[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

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler-rt/lib/fuzzer/FuzzerUtilDarwin.cpp b/compiler-rt/lib/fuzzer/FuzzerUtilDarwin.cpp
index 171db23570c..d449bc248f0 100644
--- a/compiler-rt/lib/fuzzer/FuzzerUtilDarwin.cpp
+++ b/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