #include "os/os_specific.h"
 
+RDOC_CONFIG(bool, Linux_PtraceChildProcesses, true,
+            "Use ptrace(2) to trace child processes at startup to ensure connection is made as "
+            "early as possible.");
+
 extern char **environ;
 
 // we wait 1ns, then 2ns, then 4ns, etc so our total is 0xfff etc
@@ -136,6 +148,277 @@ int GetIdentPort(pid_t childPid)
   return ret;
 }
 
+static bool ptrace_scope_ok()
+{
+  if(!Linux_PtraceChildProcesses())
+    return false;
+
+  rdcstr contents;
+  FileIO::ReadAll("/proc/sys/kernel/yama/ptrace_scope", contents);
+  contents.trim();
+  if(!contents.empty())
+  {
+    int ptrace_scope = atoi(contents.c_str());
+    if(ptrace_scope > 1)
+    {
+      if(RenderDoc::Inst().IsReplayApp())
+      {
+        static bool warned = false;
+        if(!warned)
+        {
+          warned = true;
+          RDCWARN(
+              "ptrace_scope value %d means ptrace can't be used to pause child processes while "
+              "attaching.",
+              ptrace_scope);
+        }
+      }
+      return false;
+    }
+  }
+
+  return true;
+}
+
+static uint64_t get_nanotime()
+{
+  timespec ts;
+  clock_gettime(CLOCK_MONOTONIC, &ts);
+  uint64_t ret = uint64_t(ts.tv_sec) * 1000000000ULL + uint32_t(ts.tv_nsec & 0xffffffff);
+  return ret;
+}
+
+#if ENABLED(RDOC_X64)
+#define INST_PTR_REG rip
+#else
+#define INST_PTR_REG eip
+#endif
+
+static uint64_t get_child_ip(pid_t childPid)
+{
+  user_regs_struct regs = {};
+
+  long ptraceRet = ptrace(PTRACE_GETREGS, childPid, NULL, &regs);
+  if(ptraceRet == 0)
+    return uint64_t(regs.INST_PTR_REG);
+
+  return 0;
+}
+
+static bool wait_traced_child(pid_t childPid, uint32_t timeoutMS, int &status)
+{
+  // spin waiting for the traced child, with a 100ms timeout
+  status = 0;
+  uint64_t start_nano = get_nanotime();
+  uint64_t end_nano = 0;
+  int ret = 0;
+
+  const uint64_t timeoutNanoseconds = uint64_t(timeoutMS) * 1000 * 1000;
+
+  while((ret = waitpid(childPid, &status, WNOHANG)) == 0)
+  {
+    status = 0;
+
+    // if we're in a capturing process then the process itself might have done waitpid(-1) and
+    // swallowed the wait for our child. So as an alternative we check to see if we can query the
+    // instruction pointer, which is only possible if the child is stopped.
+    uint64_t ip = get_child_ip(childPid);
+    if(ip != 0)
+    {
+      // do waitpid again in case we raced and the child stopped in between the call to waitpid and
+      // get_child_ip.
+      ret = waitpid(childPid, &status, WNOHANG);
+
+      // if it still didn't succeed, set status to 0 so we know we're earlying out and don't check
+      // the status codes.
+      if(ret == 0)
+        status = 0;
+      return true;
+    }
+
+    usleep(10);
+
+    // check the timeout
+    end_nano = get_nanotime();
+    if(end_nano - start_nano > timeoutNanoseconds)
+      break;
+  }
+
+  return WIFSTOPPED(status);
+}
+
+bool StopChildAtMain(pid_t childPid)
+{
+  // don't do this unless the ptrace scope is OK.
+  if(!ptrace_scope_ok())
+    return false;
+
+  int childStatus = 0;
+
+  // we have a low timeout for this stop since it should happen almost immediately (right after the
+  // fork). If it didn't then we want to fail relatively fast.
+  if(!wait_traced_child(childPid, 100, childStatus))
+  {
+    RDCERR("Didn't get initial stop from child PID %u", childPid);
+    return false;
+  }
+
+  if(childStatus > 0 && WSTOPSIG(childStatus) != SIGSTOP)
+  {
+    RDCERR("Initial signal from child PID %u was %x, expected %x", childPid, WSTOPSIG(childStatus),
+           SIGSTOP);
+    return false;
+  }
+
+  long ptraceRet = 0;
+
+  // continue until exec
+  ptraceRet = ptrace(PTRACE_SETOPTIONS, childPid, NULL, PTRACE_O_TRACEEXEC);
+  RDCASSERTEQUAL(ptraceRet, 0);
+
+  // continue
+  ptraceRet = ptrace(PTRACE_CONT, childPid, NULL, NULL);
+  RDCASSERTEQUAL(ptraceRet, 0);
+
+  // we're not under control of when the application calls exec() after fork() in the case of child
