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

[Josh Poimboeuf]

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