Re: module: save load_info for livepatch modules

From: Jessica Yu
Date: Thu Nov 12 2015 - 17:18:00 EST

+++ Josh Poimboeuf [12/11/15 11:05 -0600]:
On Thu, Nov 12, 2015 at 04:03:45PM +0100, Petr Mladek wrote:
On Thu 2015-11-12 14:22:28, Miroslav Benes wrote:
> On Thu, 12 Nov 2015, Petr Mladek wrote:
> > > >Maybe I am missing something but isn't it necessary to call vfree() on
> > > >info somewhere in the end?
> > >
> > > So free_copy() will call vfree(info->hdr), except in livepatch modules
> > > we want to keep all the elf section information stored there, so we
> > > avoid calling free_copy(), As for the info struct itself, if you look
> > > at the init_module and finit_module syscall definitions in
> > > kernel/module.c, you will see that info is actually a local function
> > > variable, simply passed in to the call to load_module(), and will be
> > > automatically deallocated when the syscall returns. :-) No need to
> > > explicitly free info.
> >
> > We still have to free the copied or preserved structures when
> > the module is unloaded.
> ...freeing what we allocated. We need to free info->hdr somewhere if not
> here and also mod_arch_specific struct where the patch module is removed.
> This would unfortunately lead to changes in arch-specific code in
> module.c. For example in arch/s390/kernel/module.c there is vfree call on
> part of mod_arch_specific in module_finalize. We would call it only if the
> flag mentioned above is not set and at the same time we would need to call
> it when the patch module is being removed.

Sigh, I am afraid that the flag is not enough. IMHO, we need to split
the load finalizing functions into two pieces. One will be always
called when the module load is finalized. The other part will free
the load_info. It will be called either when the load is finalized or
when the module is unloaded, depending on if we want to preserve
the load_info.

Sigh, it is getting complicated. But let's see how it looks in reality.

At the other end of the spectrum, we could do the simplest thing
possible: _always_ save the data (even if CONFIG_LIVEPATCH is disabled).

(gdb) print sizeof(*info)
$3 = 96
(gdb) p sizeof(*info->hdr)
$4 = 64
s390 mod_arch_syminfo struct: 24 bytes by my reckoning.

So between info, info->hdr, and s390 mod_arch_syminfo, we're talking
about 184 bytes on s390 and 160 bytes on x86_64. That seems like
peanuts compared to the size of a typical module. The benefit is that
the code would be simpler because we don't have any special cases and
the structs would automatically get freed with the module struct when
the module gets unloaded.

I think I agree with Josh on this one (except, I would always save
load_info if it is a livepatch module, instead of for every module in the
!CONFIG_LIVEPATCH case, and we can just check modinfo to see if it is
a livepatch module).

If the tradeoff here is between simplicity and readibility of code vs.
saving some extra space (and by the looks of it, not a lot), I think I
would choose having clear code over saving some bytes of memory. Hard
coding checks and edge cases imo might cause confusion and trouble
down the road.

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