Date:	Fri, 13 Nov 2015 01:35:47 -0500
From:	Jessica Yu <jeyu@...hat.com>
To:	Miroslav Benes <mbenes@...e.cz>
Cc:	Petr Mladek <pmladek@...e.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Kosina <jikos@...nel.org>,
	Vojtech Pavlik <vojtech@...e.com>, linux-api@...r.kernel.org,
	live-patching@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: module: save load_info for livepatch modules

+++ 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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