Adding safer permissions to PosixFilesystem::NewLogger (#8106)

Summary:
We have observed rocksdb databases creating info log files with world-writeable permissions.

The reason why the file is created like so is because stdio streams opened with fopen calls use mode 0666, and while normally most systems have a umask of 022, in some occasions (for instance, while running daemons), you may find that the application is running with a less restrictive umask. The result is that when opening the DB, the LOG file would be created with world-writeable perms:

```
$ ls -lh db/
total 6.4M
-rw-r--r-- 1 ibarba users  115 Mar 24 17:41 000004.log
-rw-r--r-- 1 ibarba users   16 Mar 24 17:41 CURRENT
-rw-r--r-- 1 ibarba users   37 Mar 24 17:41 IDENTITY
-rw-r--r-- 1 ibarba users    0 Mar 24 17:41 LOCK
-rw-rw-r-- 1 ibarba users 114K Mar 24 17:41 LOG
-rw-r--r-- 1 ibarba users  514 Mar 24 17:41 MANIFEST-000003
-rw-r--r-- 1 ibarba users  31K Mar 24 17:41 OPTIONS-000018
-rw-r--r-- 1 ibarba users  31K Mar 24 17:41 OPTIONS-000020
```

This diff replaces the fopen call with a regular open() call restricting mode, and then using fdopen to associate an stdio stream with that file descriptor. Resulting in the following files being created:

```
-rw-r--r-- 1 ibarba users   58 Mar 24 18:16 000004.log
-rw-r--r-- 1 ibarba users   16 Mar 24 18:16 CURRENT
-rw-r--r-- 1 ibarba users   37 Mar 24 18:16 IDENTITY
-rw-r--r-- 1 ibarba users    0 Mar 24 18:16 LOCK
-rw-r--r-- 1 ibarba users 111K Mar 24 18:16 LOG
-rw-r--r-- 1 ibarba users  514 Mar 24 18:16 MANIFEST-000003
-rw-r--r-- 1 ibarba users  31K Mar 24 18:16 OPTIONS-000018
-rw-r--r-- 1 ibarba users  31K Mar 24 18:16 OPTIONS-000020
```

With the correct permissions

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8106

Reviewed By: akankshamahajan15

Differential Revision: D27415377

Pulled By: mrambacher

fbshipit-source-id: 97ac6c215700a7ea306f4a1fdf9fcf64a3cbb202

env/fs_posix.cc

@@ -553,10 +553,15 @@ class PosixFileSystem : public FileSystem {
   IOStatus NewLogger(const std::string& fname, const IOOptions& /*opts*/,
                    std::shared_ptr<Logger>* result,
                    IODebugContext* /*dbg*/) override {
-    FILE* f;
+    FILE* f = nullptr;
+    int fd;
     {
       IOSTATS_TIMER_GUARD(open_nanos);
-      f = fopen(fname.c_str(),
+      fd = open(fname.c_str(),
+                cloexec_flags(O_WRONLY | O_CREAT | O_TRUNC, nullptr),
+                GetDBFileMode(allow_non_owner_access_));
+      if (fd != -1) {
+        f = fdopen(fd,
                    "w"
 #ifdef __GLIBC_PREREQ
 #if __GLIBC_PREREQ(2, 7)
@@ -565,12 +570,18 @@ class PosixFileSystem : public FileSystem {
 #endif
         );
       }
+    }
+    if (fd == -1) {
+      result->reset();
+      return status_to_io_status(
+          IOError("when open a file for new logger", fname, errno));
+    }
     if (f == nullptr) {
+      close(fd);
       result->reset();
       return status_to_io_status(
-              IOError("when fopen a file for new logger", fname, errno));
+          IOError("when fdopen a file for new logger", fname, errno));
     } else {
-      int fd = fileno(f);
 #ifdef ROCKSDB_FALLOCATE_PRESENT
       fallocate(fd, FALLOC_FL_KEEP_SIZE, 0, 4 * 1024);
 #endif