The Design and Implementation of the Anykernel and Rump Kernels

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void *handle;
const struct modinfo *const *mi_start, *const *mi_end;
int error;
if ((handle = dlopen(fname, RTLD_LAZY|RTLD_GLOBAL)) == NULL) {
/* error branch */
}
mi_start = dlsym(handle, "__start_link_set_modules");
mi_end = dlsym(handle, "__stop_link_set_modules");
if (mi_start && mi_end) {
error = rump_pub_module_init(mi_start,
(size_t)(mi_end-mi_start));
/* ... */
} else {
/* error: not a module */
}
Figure 3.16: Loading kernel modules with dlopen(). Loading is a two-part
process. First, the module is loaded and linked using a call to the host’s dlopen()
routine. Second, the rump kernel is informed about the newly loaded module.
We stated that linking libraries with “cc -o ... -lrumpfoo” and loading them
with dlopen() before calling rump_init() are equivalent. One detail deserves
discussion: weak symbols. By default, symbols in the text segment are resolved
lazily, which means they are unresolved until they are first used. To put it another
way, the dynamic linker resolves a reference to afunction() only when, or rather
if, it is called at runtime. However, lazy resolution does not apply to variables,
including function pointers, which are resolved immediately when the program is
initially linked. Any function pointer with the initial value pointing to a weak stub
will retain the same value even if a strong version of the same symbol is later loaded
with dlopen().
Function pointers must be dynamically adjusted when the rump kernel is bootstrapped
instead of relying on weak symbols. In a rump kernel, the system call
vector was the main user of function pointers to weak symbols (the only other place