Problem with init debugging under QEMU ARM

[Lukasz Majewski <l.majewski@majess.pl>]

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Dear QEMU community,

I'm now trying to fix bug in glibc which became apparent on qemu 6.0.0.

The error is caused by glibc commit:
bca0f5cbc9257c13322b99e55235c4f21ba0bd82
https://sourceware.org/git/?p=glibc.git;a=blobdiff;f=sysdeps/arm/dl-machine.h;h=eb13cb8b57496a0ec175c54a495f7e78db978fb7;hp=ff5e09e207f7986b1506b8895ae6c2aff032a380;hb=bca0f5cbc9257c13322b99e55235c4f21ba0bd82;hpb=34b4624b04fc8f038b2c329ca7560197320615b4

(reverting it causes the board to boot again)

Other components:
binutils_2.37
gcc_11.2
Yocto poky: SHA1: 1e2e9a84d6dd81d7f6dd69c0d119d0149d10ade1

Qemu start line (the problem is visible on 5.2.0 and 6.0.0):
qemu-system-arm -device
virtio-net-device,netdev=net0,mac=52:54:00:12:34:02 -netdev
tap,id=net0,ifnam e=tap0,script=no,downscript=no -object
rng-random,filename=/dev/urandom,id=rng0 -device
virtio-rng-pci,rng=rng0 -drive
id=disk0,file=y2038-image-devel-qemuarm.ext4,if=none,format
=raw -device virtio-blk-device,drive=disk0 -device qemu-xhci -device
usb-tablet -device usb-kbd  -machine virt,highmem=off -cpu cortex-a15
-smp 4 -m 256 -serial mon:stdio -serial null -nographic -device
VGA,edid=on -kernel
zImage--5.10.62+git0+bce2813b16_machine-r0-qemuarm-20210910095636.bin
-append 'root=/dev/vda rw  mem=256M
ip=192.168.7.2::192.168.7.1:255.255.255.0 console=ttyAMA0 console=hvc0
vmalloc=256


It has been tested with Cortex-A9 (Vexpress A9 2 core board) and
Cortex-A15. I've also tested the v5.1, v5.10 and v5.14 kernels. The
error is persistent.

I do add -s and -S when starting qemu-system-arm. I can use gdb to
debug the kernel without issues. Unfortunately, I'm not able to debug
/sbin/init after switching contex to user space.

Moreover, gdbserver cannot be used as the error (and kernel OOPs) is
caused when early code from ld-linux.so.3 (the _dl_start function) is
executed.


Any hints on how to debug it?

Inspecting assembler is one (awkward) option (some results presented
below). I can also inspect the VMA of the code just before starting the
/sbin/init process.

Unfortunately, when I try to break on user space code it is not very
helpful (as -S -s are supposed to be used with kernel).


Some info with the eligible code (_dl_start function):
------------------------------------------------------

I think that the problem may be with having the negative value
calculated.

The relevant snipet:

    116c:       bf00            nop
    116e:       bf00            nop
    1170:       bf00            nop
    1172:       f8df 3508       ldr.w   r3, [pc, #1288] ; 167c
    <_dl_start+0x520>

    1176:       f8df 1508       ldr.w   r1, [pc, #1288] ; 1680
    <_dl_start+0x524>

    117a:       447b            add     r3, pc
    117c:       4479            add     r1, pc
    117e:       f8c3 1598       str.w   r1, [r3, #1432] ; 0x598
    1182:       bf00            nop
    1184:       bf00            nop
    1186:       bf00            nop
    1188:       bf00            nop
    118a:       bf00            nop
    118c:       bf00            nop
    118e:       f8df 24f4       ldr.w   r2, [pc, #1268] ; 1684
    <_dl_start+0x528> 1192:       f8d3 5598       ldr.w   r5, [r3,
    #1432] ; 0x598 1196:       447a            add     r2, pc
    1198:       442a            add     r2, r5
    119a:       1a52            subs    r2, r2, r1
    119c:       f8c3 25a0       str.w   r2, [r3, #1440] ; 0x5a0
    11a0:       6813            ldr     r3, [r2, #0]


    167c:       0002be92        .word   0x0002be92
    1680:       ffffee80        .word   0xffffee80

The r1 gets the 0xffffee80 (negative offset) value. It is then added to
pc and used to calculate r2.

For working code (aforementioned patch reverted) - there are NO such
large values (like aforementioned 0xffffee80). The arithmetic is done
on 

   1690:       00000020        .word   0x00000020
   1694:       0002be7e        .word   0x0002be7e

which seems to work.

Maybe I'm missing some flag when I do start qemu-system-arm?

Thanks in advance for help and hints.

-- 
Best regards,

Łukasz Majewski

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