Refactor host.canMap.

Simplify the canMap check. We do not have plans to allow mmap for anything
beyond regular files, so we can just inline canMap() as a simple file mode
check.

Updates #1672.

PiperOrigin-RevId: 316929654

pkg/sentry/fsimpl/host/host.go

@@ -91,7 +91,9 @@ func NewFD(ctx context.Context, mnt *vfs.Mount, hostFD int, opts *NewFDOptions)
 		isTTY:      opts.IsTTY,
 		wouldBlock: wouldBlock(uint32(fileType)),
 		seekable:   seekable,
-		canMap:     canMap(uint32(fileType)),
+		// NOTE(b/38213152): Technically, some obscure char devices can be memory
+		// mapped, but we only allow regular files.
+		canMap: fileType == linux.S_IFREG,
 	}
 	i.pf.inode = i
 

pkg/sentry/fsimpl/host/util.go

@@ -49,16 +49,6 @@ func wouldBlock(fileType uint32) bool {
 	return fileType == syscall.S_IFIFO || fileType == syscall.S_IFCHR || fileType == syscall.S_IFSOCK
 }
 
-// canMap returns true if a file with fileType is allowed to be memory mapped.
-// This is ported over from VFS1, but it's probably not the best way for us
-// to check if a file can be memory mapped.
-func canMap(fileType uint32) bool {
-	// TODO(gvisor.dev/issue/1672): Also allow "special files" to be mapped (see fs/host:canMap()).
-	//
-	// TODO(b/38213152): Some obscure character devices can be mapped.
-	return fileType == syscall.S_IFREG
-}
-
 // isBlockError checks if an error is EAGAIN or EWOULDBLOCK.
 // If so, they can be transformed into syserror.ErrWouldBlock.
 func isBlockError(err error) bool {