Clean up gofer handle caching.

- Document fsutil.CachedFileObject.FD() requirements on access
permissions, and change gofer.inodeFileState.FD() to honor them.
Fixes #147.

- Combine gofer.inodeFileState.readonly and
gofer.inodeFileState.readthrough, and simplify handle caching logic.

- Inline gofer.cachePolicy.cacheHandles into
gofer.inodeFileState.setSharedHandles, because users with access to
gofer.inodeFileState don't necessarily have access to the fs.Inode
(predictably, this is a save/restore problem).

Before this CL:

$ docker run --runtime=runsc-d -v $(pwd)/gvisor/repro:/root/repro -it ubuntu bash
root@34d51017ed67:/# /root/repro/runsc-b147
mmap: 0x7f3c01e45000
Segmentation fault

After this CL:

$ docker run --runtime=runsc-d -v $(pwd)/gvisor/repro:/root/repro -it ubuntu bash
root@d3c3cb56bbf9:/# /root/repro/runsc-b147
mmap: 0x7f78987ec000
o
PiperOrigin-RevId: 240818413
Change-Id: I49e1d4a81a0cb9177832b0a9f31a10da722a896b

1 files changed, 23 insertions, 0 deletions

diff --git a/test/syscalls/linux/mmap.cc b/test/syscalls/linux/mmap.cc
index afe060d33..b500e79a4 100644
--- a/test/syscalls/linux/mmap.cc
+++ b/test/syscalls/linux/mmap.cc
@@ -1696,6 +1696,29 @@ TEST(MMapDeathTest, TruncateAfterCOWBreak) {
               ::testing::KilledBySignal(SIGBUS), "");
 }
 
+// Regression test for #147.
+TEST(MMapNoFixtureTest, MapReadOnlyAfterCreateWriteOnly) {
+  std::string filename = NewTempAbsPath();
+
+  // We have to create the file O_RDONLY to reproduce the bug because
+  // fsgofer.localFile.Create() silently upgrades O_WRONLY to O_RDWR, causing
+  // the cached "write-only" FD to be read/write and therefore usable by mmap().
+  auto const ro_fd = ASSERT_NO_ERRNO_AND_VALUE(
+      Open(filename, O_RDONLY | O_CREAT | O_EXCL, 0666));
+
+  // Get a write-only FD for the same file, which should be ignored by mmap()
+  // (but isn't in #147).
+  auto const wo_fd = ASSERT_NO_ERRNO_AND_VALUE(Open(filename, O_WRONLY));
+  ASSERT_THAT(ftruncate(wo_fd.get(), kPageSize), SyscallSucceeds());
+
+  auto const mapping = ASSERT_NO_ERRNO_AND_VALUE(
+      Mmap(nullptr, kPageSize, PROT_READ, MAP_SHARED, ro_fd.get(), 0));
+  std::vector<char> buf(kPageSize);
+  // The test passes if this survives.
+  std::copy(static_cast<char*>(mapping.ptr()),
+            static_cast<char*>(mapping.endptr()), buf.data());
+}
+
 // Conditional on MAP_32BIT.
 #ifdef __x86_64__