+  // processes, so be a little more generous with the timeout
+  if(!wait_traced_child(childPid, 250, childStatus))
+  {
+    RDCERR("Didn't get to execve in child PID %u", childPid);
+    return false;
+  }
+
+  if(childStatus > 0 && (childStatus >> 8) != (SIGTRAP | (PTRACE_EVENT_EXEC << 8)))
+  {
+    RDCERR("Exec wait event from child PID %u was status %x, expected %x", childPid,
+           (childStatus >> 8), (SIGTRAP | (PTRACE_EVENT_EXEC << 8)));
+    return false;
+  }
+
+  rdcstr exepath;
+  long basePointer = 0;
+  uint32_t sectionOffset = 0;
+
+  rdcstr mapsName = StringFormat::Fmt("/proc/%u/maps", childPid);
+
+  FILE *maps = FileIO::fopen(mapsName.c_str(), "r");
+
+  if(!maps)
+  {
+    RDCERR("Couldn't open %s", mapsName.c_str());
+    return false;
+  }
+
+  while(!feof(maps))
+  {
+    char line[512] = {0};
+    if(fgets(line, 511, maps))
+    {
+      if(strstr(line, "r-xp"))
+      {
+        RDCCOMPILE_ASSERT(sizeof(long) == sizeof(void *), "Expected long to be pointer sized");
+        int pathOffset = 0;
+        int num = sscanf(line, "%lx-%*x r-xp %x %*x:%*x %*u %n", &basePointer, &sectionOffset,
+                         &pathOffset);
+
+        if(num != 2 || pathOffset == 0)
+        {
+          RDCERR("Couldn't parse first executable mapping '%s'", rdcstr(line).trimmed().c_str());
+          return false;
+        }
+
+        exepath = line + pathOffset;
+        exepath.trim();
+        break;
+      }
+    }
+  }
+
+  if(basePointer == 0)
+  {
+    RDCERR("Couldn't find executable mapping in maps file");
+    return false;
+  }
+
+  FileIO::fclose(maps);
+
+  FILE *elf = FileIO::fopen(exepath.c_str(), "r");
+
+  if(!elf)
+  {
+    RDCERR("Couldn't open %s to parse ELF header", exepath.c_str());
+    return false;
+  }
+
+  Elf64_Ehdr elf_header;
+  size_t read = FileIO::fread(&elf_header, sizeof(elf_header), 1, elf);
+  FileIO::fclose(elf);
+
+  if(read != 1)
+  {
+    RDCERR("Couldn't read ELF header from %s", exepath.c_str());
+    return false;
+  }
+
+  void *entry = (void *)(basePointer + elf_header.e_entry - sectionOffset);
+
+  long origEntryWord = ptrace(PTRACE_PEEKTEXT, childPid, entry, 0);
+
+  long breakpointWord = (origEntryWord & 0xffffff00) | 0xcc;
+  ptraceRet = ptrace(PTRACE_POKETEXT, childPid, entry, breakpointWord);
+  RDCASSERTEQUAL(ptraceRet, 0);
+
+  // continue
+  ptraceRet = ptrace(PTRACE_CONT, childPid, NULL, NULL);
+  RDCASSERTEQUAL(ptraceRet, 0);
+
+  // it could take a long time to hit main so we have a large timeout here
+  if(!wait_traced_child(childPid, 2000, childStatus))
+  {
+    RDCERR("Didn't hit breakpoint in PID %u (%x)", childPid, childStatus);
+    return false;
+  }
+
+  // we're now at main! now just need to clean up after ourselves
+
+  user_regs_struct regs = {};
+
+  ptraceRet = ptrace(PTRACE_GETREGS, childPid, NULL, &regs);
+  RDCASSERTEQUAL(ptraceRet, 0);
+
+  // step back past the byte we inserted the breakpoint on
+  regs.INST_PTR_REG--;
+  ptraceRet = ptrace(PTRACE_SETREGS, childPid, NULL, &regs);
+  RDCASSERTEQUAL(ptraceRet, 0);
+
+  // restore the function
+  ptraceRet = ptrace(PTRACE_POKETEXT, childPid, entry, origEntryWord);
+  RDCASSERTEQUAL(ptraceRet, 0);
+
+  // we'll resume after reading the ident port in the calling function
+  return true;
+}
+
+void StopAtMainInChild()
+{
+  // don't do this unless the ptrace scope is OK.
+  if(!ptrace_scope_ok())
+    return;
+
+  // allow parent tracing, and immediately stop so the parent process can attach
+  ptrace(PTRACE_TRACEME, 0, 0, 0);
+  raise(SIGSTOP);
+}
+
+void ResumeProcess(pid_t childPid)
+{
+  if(childPid != 0)
+  {
+    // try to detach and resume the process, ignoring any errors if we weren't tracing
+    ptrace(PTRACE_DETACH, childPid, NULL, NULL);
+  }
+}
+
 // because OSUtility::DebuggerPresent is called often we want it to be
 // cheap. Opening and parsing a file would cause high overhead on each
 // call, so instead we just cache it at startup. This fails in the case

