From 6bc7929d0598cacceeeecbb5e1e926dea4aa03cb Mon Sep 17 00:00:00 2001
From: Jeremy Rifkin <51220084+jeremy-rifkin@users.noreply.github.com>
Date: Thu, 5 Sep 2024 20:53:31 -0500
Subject: [PATCH] Some signal safety and refactoring, also don't LD_PRELOAD for
 the tracer program (#4)

---
 libSegFault/config.cpp            | 11 -------
 libSegFault/config.hpp            |  1 -
 libSegFault/segfault.cpp          |  8 +++--
 libSegFault/signal-safe-trace.cpp | 50 ++++++++++++++++++++++++++-----
 4 files changed, 47 insertions(+), 23 deletions(-)

diff --git a/libSegFault/config.cpp b/libSegFault/config.cpp
index 844f15d..03b993f 100644
--- a/libSegFault/config.cpp
+++ b/libSegFault/config.cpp
@@ -1,17 +1,6 @@
 #include <cstdlib>
 #include <cstring>
 
-const char *getTracerProgram()
-{
-    static const char *emptystr = "";
-    if (const char *value = getenv("LIBSEGFAULT_TRACER"))
-    {
-        return value;
-    }
-
-    return emptystr;
-}
-
 bool isDebugMode()
 {
     if (const char *value = getenv("LIBSEGFAULT_DEBUG"))
diff --git a/libSegFault/config.hpp b/libSegFault/config.hpp
index 7a4a23a..25c0691 100644
--- a/libSegFault/config.hpp
+++ b/libSegFault/config.hpp
@@ -1,7 +1,6 @@
 #pragma once
 
 bool isDebugMode();
-const char *getTracerProgram();
 bool useAlternativeStack();
 bool dumpRegisters();
 bool dumpMemory();
diff --git a/libSegFault/segfault.cpp b/libSegFault/segfault.cpp
index 8733f0c..722fd9d 100644
--- a/libSegFault/segfault.cpp
+++ b/libSegFault/segfault.cpp
@@ -32,8 +32,8 @@
 #include <register-dump.h>
 #include <sigcontextinfo.h>
 
-#include "signal-safe-trace.hpp"
 #include "config.hpp"
+#include "signal-safe-trace.hpp"
 
 #ifdef SA_SIGINFO
 #define SIGCONTEXT siginfo_t *info, void *
@@ -59,6 +59,8 @@ static void write_strsignal(int fd, int signal)
     TEMP_FAILURE_RETRY(write(fd, ptr, &buf[sizeof(buf)] - ptr));
 }
 
+const auto dump_registers = dumpRegisters();
+const auto dump_memory = dumpMemory();
 
 /* This function is called when a segmentation fault is caught.  The system
    is in an unstable state now.  This means especially that malloc() might
@@ -85,7 +87,7 @@ static void catch_segfault(int signal, SIGCONTEXT ctx)
     WRITE_STRING("\n");
 
 #ifdef REGISTER_DUMP
-    if (dumpRegisters())
+    if (dump_registers)
     {
         REGISTER_DUMP;
     }
@@ -96,7 +98,7 @@ static void catch_segfault(int signal, SIGCONTEXT ctx)
     do_signal_safe_trace();
 
 #ifdef HAVE_PROC_SELF
-    if (dumpMemory())
+    if (dump_memory)
     {
         /* Now the link map.  */
         int mapfd = open("/proc/self/maps", O_RDONLY);
diff --git a/libSegFault/signal-safe-trace.cpp b/libSegFault/signal-safe-trace.cpp
index 16b0067..8c2947b 100644
--- a/libSegFault/signal-safe-trace.cpp
+++ b/libSegFault/signal-safe-trace.cpp
@@ -8,6 +8,7 @@
 #include <iostream>
 #include <string>
 #include <string_view>
+#include <vector>
 
 #include <csignal>
 #include <cstring>
@@ -41,6 +42,40 @@ void warmup_cpptrace()
     cpptrace::get_safe_object_frame(buffer[0], &frame);
 }
 
+std::string tracer_program = []
+{
+    const char *value = getenv("LIBSEGFAULT_TRACER");
+    return value ? value : "";
+}();
+
+extern char **environ;
+
+std::vector<std::string> tracer_env = []
+{
+    std::vector<std::string> tracer_env;
+    for (char **s = environ; *s; s++)
+    {
+        std::string_view var = *s;
+        if (!var.starts_with("LD_PRELOAD="))
+        {
+            tracer_env.emplace_back(var);
+        }
+    }
+    return tracer_env;
+}();
+
+const std::vector<char *> tracer_env_buffer = []
+{
+    std::vector<char *> tracer_env_buffer;
+    for (auto &var : tracer_env)
+    {
+        tracer_env_buffer.emplace_back(var.data());
+    }
+    tracer_env_buffer.emplace_back(nullptr);
+    return tracer_env_buffer;
+}();
+
+const bool is_debug = isDebugMode();
 
 [[gnu::constructor]] void init()
 {
@@ -48,10 +83,11 @@ void warmup_cpptrace()
 }
 
 const auto fork_failure_message = "fork() failed, unable to collect trace\n"sv;
-const auto no_tracer_message = "exec(signal_tracer) failed: Please supply the environment variable LIBSEGFAULT_TRACER.\n"sv;
+const auto no_tracer_message = "exec(signal_tracer) failed: Please supply the environment variable "
+                               "LIBSEGFAULT_TRACER.\n"sv;
 
-const auto exec_failure_message =
-"exec(signal_tracer) failed: Make sure the signal_tracer exists and the executable permissions are correct.\n"sv;
+const auto exec_failure_message = "exec(signal_tracer) failed: Make sure the signal_tracer exists and the executable "
+                                  "permissions are correct.\n"sv;
 
 void do_signal_safe_trace()
 {
@@ -74,21 +110,19 @@ void do_signal_safe_trace()
         close(input_pipe.read_end);
         close(input_pipe.write_end);
 
-        bool is_debug = isDebugMode();
-        const char *tracer_program = getTracerProgram();
-        if (strlen(tracer_program) == 0)
+        if (tracer_program.size() == 0)
         {
             std::ignore = write(STDERR_FILENO, no_tracer_message.data(), no_tracer_message.size());
         }
         else
         {
-            execl(tracer_program, tracer_program, nullptr);
+            execle(tracer_program.c_str(), tracer_program.c_str(), nullptr, tracer_env_buffer.data());
 
             if (is_debug)
             {
                 auto errcode = errno;
                 fprintf(stderr, "errno: %d\n", errcode);
-                fprintf(stderr, "tried to execute: %s\n", tracer_program);
+                fprintf(stderr, "tried to execute: %s\n", tracer_program.c_str());
             }
 
             // https://linux.die.net/man/3/execl - execl() only returns when an error has occured