ARM: pxa/corgi: armv5te kernel 4.12 fails to decompress compiled with gcc7

[Aaro Koskinen]

Hi,

On Sun, Oct 15, 2017 at 02:01:19PM +0100, Russell King - ARM Linux wrote:
> On Sun, Oct 15, 2017 at 01:46:40PM +0300, Aaro Koskinen wrote:
> > This seems to be simpler to debug/reproduce using busybox. Compiling
> > just busybox with GCC 7.2 and march=armv5te/mtune=xscale already
> > produces failing xz decompression:
> > 
> > root at thecus-n2100:~$ echo foo > foo.txt
> > root at thecus-n2100:~$ xz foo.txt 
> > root at thecus-n2100:~$ ./busybox.gcc-7.2 xz -d foo.txt.xz 
> > xz: corrupted data

[...]

> What mode do you have for alignment handling?
> 
> cat /proc/cpu/alignment

Oh right, thanks, yes it seems GCC generates unaligned access:

root at thecus-n2100:~$ echo 0 > /proc/cpu/alignment 
root at thecus-n2100:~$ ./busybox.gcc-7.2 gzip -d foo.txt.gz 
gzip: crc error
root at thecus-n2100:~$ echo 2 > /proc/cpu/alignment 
root at thecus-n2100:~$ ./busybox.gcc-7.2 gzip -d foo.txt.gz 
root at thecus-n2100:~$ 

> Default for older CPUs is to "ignore" then, which basically means that
> unaligned loads are rotated - in other words, the 32-bit naturally
> aligned word is loaded and then rotated such that the addressed byte
> is in bits 0-7.  This is standard behaviour for older CPUs - it's
> how the CPU works when there's no trapping of unaligned accesses, and
> it should really be the way that GCC expects these CPUs to work.
> 
> Since there have been moves to drop older architecture support from GCC,
> I wouldn't be surprised if handling for pre-ARMv6 CPUs was bitrotting
> and GCC was producing code that fails.
> 
> I also know that various folk have put out calls for help with tooling
> for older CPUs (without which support gets dropped), but I don't think
> anyone stepped up.
> 
> So... you might be able to work around the userspace issues by setting
> the alignment fault handling to "fixup" but that really won't help the
> decompressor - we have no ways to take any faults in the decompressor,
> so we can't fix up an unaligned access.

Yes, the fixup is not really a solution. What is confusing is that I
was able to compile bootable kernels for OMAP1 boards, so there GCC is
compiling the decompressor correctly.

A.