Improve fork-safety by opening files with O_CLOEXEC.

PiperOrigin-RevId: 707791841
Change-Id: If422c8246c33617e3b103ce5894234d5ee96071f

Author: Abseil Team

absl/debugging/symbolize_elf.inc

@@ -378,8 +378,8 @@ class Symbolizer {
 };
 
 // Protect against client code closing low-valued file descriptors it doesn't
-// actually own, as has happened in b/384213477.
-int SaferOpen(const char *fname, int flags) {
+// actually own.
+int OpenReadOnlyWithHighFD(const char *fname) {
   static int high_fd = [] {
     struct rlimit rlim{};
     const int rc = getrlimit(RLIMIT_NOFILE, &rlim);
@@ -393,22 +393,28 @@ int SaferOpen(const char *fname, int flags) {
                  rc, static_cast<long>(rlim.rlim_cur));
     return -1;
   }();
+  constexpr int kOpenFlags = O_RDONLY | O_CLOEXEC;
   if (high_fd >= 1000) {
-    const int fd = open(fname, flags);
+    const int fd = open(fname, kOpenFlags);
     if (fd != -1 && fd < high_fd) {
       // Try to relocate fd to high range.
-      const int fd2 = fcntl(fd, F_DUPFD, high_fd);
+      static_assert(kOpenFlags & O_CLOEXEC,
+                    "F_DUPFD_CLOEXEC assumes O_CLOEXEC");
+      const int fd2 = fcntl(fd, F_DUPFD_CLOEXEC, high_fd);
       if (fd2 != -1) {
         // Successfully obtained high fd. Use it.
         close(fd);
         return fd2;
+      } else {
+        ABSL_RAW_LOG(WARNING, "Unable to dup fd=%d above %d, errno=%d", fd,
+                     high_fd, errno);
       }
     }
     // Either open failed and fd==-1, or fd is already above high_fd, or fcntl
     // failed and fd is valid (but low).
     return fd;
   }
-  return open(fname, flags);
+  return open(fname, kOpenFlags);
 }
 
 static std::atomic<Symbolizer *> g_cached_symbolizer;
@@ -1099,7 +1105,7 @@ static ABSL_ATTRIBUTE_NOINLINE bool ReadAddrMap(
   snprintf(maps_path, sizeof(maps_path), "/proc/self/task/%d/maps", getpid());
 
   int maps_fd;
-  NO_INTR(maps_fd = SaferOpen(maps_path, O_RDONLY));
+  NO_INTR(maps_fd = OpenReadOnlyWithHighFD(maps_path));
   FileDescriptor wrapped_maps_fd(maps_fd);
   if (wrapped_maps_fd.get() < 0) {
     ABSL_RAW_LOG(WARNING, "%s: errno=%d", maps_path, errno);
@@ -1373,7 +1379,7 @@ static void MaybeOpenFdFromSelfExe(ObjFile *obj) {
   if (memcmp(obj->start_addr, ELFMAG, SELFMAG) != 0) {
     return;
   }
-  int fd = SaferOpen("/proc/self/exe", O_RDONLY);
+  int fd = OpenReadOnlyWithHighFD("/proc/self/exe");
   if (fd == -1) {
     return;
   }
@@ -1397,15 +1403,15 @@ static void MaybeOpenFdFromSelfExe(ObjFile *obj) {
 
 static bool MaybeInitializeObjFile(ObjFile *obj) {
   if (obj->fd < 0) {
-    obj->fd = SaferOpen(obj->filename, O_RDONLY);
+    obj->fd = OpenReadOnlyWithHighFD(obj->filename);
 
     if (obj->fd < 0) {
       // Getting /proc/self/exe here means that we were hinted.
       if (strcmp(obj->filename, "/proc/self/exe") == 0) {
         // /proc/self/exe may be inaccessible (due to setuid, etc.), so try
         // accessing the binary via argv0.
         if (argv0_value != nullptr) {
-          obj->fd = SaferOpen(argv0_value, O_RDONLY);
+          obj->fd = OpenReadOnlyWithHighFD(argv0_value);
         }
       } else {
         MaybeOpenFdFromSelfExe(obj);