[msan] Fix getmntent{_r} for empty /etc/fstab

[zatrazz]

Summary

Some configuration (for instance default docker ubuntu images) uses
a default empty and invalid /etc/fstab configuration file. It makes
any call to getmntent return NULL and it leads to failures on
Msan-aarch64{-with-call}-Test/MemorySanitizer.getmntent{_r}.

This patch fixes it by creating a temporary file with some valid
entries (although not valid for the system) to use along with
setmntent/getmntent.

[eugenis]

lib/msan/tests/msan_test.cc

2213

Either make this a constructor, or call it from the constructor, or add a check in the destructor so that it does not try to unlink an uninitialized path if Create was never called.

What happens if the test fails in one of the ASSERTs, will the file be unlinked? We build msan tests with -fno-exceptions.

[zatrazz]

lib/msan/tests/msan_test.cc

2213

We can't really add the EXPECT_* macros on constructor, so I changed to a different logic: the Create returns a bool and set an internal file descriptor indicating the file was open correctly. The destructor close the file assuring the removal since it was unlinked after successful opening.

[eugenis]

lib/msan/tests/msan_test.cc

2213

if (fd != -1)

0 is a valid file descriptor

[zatrazz]

lib/msan/tests/msan_test.cc

2213

Indeed, I will fix it.

lib/msan/tests/msan_test.cc

TEST(MemorySanitizer, localtime_r) {
   ASSERT_TRUE(res != NULL);
   EXPECT_NOT_POISONED(time.tm_sec);
   EXPECT_NOT_POISONED(time.tm_hour);
   EXPECT_NOT_POISONED(time.tm_year);
   EXPECT_NOT_POISONED(time.tm_isdst);
   EXPECT_NE(0U, strlen(time.tm_zone));
 }
 
+#if !defined(__FreeBSD__)
+/* Creates a temporary file with contents similar to /etc/fstab to be used
+   with getmntent{_r}.  */
+class TempFstabFile {
+ public:
+   TempFstabFile() : fd (-1) { }
+   ~TempFstabFile() {
+     if (fd > 0)
+       close (fd);
+   }
+
+   bool Create(void) {
+     snprintf(tmpfile, sizeof(tmpfile), "/tmp/msan.getmntent.tmp.XXXXXX");
+
+     fd = mkstemp(tmpfile);
+     if (fd == -1)
+       return false;
+
+     const char entry[] = "/dev/root / ext4 errors=remount-ro 0 1";
+     size_t entrylen = sizeof(entry);
+
+     size_t bytesWritten = write(fd, entry, entrylen);
+     if (entrylen != bytesWritten)
+       return false;
+
+     return true;
+   }
+
+   const char* FileName(void) {
+     return tmpfile;
+   }
+
+ private:
+  char tmpfile[128];
+  int fd;
+};
+#endif
+
 // There's no getmntent() on FreeBSD.
 #if !defined(__FreeBSD__)
 TEST(MemorySanitizer, getmntent) {
-  FILE *fp = setmntent("/etc/fstab", "r");
+  TempFstabFile fstabtmp;
+  ASSERT_TRUE(fstabtmp.Create());
+  FILE *fp = setmntent(fstabtmp.FileName(), "r");
+
   struct mntent *mnt = getmntent(fp);
   ASSERT_TRUE(mnt != NULL);
   ASSERT_NE(0U, strlen(mnt->mnt_fsname));
   ASSERT_NE(0U, strlen(mnt->mnt_dir));
   ASSERT_NE(0U, strlen(mnt->mnt_type));
   ASSERT_NE(0U, strlen(mnt->mnt_opts));
   EXPECT_NOT_POISONED(mnt->mnt_freq);
   EXPECT_NOT_POISONED(mnt->mnt_passno);
   fclose(fp);
 }
 #endif
 
 // There's no getmntent_r() on FreeBSD.
 #if !defined(__FreeBSD__)
 TEST(MemorySanitizer, getmntent_r) {
-  FILE *fp = setmntent("/etc/fstab", "r");
+  TempFstabFile fstabtmp;
+  ASSERT_TRUE(fstabtmp.Create());
+  FILE *fp = setmntent(fstabtmp.FileName(), "r");
+
   struct mntent mntbuf;
   char buf[1000];
   struct mntent *mnt = getmntent_r(fp, &mntbuf, buf, sizeof(buf));
   ASSERT_TRUE(mnt != NULL);
   ASSERT_NE(0U, strlen(mnt->mnt_fsname));
   ASSERT_NE(0U, strlen(mnt->mnt_dir));
   ASSERT_NE(0U, strlen(mnt->mnt_type));
   ASSERT_NE(0U, strlen(mnt->mnt_opts));