Messages in this thread Patch in this message | | | Date | Fri, 13 Nov 2015 01:35:47 -0500 | From | Jessica Yu <> | Subject | Re: module: save load_info for livepatch modules |
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+++ 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.
Thanks, Jessica
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