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

[Gregory CLEMENT]

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 ?

Gregory

>
> 3. Ard's patch "efi/libstub: arm: omit sorting of the UEFI memory map"
>    gets rid of the additional sections that (a) change the alignment
>    of the compressed data, and (b) add additional unexpected bytes on
>    the end of zImage.
>
> So, merely applying Ard's patch will appear to fix the problem, but leave
> the actual underlying issues - so next time we have an additional section
> appear in the zImage, we'd go through all this pain again.
>
> These other patches are required to either make the decompressor tolerant
> or catch the problem cases while they're still in the developer's tree.
>
> -- 
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-- 
Gregory Clement, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com