alignment issues on armhf in chunked items

[dormando]

my rpi3 with alignment checking (Aarch64) fails all over the place, but never in t/chunked-items.t no matter what I do. for some reason that alignment is fine; or gcc/glibc is no longer doing something clever that it used to do.

https://wiki.ubuntu.com/ARM/RaspberryPi
^ note for self, ARMHF is the specific platform failing on their end. I can try this image on my rpi2 and see if the specific failure can be reproduced... else try the arm64 images. think I found a 4G sd card I can use for this.

Probably have to stick an #ifdef NEED_ALIGN to pad bytes before the chunked item header, given that key comes before ITEM_data() it can't be naturally aligned to the struct.

https://bugs.launchpad.net/memcached/+bug/1780838 (ubuntu bug for ref)

[basak]

With dormando's help we eventually pinned this down to:

(gdb) bt
#0  do_item_alloc (key=0xf5a22a70 "t", 'd' <repeats 199 times>..., nkey=1, 
    flags=0, exptime=0, nbytes=nbytes@entry=7905) at items.c:344
#1  0x00430a10 in item_alloc (key=<optimized out>, nkey=<optimized out>, 
    flags=<optimized out>, exptime=<optimized out>, nbytes=7905)
    at thread.c:549
#2  0x00415e36 in process_bin_update (c=<optimized out>) at memcached.c:2432
#3  complete_nread_binary (c=<optimized out>) at memcached.c:2640
#4  0x0041f1b6 in complete_nread (c=0xf5a20670) at memcached.c:2697
#5  drive_machine (c=0xf5a20670) at memcached.c:5511
#6  0x00421134 in event_handler (fd=<optimized out>, which=<optimized out>, 
    arg=0xf5a20670) at memcached.c:5755
#7  0xf7799558 in ?? () from /usr/lib/arm-linux-gnueabihf/libevent-2.1.so.6
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) 

This is with the released items.c from released 1.5.9.

[dormando]

can't get my rpi2 to boot that armhf image :( tried the contrib rpi3 image and it's not running with restricted alignment. NEED_ALIGN isn't detected and it passes all tests.

debating what to try next. getting arm64 to boot looks a little involved, need to reserve more time for it.

[dormando]

finally

armhf rpi3 experimental build for ubuntu 18.04 specifically (debian's never trip somehow).

echo 4 > /proc/cpu/alignment

[Switching to Thread 0x75c30460 (LWP 7762)]
do_item_alloc (key=0x75322a70 "t", 'd' <repeats 199 times>..., nkey=1, flags=0, 
    exptime=0, nbytes=nbytes@entry=7905) at items.c:344
344	        chunk->size = 0;

sadly, fixing this looks painful. subsequent chunks are dealt with properly, but the "first chunk" is dereferenced in a lot of places. trying to come up with a fetch macro that auto-pads.

[dormando]

fix released. closing this out, hopefully it won't bounce back :)