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, 9 insertions, 2 deletions

diff --git a/pkg/sentry/fs/fsutil/inode_cached.go b/pkg/sentry/fs/fsutil/inode_cached.go
index 6ca51ab0d..b690cfe93 100644
--- a/pkg/sentry/fs/fsutil/inode_cached.go
+++ b/pkg/sentry/fs/fsutil/inode_cached.go
@@ -138,8 +138,15 @@ type CachedFileObject interface {
 	// Sync instructs the remote filesystem to sync the file to stable storage.
 	Sync(ctx context.Context) error
 
-	// FD returns a host file descriptor. Return value must be -1 or not -1
-	// for the lifetime of the CachedFileObject.
+	// FD returns a host file descriptor. If it is possible for
+	// CachingInodeOperations.AddMapping to have ever been called with writable
+	// = true, the FD must have been opened O_RDWR; otherwise, it may have been
+	// opened O_RDONLY or O_RDWR. (mmap unconditionally requires that mapped
+	// files are readable.) If no host file descriptor is available, FD returns
+	// a negative number.
+	//
+	// For any given CachedFileObject, if FD() ever succeeds (returns a
+	// non-negative number), it must always succeed.
 	//
 	// FD is called iff the file has been memory mapped. This implies that
 	// the file was opened (see fs.InodeOperations.GetFile).