Re: [PATCH] ARM: add a private asm/unaligned.h

[Ard Biesheuvel]

On 30 October 2017 at 13:48, Gregory CLEMENT
<gregory.clement@xxxxxxxxxxxxxxxxxx> wrote:
> Hi Russell,
>
>  On ven., oct. 27 2017, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote:
>
>> On Fri, Oct 27, 2017 at 05:19:55PM +0200, Gregory CLEMENT wrote:
>>> Hi Arnd,
>>>
>>>  On ven., oct. 20 2017, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>>
>>> > The asm-generic/unaligned.h header provides two different implementations
>>> > for accessing unaligned variables: the access_ok.h version used when
>>> > CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is set pretends that all pointers
>>> > are in fact aligned, while the le_struct.h version convinces gcc that the
>>> > alignment of a pointer is '1', to make it issue the correct load/store
>>> > instructions depending on the architecture flags.
>>> >
>>> > On ARMv5 and older, we always use the second version, to let the compiler
>>> > use byte accesses. On ARMv6 and newer, we currently use the access_ok.h
>>> > version, so the compiler can use any instruction including stm/ldm and
>>> > ldrd/strd that will cause an alignment trap. This trap can significantly
>>> > impact performance when we have to do a lot of fixups and, worse, has
>>> > led to crashes in the LZ4 decompressor code that does not have a trap
>>> > handler.
>>> >
>>> > This adds an ARM specific version of asm/unaligned.h that uses the
>>> > le_struct.h/be_struct.h implementation unconditionally. This should lead
>>> > to essentially the same code on ARMv6+ as before, with the exception of
>>> > using regular load/store instructions instead of the trapping instructions
>>> > multi-register variants.
>>> >
>>> > The crash in the LZ4 decompressor code was probably introduced by the
>>> > patch replacing the LZ4 implementation, commit 4e1a33b105dd ("lib: update
>>> > LZ4 compressor module"), so linux-4.11 and higher would be affected most.
>>> > However, we probably want to have this backported to all older stable
>>> > kernels as well, to help with the performance issues.
>>> >
>>> > There are two follow-ups that I think we should also work on, but not
>>> > backport to stable kernels, first to change the asm-generic version of
>>> > the header to remove the ARM special case, and second to review all
>>> > other uses of CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS to see if they
>>> > might be affected by the same problem on ARM.
>>> >
>>> > Cc: stable@xxxxxxxxxxxxxxx
>>> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
>>> > ---
>>> > Untested so far, please verify that this fixes all the known problems
>>> > with the alignment traps.
>>>
>>> I think Russell already find this conclusion but this patch didn't solve
>>> my boot issue with dtb append.
>>>
>>> I tested this patch onto a v4.14-rc6.
>>>
>>> Then at least with the patch from Ard: "efi/libstub: arm: omit sorting
>>> of the UEFI memory map", it didn't prevent booting.
>>
>> There's three things wrong, all of which I have patches to address:
>>
>> 1. The decompressor code reading the image data sometimes issues unaligned
>>    reads.  Some compilers get this wrong and cause an abort.  Arnds patch
>>    addresses this.
>>
>> 2. Additional sections can appear in the zImage binary which adds extra
>>    bytes on the end of the image.  Concatenating the zImage with the
>>    extra bytes onto a DTB is the same thing as doing this:
>>
>>       cat zImage extrabytes foo.dtb > image
>>
>>    and the decompressor tolerates no additional bytes between the
>>    _official_ end of the zImage and the DTB.  I've added a patch which
>>    detects this situation and fails the kernel build when it happens.
>
> So I tested the branch fixes in your git tree.
>
> After doing a "make multi_v7_defconfig; make zImage", I got the message
> "arm-linux-gnueabi-ld: error: zImage file size is incorrect" you added
> in the commit "ARM: verify size of zImage".
>
> It is the same with mvebu_v7_defconfig, so I wonder wich with
> configuration this patch was tested ?
>

Could you please share the output of 'readelf -S' for those vmlinux
decompressor images?