The internal _page_offset() helper gets the value of PAGE_OFFSET, but
the fallback when KASLR is disabled has been out of date since Linux
v4.20 and never handled 5-level page tables. Additionally, it makes more
sense as part of the Linux kernel (formerly vmcoreinfo) object finder so
that it's cleanly accessible outside of drgn internals.
For operations where we don't have a type available, we currently fall
back to C. Instead, we should guess the language of the program and use
that as the default. The heurisitic implemented here gets the language
of the CU containing "main" (except for the Linux kernel, which is
always C). In the future, we should allow manually overriding the
automatically determined language.
UTS_RELEASE is currently only accessible once debug info is loaded with
prog.load_debug_info(main=True). This makes it difficult to get the
release, find the appropriate vmlinux, then load the found vmlinux. We
can add vmcoreinfo_object_find as part of set_core_dump(), which makes
it possible to do the following:
prog = drgn.Program()
prog.set_core_dump(core_dump_path)
release = prog['UTS_RELEASE'].string_()
vmlinux_path = find_vmlinux(release)
prog.load_debug_info([vmlinux_path])
The only downside is that this ends up using the default definition of
char rather than what we would get from the debug info, but that
shouldn't be a big problem.
