The Design and Implementation of the Anykernel and Rump Kernels

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Note: the categories are not mutually exclusive. For example, socket() and open()
belong to several of them. In case open() is given a filename under a configurable
prefix (e.g. /rump), it will call the rump kernel to handle the request and new rump
kernel file descriptor will be returned to the application as a result.
The rest of this section describes advanced rumphijack features beyond simple system
call routing. Nevertheless, those features are commonly required for supporting
many real-world applications.
File Descriptor Games
A rump kernel file descriptor is differentiated from a host kernel file descriptor by the
numerical value of the file descriptor. Before a rump kernel descriptor is returned
to the application, it is offset by a per-process configurable constant. Generally
speaking, if the file descriptor parameter for a system call is greater than the offset,
it belongs to the rump kernel and the system call should be directed to the rump
kernel.
The default offset was selected to be half of select()’s FD_SETSIZE and is 128.
This value allows almost all applications to work, including historic ones that use
select() and modern ones that use a fairly large number of file descriptors. In
case the host returns a file descriptor which is equal to or greater than the process’s
hijack fd offset, rumphijack closes the fd and sets errno to ENFILE.
A problem arises from the dup2() interface which does not fit the above model: in
dup2 the new file descriptor number is decided by the caller. For example, a common
scheme used e.g. by certain web servers is accepting a connection on a socket, forking
a handler, and dup2’ing the accepted socket connection to stdin/stdout. The new
file descriptor must belong to the same kernel as the old descriptor, but in case of