[libFuzzer] don't use /dev/null for DiscardOuput in Fuchsia.

Summary:

This commit moves the `DiscardOutput` function in FuzzerIO to
FuzzerUtil, so fuchsia can have its own specialized version.

In fuchsia, accessing `/dev/null` is not supported, and there's nothing
similar to a file that discards everything that is written to it. The
way of doing something similar in fuchsia is by using `fdio_null_create`
and binding that to a file descriptor with `fdio_bind_to_fd`.

This change should fix one of the issues with the `-close_fd_mask` flag
in libfuzzer, in which closing stdout was not working due to
`fopen("/dev/null", "w")` returning `NULL`.

Reviewers: kcc, aarongreen

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69593

compiler-rt/lib/fuzzer/FuzzerIO.h

@@ -94,8 +94,6 @@ int DuplicateFile(int Fd);
 void RemoveFile(const std::string &Path);
 void RenameFile(const std::string &OldPath, const std::string &NewPath);
 
-void DiscardOutput(int Fd);
-
 intptr_t GetHandleFromFd(int fd);
 
 void MkDir(const std::string &Path);

compiler-rt/lib/fuzzer/FuzzerIOPosix.cpp

@@ -124,14 +124,6 @@ void RenameFile(const std::string &OldPath, const std::string &NewPath) {
   rename(OldPath.c_str(), NewPath.c_str());
 }
 
-void DiscardOutput(int Fd) {
-  FILE* Temp = fopen("/dev/null", "w");
-  if (!Temp)
-    return;
-  dup2(fileno(Temp), Fd);
-  fclose(Temp);
-}
-
 intptr_t GetHandleFromFd(int fd) {
   return static_cast<intptr_t>(fd);
 }

compiler-rt/lib/fuzzer/FuzzerIOWindows.cpp

@@ -223,14 +223,6 @@ void RenameFile(const std::string &OldPath, const std::string &NewPath) {
   rename(OldPath.c_str(), NewPath.c_str());
 }
 
-void DiscardOutput(int Fd) {
-  FILE* Temp = fopen("nul", "w");
-  if (!Temp)
-    return;
-  _dup2(_fileno(Temp), Fd);
-  fclose(Temp);
-}
-
 intptr_t GetHandleFromFd(int fd) {
   return _get_osfhandle(fd);
 }

compiler-rt/lib/fuzzer/FuzzerUtil.h

@@ -79,6 +79,8 @@ inline std::pair<std::string, std::string> SplitBefore(std::string X,
   return std::make_pair(S.substr(0, Pos), S.substr(Pos));
 }
 
+void DiscardOutput(int Fd);
+
 std::string DisassembleCmd(const std::string &FileName);
 
 std::string SearchRegexCmd(const std::string &Regex);

compiler-rt/lib/fuzzer/FuzzerUtilDarwin.cpp

@@ -17,6 +17,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/wait.h>
+#include <unistd.h>
 
 // There is no header for this on macOS so declare here
 extern "C" char **environ;
@@ -156,6 +157,14 @@ int ExecuteCommand(const Command &Cmd) {
   return ProcessStatus;
 }
 
+void DiscardOutput(int Fd) {
+  FILE* Temp = fopen("/dev/null", "w");
+  if (!Temp)
+    return;
+  dup2(fileno(Temp), Fd);
+  fclose(Temp);
+}
+
 } // namespace fuzzer
 
 #endif // LIBFUZZER_APPLE

compiler-rt/lib/fuzzer/FuzzerUtilFuchsia.cpp

@@ -18,6 +18,7 @@
 #include <cinttypes>
 #include <cstdint>
 #include <fcntl.h>
+#include <lib/fdio/fdio.h>
 #include <lib/fdio/spawn.h>
 #include <string>
 #include <sys/select.h>
@@ -529,6 +530,18 @@ const void *SearchMemory(const void *Data, size_t DataLen, const void *Patt,
   return memmem(Data, DataLen, Patt, PattLen);
 }
 
+// In fuchsia, accessing /dev/null is not supported. There's nothing
+// similar to a file that discards everything that is written to it.
+// The way of doing something similar in fuchsia is by using
+// fdio_null_create and binding that to a file descriptor.
+void DiscardOutput(int Fd) {
+  fdio_t *fdio_null = fdio_null_create();
+  if (fdio_null == nullptr) return;
+  int nullfd = fdio_bind_to_fd(fdio_null, -1, 0);
+  if (nullfd < 0) return;
+  dup2(nullfd, Fd);
+}
+
 } // namespace fuzzer
 
 #endif // LIBFUZZER_FUCHSIA

compiler-rt/lib/fuzzer/FuzzerUtilLinux.cpp

@@ -15,6 +15,7 @@
 #include <stdlib.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <unistd.h>
 
 
 namespace fuzzer {
@@ -27,6 +28,14 @@ int ExecuteCommand(const Command &Cmd) {
   return exit_code;
 }
 
+void DiscardOutput(int Fd) {
+  FILE* Temp = fopen("/dev/null", "w");
+  if (!Temp)
+    return;
+  dup2(fileno(Temp), Fd);
+  fclose(Temp);
+}
+
 } // namespace fuzzer
 
 #endif

compiler-rt/lib/fuzzer/FuzzerUtilWindows.cpp

@@ -16,6 +16,7 @@
 #include <chrono>
 #include <cstring>
 #include <errno.h>
+#include <io.h>
 #include <iomanip>
 #include <signal.h>
 #include <stdio.h>
@@ -190,6 +191,14 @@ std::string SearchRegexCmd(const std::string &Regex) {
   return "findstr /r \"" + Regex + "\"";
 }
 
+void DiscardOutput(int Fd) {
+  FILE* Temp = fopen("nul", "w");
+  if (!Temp)
+    return;
+  _dup2(_fileno(Temp), Fd);
+  fclose(Temp);
+}
+
 } // namespace fuzzer
 
 #endif // LIBFUZZER_WINDOWS