The Design and Implementation of the Anykernel and Rump Kernels

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any functions they desire. We demonstrated the ability to call arbitrary kernel
interfaces with the example on how to access the BPF driver without going through
the file system (Figure 2.3). In that example we had to provide our own prototype
and execute the entry point manually, since we did not use predefined application
interfaces.
3.6.1 System Calls
On a regular NetBSD system, a user process calls the kernel through a stub in libc.
The libc stub’s task is to trap into the kernel. The kernel examines the trapframe
to see which system call was requested and proceeds to call the system call handler.
After the call returns from the kernel, the libc stub sets errno.
We are interested in preserving the standard libc application interface signature for
rump kernel clients. Preserving the signature will make using existing code in rump
kernel clients easier, since the calling convention for system calls will remain the
same. In this section we will examine how to generate handlers for rump kernels
with minimal manual labor. All of our discussion is written against how system
calls are implemented in NetBSD. We use lseek() as an example of the problem
and our solution.
The signature of the lseek() system call stub in libc is as follows:
off_t
lseek(int fildes, off_t offset, int whence)
Prototypes are provided in header files. The header file varies from call to call. For
example, the prototype of lseek() is made available to an application by including