Re: [PATCH v2 6/9] s390/module: Use s390_kernel_write() for late relocations

Josh Poimboeuf
2020-04-23T18:10:45Z

On Thu, Apr 23, 2020 at 09:12:28AM -0500, Josh Poimboeuf wrote:
> > > this is strange. While I would have expected an exception similar to
> > > this, it really should have happened on the "sturg" instruction which
> > > does the DAT-off store in s390_kernel_write(), and certainly not with
> > > an ID of 0004 (protection). However, in your case, it happens on a
> > > normal store instruction, with 0004 indicating a protection exception.
> > >
> > > This is more like what I would expect e.g. in the case where you do
> > > _not_ use the s390_kernel_write() function for RO module text patching,
> > > but rather normal memory access. So I am pretty sure that this is not
> > > related to the s390_kernel_write(), but some other issue, maybe some
> > > place left where you still use normal memory access?
> >
> > The call trace above also suggests that it is not a late relocation, no?
> > The path is from KLP module init function through klp_enable_patch. It should
> > mean that the to-be-patched object is loaded (it must be a module thanks
> > to a check klp_init_object_loaded(), vmlinux relocations were processed
> > earlier in apply_relocations()).
> >
> > However, the KLP module state here must be COMING, so s390_kernel_write()
> > should be used. What are we missing?
>
> I'm also scratching my head. It _should_ be using s390_kernel_write()
> based on the module state, but I don't see that on the stack trace.
>
> This trace (and Gerald's comment) seem to imply it's using
> __builtin_memcpy(), which might expected for UNFORMED state.
>
> Weird...

Mystery solved:

$ CROSS_COMPILE=s390x-linux-gnu- scripts/faddr2line vmlinux apply_rela+0x16a/0x520
apply_rela+0x16a/0x520:
apply_rela at arch/s390/kernel/module.c:336

which corresponds to the following code in apply_rela():

case R_390_PLTOFF64: /* 16 bit offset from GOT to PLT. */
if (info->plt_initialized == 0) {
unsigned int *ip;
ip = me->core_layout.base + me->arch.plt_offset +
info->plt_offset;
ip[0] = 0x0d10e310; /* basr 1,0 */
ip[1] = 0x100a0004; /* lg 1,10(1) */

Notice how it's writing directly to text... oops.

--
Josh