Alexey Kardashevskiy <a...@ozlabs.ru> writes:
> On 3/22/22 13:12, Michael Ellerman wrote:
>> Alexey Kardashevskiy <a...@ozlabs.ru> writes:
>>> So far the RELACOUNT tag from the ELF header was containing the exact
>>> number of R_PPC_RELATIVE/R_PPC64_RELATIVE relocations. However the LLVM's
>>> recent change [1] make it equal-or-less than the actual number which
>>> makes it useless.
>>>
>>> This replaces RELACOUNT in zImage loader with a pair of RELASZ and RELAENT.
>>> The vmlinux relocation code is fixed in [2].
>>
>> That's committed so you can say:
>> in commit d79976918852 ("powerpc/64: Add UADDR64 relocation support")
>>
>>> To make it more future proof, this walks through the entire .rela.dyn
>>> section instead of assuming that the section is sorter by a relocation
>>> type. Unlike [1], this does not add unaligned UADDR/UADDR64 relocations
>> ^
>> that should be 2?
>
> Yes.
>
>>
>>> as in hardly possible to see those in arch-specific zImage.
>>
>> I don't quite parse that. Is it true we can never see them in zImage?
>> Maybe it's true that we don't see them in practice.
>
> I can force UADDR64 in zImage as I did for d79976918852 but zImage is
> lot smaller and more arch-specific than vmlinux and so far only
> PRINT_INDEX triggered UADDR64 in vmlinux and chances of the same thing
> happening in zImage are small.
OK. Just update the change log to say something like that. ie. they're
not impossible, but not seen in practice.
>>> @@ -75,34 +76,38 @@ p_base: mflr r10 /* r10 now points to
>>> runtime addr of p_base */
>>> bne 11f
>>> lwz r9,4(r12) /* get RELA pointer in r9 */
>>> b 12f
>>> -11: addis r8,r8,(-RELACOUNT)@ha
>>> - cmpwi r8,RELACOUNT@l
>>> +11: cmpwi r8,RELASZ
>>> + bne 111f
>>> + lwz r0,4(r12) /* get RELASZ value in r0 */
>>> + b 12f
>>> +111: cmpwi r8,RELAENT
>>
>> Can you use named local labels for new labels you introduce?
>>
>> This could be .Lcheck_for_relaent: perhaps.
>
> Then I'll need to rename them all/most and add more noise to the patch
> which reduces chances of it being reviewed. But sure, I can rename labels.
I said for new labels you introduce :) We can do a follow-up to rename
existing labels if we want to.
>>> bne 12f
>>> - lwz r0,4(r12) /* get RELACOUNT value in r0 */
>>> + lwz r14,4(r12) /* get RELAENT value in r14 */
>>> 12: addi r12,r12,8
>>> b 9b
>>>
>>> /* The relocation section contains a list of relocations.
>>> * We now do the R_PPC_RELATIVE ones, which point to words
>>> - * which need to be initialized with addend + offset.
>>> - * The R_PPC_RELATIVE ones come first and there are RELACOUNT
>>> - * of them. */
>>> + * which need to be initialized with addend + offset */
>>> 10: /* skip relocation if we don't have both */
>>> cmpwi r0,0
>>> beq 3f
>>> cmpwi r9,0
>>> beq 3f
>>> + cmpwi r14,0
>>> + beq 3f
>>>
>>> add r9,r9,r11 /* Relocate RELA pointer */
>>> + divd r0,r0,r14 /* RELASZ / RELAENT */
>>
>> This is in the 32-bit portion isn't it. AFAIK 32-bit CPUs don't
>> implement divd. I'm not sure why the toolchain allowed it. I would
>> expect it to trap if run on real 32-bit hardware.
>
>
> Uff, my bad, "divw", right?
AFAIK yes.
> I am guessing it works as zImage for 64bit BigEndian is still ELF32
> which runs in 64bit CPU and I did not test on real PPC32 as I'm not
> quite sure how and I hoped your farm will do this for me :)
Yeah I was hoping they would catch it too. I build pmac32 which should
build a 32-bit zImage, but I build it with a 64-bit compiler using -m32,
so maybe that's why it's accepted. Or maybe we're passing the wrong
options to the assembler.
I don't have any tests of actually booting a 32-bit zImage, my automated
tests all use the vmlinux.
cheers
