Re: [PATCH v3 3/3] module: Make __tracepoints_ptrs as read-only

Paul E. McKenney
2019-04-11T13:44:38Z

On Thu, Apr 11, 2019 at 04:21:06AM -0400, Joel Fernandes wrote:
> On Wed, Apr 10, 2019 at 08:44:01PM -0400, Steven Rostedt wrote:
> > On Wed, 10 Apr 2019 16:29:02 -0400
> > Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > > The srcu structure pointer array is modified at module load time because the
> > > array is fixed up by the module loader at load-time with the final locations
> > > of the tracepoints right? Basically relocation fixups. At compile time, I
> > > believe it is not know what the values in the ptr array are. I believe same
> > > is true for the tracepoint ptrs array.
> > >
> > > Also it needs to be in a separate __tracepoint_ptrs so that this code works:
> > >
> > >
> > > #ifdef CONFIG_TRACEPOINTS
> > > mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
> > > sizeof(*mod->tracepoints_ptrs),
> > > &mod->num_tracepoints);
> > > #endif
> > >
> > > Did I miss some point? Thanks,
> >
> > But there's a lot of others too. Hmm, does this mean that the RO data
> > sections that are in modules are not set to RO?
> >
> > There's a bunch of separate sections that are RO. Just look in
> > include/asm-generic/vmlinux.lds.h under the RO_DATA_SECTION() macro.
> >
> > A lot of the sections saved in module.c:find_module_sections() are in
> > that RO_DATA when compiled as a builtin. Are they not RO when loaded via
> > a module?
> >
> > If this is the case, there probably is going to be a lot more sections
> > added to your list.
>
> Hi Steven,
>
> You are right. It turns out that this patch for tracepoint is not needed
> since each tracepoint pointer is marked as a const which automatically makes
> the section non-writable after relocations..
>
> #ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
> typedef const int tracepoint_ptr_t;
> #else
> typedef struct tracepoint * const tracepoint_ptr_t;
> #endif
>
> So the fix for SRCU could just be the following. I verified with the change
> that the ELF section section is marked only with the ALLOCATE flag, not the
> WRITE flag which should automatically put the srcu pointer array in rodata.
> I'll test this out tomorrow.
>
> Patch 2/3 and 3/3 would not be nececessary if this works out. 1/3 may be a
> nice clean up but is not something urgent and we could do that in the future
> if needed.
>
> Any thoughts? Thanks a lot for the review!
>
> (I believe it is still worth auditing other sections in built-in RODATA and
> making sure they are non-writable for modules as well).

Nice and simple change! ;-)

If it works and Steve is OK with it, I will be happy to take the
corresponding formal patch.

Thanx, Paul

> ---8<-----------------------
>
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index 8af1824c46a8..9cfcc8a756ae 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -123,7 +123,7 @@ struct srcu_struct {
> #ifdef MODULE
> # define __DEFINE_SRCU(name, is_static) \
> is_static struct srcu_struct name; \
> - struct srcu_struct *__srcu_struct_##name \
> + struct srcu_struct * const __srcu_struct_##name \
> __section("___srcu_struct_ptrs") = &name
> #else
> # define __DEFINE_SRCU(name, is_static) \
>