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

[Jessica Yu]

+++ Steven Rostedt [10/04/19 20:44 -0400]:
> On Wed, 10 Apr 2019 16:29:02 -0400
> Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> 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?

Unlike the kernel, the module loader does not rely on a linker script
to determine which sections get what protections. On module load, all
sections in a module are looped through and those sections without the
SHF_WRITE flag will be set to RO. For example, when there is a section
filled with structs declared as const or if the section was explicitly
given only the SHF_ALLOC attribute, those will be read-only. As long
as the sections were given the correct section attributes for
read-only, it'll have read-only protection. I see this is already the
case for __param and  __ksymtab*/__kcrctab* sections, but I agree that
a full audit would be useful to be consistent with builtin RO
protections.

Hope that helps,

Jessica