linux-kernel - Re: [PATCH v2 6/9] s390/module: Use s390_kernel

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Date:   Thu, 30 Apr 2020 11:48:42 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Joe Lawrence <joe.lawrence@...hat.com>
Cc:     Miroslav Benes <mbenes@...e.cz>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Jessica Yu <jeyu@...nel.org>, linux-s390@...r.kernel.org,
        heiko.carstens@...ibm.com, Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v2 6/9] s390/module: Use s390_kernel_write() for late
 relocations

On Thu, Apr 30, 2020 at 10:38:21AM -0400, Joe Lawrence wrote:
> On Thu, Apr 23, 2020 at 01:10:30PM -0500, Josh Poimboeuf wrote:
> > 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.
> > 
> 
> This is more of note for the future, but when/if we add livepatch
> support on arm64 we'll need to make the very same adjustment there as
> well.  See the following pattern:
> 
> arch/arm64/kernel/module.c:
> 
>   reloc_insn_movw()
>   reloc_insn_imm()
>   reloc_insn_adrp()
> 
>     *place = cpu_to_le32(insn);
> 
> maybe something like aarch64_insn_patch_text_nosync() could be used
> there, I dunno. (It looks like ftrace and jump_labels are using that
> interface.)
> 
> This is outside the scope of the patchset, but I thought I'd mention it
> as I was curious to see how other arches were currently handling their
> relocation updates.

True... I suspect your klp-convert selftests will catch that?

-- 
Josh