> >> Ad relocations. I checked that everything in struct mod_arch_specific > >> stays after the module is load. Both core and init get SHF_ALLOC set > >> (mod->arch.core.plt->sh_flags in module_frob_arch_sections(). It is > >> important because apply_relocate_add() may use those sections > >> through module_emit_plt_entry() call. > > > > > > Yes, it looks like the needed .plt sections will remain in module > > memory. However, I think I found a slight issue... :/ > > > > In module_frob_arch_sections(), mod->arch.core.plt is set to an offset > > within info->sechdrs: > > > > if (!strcmp(secstrings + sechdrs[i].sh_name, ".plt")) > > mod->arch.core.plt = sechdrs + i; > > > > sechdrs is from info->sechdrs, which is freed at the end of > > load_module() via free_copy(). So although the relevant plt section(s) > > are in module memory, mod->arch.core.plt will point to invalid memory > > after info is freed. In other words, the section (.plt) *is* in memory > > but the section header (struct elf64_shdr) containing section metadata > > like sh_addr, sh_size etc., is not. > > > > Just for my understanding: this only matters for live patching, right? Yes. Live patching can do deferred relocations. When a module is loaded and livepatched, the relevant symbols in the patching module are resolved. We call apply_relocate_add(), which is arch-specific, and we must be sure that everything used by apply_relocate_add() is preserved in the patching module after it is loaded. > The original PLT support was implemented to support loading modules > outside of the -/+ 128 MB range of an arm64 relative branch/jump > instruction, and was later enhanced [for the same reason] to support > emitting a trampoline for the ftrace entrypoint. Regards, Miroslav