renderdoc/os/posix/posix_process.cpp

@@ -44,6 +44,12 @@
 char **GetCurrentEnvironment();
 int GetIdentPort(pid_t childPid);
 
+// functions to try and let the child run just far enough to get to main() but no further. This lets
+// us check the ident port and resume.
+void StopAtMainInChild();
+bool StopChildAtMain(pid_t childPid);
+void ResumeProcess(pid_t childPid);
+
 #if ENABLED(RDOC_APPLE)
 
 #define PRELOAD_ENV_VAR "DYLD_INSERT_LIBRARIES"
@@ -572,6 +578,8 @@ static pid_t RunProcess(const char *app, const char *workingDir, const char *cmd
     childPid = fork();
     if(childPid == 0)
     {
+      StopAtMainInChild();
+
       FileIO::ReleaseFDAfterFork();
       if(stdoutPipe)
       {
@@ -593,22 +601,27 @@ static pid_t RunProcess(const char *app, const char *workingDir, const char *cmd
       fprintf(stderr, "exec failed\n");
       _exit(1);
     }
-    else if(!stdoutPipe)
+    else
     {
-      // remember this PID so we can wait on it later
-      SCOPED_SPINLOCK(zombieLock);
+      if(!stdoutPipe)
+      {
+        // remember this PID so we can wait on it later
+        SCOPED_SPINLOCK(zombieLock);
 
-      PIDNode *node = NULL;
+        PIDNode *node = NULL;
 
-      // take a child from the free list if available, otherwise allocate a new one
-      if(freeChildren.head)
-        node = freeChildren.pop_front();
-      else
-        node = new PIDNode();
+        // take a child from the free list if available, otherwise allocate a new one
+        if(freeChildren.head)
+          node = freeChildren.pop_front();
+        else
+          node = new PIDNode();
 
-      node->pid = childPid;
+        node->pid = childPid;
 
-      children.append(node);
+        children.append(node);
+      }
+
+      StopChildAtMain(childPid);
     }
   }
 
@@ -650,8 +663,10 @@ uint32_t Process::LaunchProcess(const char *app, const char *workingDir, const c
   }
 
   char **currentEnvironment = GetCurrentEnvironment();
-  uint32_t ret = (uint32_t)RunProcess(app, workingDir, cmdLine, currentEnvironment,
-                                      result ? stdoutPipe : NULL, result ? stderrPipe : NULL);
+  pid_t ret = RunProcess(app, workingDir, cmdLine, currentEnvironment, result ? stdoutPipe : NULL,
+                         result ? stderrPipe : NULL);
+
+  ResumeProcess(ret);
 
   if(result)
   {
@@ -698,7 +713,7 @@ uint32_t Process::LaunchProcess(const char *app, const char *workingDir, const c
     close(stderrPipe[0]);
   }
 
-  return ret;
+  return (uint32_t)ret;
 }
 
 uint32_t Process::LaunchScript(const char *script, const char *workingDir, const char *argList,
@@ -826,17 +841,20 @@ rdcpair<ReplayStatus, uint32_t> Process::LaunchAndInjectIntoProcess(
     i++;
   }
 
+  RDCLOG("Running process %s for injection", app);
+
   pid_t childPid = RunProcess(app, workingDir, cmdLine, envp);
 
   int ret = 0;
 
   if(childPid != (pid_t)0)
   {
-    // wait for child to have opened its socket
-    usleep(1000);
-
+    // ideally we stopped at main so we can check the port immediately. Otherwise this will do an
+    // exponential wait to get it as soon as possible
     ret = GetIdentPort(childPid);
 
+    ResumeProcess(ret);
+
     if(waitForExit)
     {
       int dummy = 0;