On Fri, 13 Nov 2015, Jessica Yu wrote: > +++ Miroslav Benes [12/11/15 15:19 +0100]: > > On Thu, 12 Nov 2015, Petr Mladek wrote: > > > > > On Wed 2015-11-11 23:44:08, Jessica Yu wrote: > > > > +++ Petr Mladek [11/11/15 15:31 +0100]: > > > > >On Mon 2015-11-09 23:45:52, Jessica Yu wrote: > > > > >>diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c > > > > >>index 6e53441..087a8c7 100644 > > > > >>--- a/kernel/livepatch/core.c > > > > >>+++ b/kernel/livepatch/core.c > > > > >>@@ -1001,6 +1001,23 @@ static struct notifier_block klp_module_nb = { > > > > >> .priority = INT_MIN+1, /* called late but before ftrace > > > notifier */ > > > > >> }; > > > > >> > > > > >>+/* > > > > >>+ * Save necessary information from info in order to be able to > > > > >>+ * patch modules that might be loaded later > > > > >>+ */ > > > > >>+void klp_prepare_patch_module(struct module *mod, struct load_info > > > *info) > > > > >>+{ > > > > >>+ Elf_Shdr *symsect; > > > > >>+ > > > > >>+ symsect = info->sechdrs + info->index.sym; > > > > >>+ /* update sh_addr to point to symtab */ > > > > >>+ symsect->sh_addr = (unsigned long)info->hdr + > > > symsect->sh_offset; > > > > > > > > > >Is livepatch the only user of this value? By other words, is this safe? > > > > > > > > I think it is safe to say yes. klp_prepare_patch_module() is only > > > > called at the very end of load_module(), right before > > > > do_init_module(). Normally, at that point, info->hdr will have already > > > > been freed by free_copy() along with the elf section information > > > > associated with it. But if we have a livepatch module, we don't free. > > > > So we should be the very last user, and there should be nobody > > > > utilizing the memory associated with the load_info struct anymore at > > > > that point. > > > > > > I see. It looks safe at this point. But still I wonder if it would be > > > possible to calculate this later in the livepatch code. It will allow > > > to potentially use the info structure also by other subsystem. > > > > > > BTW: Where is "sh_addr" value used, please? I see it used only > > > in the third patch as info->sechdrs[relindex].sh_addr. But it is > > > an array. I am not sure if it is the same variable. > > > > Jessica, why do we need to update sh_addr for symtab? It is not clear to > > me. > > Ah, I definitely need to make that comment a lot more informative. > Will make sure to add that in v2. > So, the sh_addr field tells us where a certain section is in memory. > Here, we need to update the symbol table section's sh_addr because if > we don't, it will eventually point to freed module init memory, which > is freed in do_init_module(). Let me explain what happens. > > At the beginning of load_module(), the sh_addr fields of each section > initially point to the vmalloc'd memory within info->hdr (which is > allocated in copy_module_from_{fd,user}() in module.c). The sh_addr's > are first assigned in rewrite_section_headers(), called from > setup_load_info(). These sh_addr's initially just point to an offset > within info->hdr depending on each section's sh_offset. > > However, in move_module(), where we layout and allocate the memory > where the module will finally reside, these sh_addr's will get > reassigned. For the symtab section's sh_addr, it gets reassigned to > module init memory. (In layout_symtab(), you'll see that the symtab > section gets marked with INIT_OFFSET_MASK, which indicates that it > will get an address in init memory when the sh_addr's get reassigned > in move_module()). Thus the symbol table that simplify_symbols() uses > is actually in init memory, and will be freed later in > do_init_module(). > > info->hdr is just a temporary holding place for module elf section > data in memory. Normally, we would get rid of info->hdr and free the > memory associated with it at the end of the module loading process > (via free_copy()). But in this patchset, we save all the original elf > section information because we need it (along with the original > symtab) in order to make the call to apply_relocate_add(). If you look > at apply_relocate_add() for x86, s390, etc you'll see that it expects > a symbol table at the symbol section's sh_addr field (basically, an > array of Elf_Sym's). This is why we fix up the sh_addr of the symtab > section to point back to the memory associated with info->hdr (and not > module init memory). I hope that makes sense. Great explanation. Thanks. Only info->hdr makes me worried. See my other mail. Miroslav -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html