Deflake exec test.

- Only use MAXSYMLINKS/2+1 symlinks for each of the interpreter and script
  paths in SymlinkLimitRefreshedForInterpreter to tolerate cases where the
  original paths (/tmp, /bin, or /bin/echo) themselves contain symlinks.

- Ensure that UnshareFiles performs execve immediately after clone(CLONE_VFORK)
  (no heap allocation for ExecveArray/RunfilesPath).

- Use lstat() rather than stat() for the existence check in fs_util's Exists;
  the latter will fail if the symlink target does not exist, even if the
  symlink does.

PiperOrigin-RevId: 320110156

test/syscalls/linux/exec.cc

@@ -553,7 +553,12 @@ TEST(ExecTest, SymlinkLimitRefreshedForInterpreter) {
   // Hold onto TempPath objects so they are not destructed prematurely.
   std::vector<TempPath> interpreter_symlinks;
   std::vector<TempPath> script_symlinks;
-  for (int i = 0; i < kLinuxMaxSymlinks; i++) {
+  // Replace both the interpreter and script paths with symlink chains of just
+  // over half the symlink limit each; this is the minimum required to test that
+  // the symlink limit applies separately to each traversal, while tolerating
+  // some symlinks in the resolution of (the original) interpreter_path and
+  // script_path.
+  for (int i = 0; i < (kLinuxMaxSymlinks / 2) + 1; i++) {
     interpreter_symlinks.push_back(ASSERT_NO_ERRNO_AND_VALUE(
         TempPath::CreateSymlinkTo(tmp_dir, interpreter_path)));
     interpreter_path = interpreter_symlinks[i].path();
@@ -679,18 +684,16 @@ TEST(ExecveatTest, UnshareFiles) {
   const FileDescriptor fd_closed_on_exec =
       ASSERT_NO_ERRNO_AND_VALUE(Open(tempFile.path(), O_RDONLY | O_CLOEXEC));
 
-  pid_t child;
-  EXPECT_THAT(child = syscall(__NR_clone, SIGCHLD | CLONE_VFORK | CLONE_FILES,
-                              0, 0, 0, 0),
-              SyscallSucceeds());
+  ExecveArray argv = {"test"};
+  ExecveArray envp;
+  std::string child_path = RunfilePath(kBasicWorkload);
+  pid_t child =
+      syscall(__NR_clone, SIGCHLD | CLONE_VFORK | CLONE_FILES, 0, 0, 0, 0);
   if (child == 0) {
-    ExecveArray argv = {"test"};
-    ExecveArray envp;
-    ASSERT_THAT(
-        execve(RunfilePath(kBasicWorkload).c_str(), argv.get(), envp.get()),
-        SyscallSucceeds());
+    execve(child_path.c_str(), argv.get(), envp.get());
     _exit(1);
   }
+  ASSERT_THAT(child, SyscallSucceeds());
 
   int status;
   ASSERT_THAT(RetryEINTR(waitpid)(child, &status, 0), SyscallSucceeds());

test/util/fs_util.cc

@@ -125,12 +125,12 @@ PosixErrorOr<struct stat> Fstat(int fd) {
 
 PosixErrorOr<bool> Exists(absl::string_view path) {
   struct stat stat_buf;
-  int res = stat(std::string(path).c_str(), &stat_buf);
+  int res = lstat(std::string(path).c_str(), &stat_buf);
   if (res < 0) {
     if (errno == ENOENT) {
       return false;
     }
-    return PosixError(errno, absl::StrCat("stat ", path));
+    return PosixError(errno, absl::StrCat("lstat ", path));
   }
   return true;
 }

test/util/fs_util.h

@@ -44,9 +44,14 @@ PosixErrorOr<std::string> GetCWD();
 // can't be determined.
 PosixErrorOr<bool> Exists(absl::string_view path);
 
-// Returns a stat structure for the given path or an error.
+// Returns a stat structure for the given path or an error. If the path
+// represents a symlink, it will be traversed.
 PosixErrorOr<struct stat> Stat(absl::string_view path);
 
+// Returns a stat structure for the given path or an error. If the path
+// represents a symlink, it will not be traversed.
+PosixErrorOr<struct stat> Lstat(absl::string_view path);
+
 // Returns a stat struct for the given fd.
 PosixErrorOr<struct stat> Fstat(int fd);
 

test/util/temp_path.cc

@@ -56,7 +56,7 @@ void TryDeleteRecursively(std::string const& path) {
     if (undeleted_dirs || undeleted_files || !status.ok()) {
       std::cerr << path << ": failed to delete " << undeleted_dirs
                 << " directories and " << undeleted_files
-                << " files: " << status;
+                << " files: " << status << std::endl;
     }
   }
 }