Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753215AbbKMGfy (ORCPT ); Fri, 13 Nov 2015 01:35:54 -0500 Received: from mx1.redhat.com ([209.132.183.28]:39202 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750779AbbKMGfw (ORCPT ); Fri, 13 Nov 2015 01:35:52 -0500 Date: Fri, 13 Nov 2015 01:35:47 -0500 From: Jessica Yu To: Miroslav Benes Cc: Petr Mladek , Rusty Russell , Josh Poimboeuf , Seth Jennings , Jiri Kosina , Vojtech Pavlik , linux-api@vger.kernel.org, live-patching@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org Subject: Re: module: save load_info for livepatch modules Message-ID: <20151113063547.GC13513@packer-debian-8-amd64.digitalocean.com> References: <1447130755-17383-1-git-send-email-jeyu@redhat.com> <1447130755-17383-3-git-send-email-jeyu@redhat.com> <20151111143152.GG4431@pathway.suse.cz> <20151112044407.GC30025@packer-debian-8-amd64.digitalocean.com> <20151112100548.GK4431@pathway.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: X-OS: Linux eisen.io 3.16.0-4-amd64 x86_64 User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4432 Lines: 92 +++ 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/