[fuzzer] Use puts() rather than printf() in CopyFileToErr()

CopyFileToErr() uses Printf("%s", ...) which fails with a negative size on
files >2Gb (Its path is through var-args wrappers to an unnecessary "%s"
expansion and subject to int overflows) Using puts() in place of printf()
bypasses this path and writes the string directly to stderr. This avoids the
present loss of data when a crashed worker has generated >2Gb of output.

rdar://99384640

Reviewed By: yln, rsundahl

Differential Revision: https://reviews.llvm.org/D146189

compiler-rt/lib/fuzzer/FuzzerIO.cpp

@@ -65,7 +65,7 @@ std::string FileToString(const std::string &Path) {
 }
 
 void CopyFileToErr(const std::string &Path) {
-  Printf("%s", FileToString(Path).c_str());
+  Puts(FileToString(Path).c_str());
 }
 
 void WriteToFile(const Unit &U, const std::string &Path) {
@@ -151,6 +151,11 @@ void CloseStdout() {
   DiscardOutput(1);
 }
 
+void Puts(const char *Str) {
+  fputs(Str, OutputFile);
+  fflush(OutputFile);
+}
+
 void Printf(const char *Fmt, ...) {
   va_list ap;
   va_start(ap, Fmt);

compiler-rt/lib/fuzzer/FuzzerIO.h

@@ -58,6 +58,7 @@ void CloseStdout();
 FILE *GetOutputFile();
 void SetOutputFile(FILE *NewOutputFile);
 
+void Puts(const char *Str);
 void Printf(const char *Fmt, ...);
 void VPrintf(bool Verbose, const char *Fmt, ...);
 

compiler-rt/test/fuzzer/BigFileCopy.cpp

@@ -0,0 +1,31 @@
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cstddef>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <cstring>
+
+#include "FuzzerIO.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+  const char *FileName = "big-file.txt";
+  FILE *f = fopen(FileName, "w");
+
+  // This is the biggest file possible unless CopyFileToErr() uses Puts()
+  fprintf(f, "%2147483646s", "2Gb-2");
+
+  // This makes the file too big if CopyFileToErr() uses fprintf("%s", <file>)
+  fprintf(f, "THIS LINE RESPONSIBLE FOR EXCEEDING 2Gb FILE SIZE\n");
+  fclose(f);
+
+  // Should now because CopyFileToErr() now uses Puts()
+  fuzzer::CopyFileToErr(FileName);
+
+  // File is >2Gb so clean up
+  remove(FileName);
+
+  return 0;
+}

compiler-rt/test/fuzzer/big-file-copy.test

@@ -0,0 +1,4 @@
+RUN: %cpp_compiler %S/BigFileCopy.cpp -o %t
+RUN: %run %t -runs=1 -rss_limit_mb=4096 2>big-file-out.txt; result=$?
+RUN: %run rm -f big-file.txt big-file-out.txt
+RUN: %run (exit $result)