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

    -- Joe

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