Hi,

Recently, with the Linux 5.10 release, I have run into the following
segfault on UML. I was a little disappointed in myself that this
slipped my regular set of tests, before being pushed to Debian. It is
right now part of Debian Testing too and I'd hate to have it removed
from the Bullseye release.

What is worse is that (to do some quick tests) I reverted to an older
UML (5.9) which I recollect to have working, and that too failed on the
setups.

In regard to setups, I tried and reproduced the issue on 3 different
machines, but all running Intel hardware. And all running 5.10 host
kernel


It would really help if others on this mailing list can check and
validate if they run into this problem. So far I have had 1 report of
being able to reproduce this bug other than me. I have also had 1
report of not being able to reproduce this bug.


Thanks,
Ritesh


```
rrs@priyasi:~$ linux ubd0=~/rrs-home/Libvirt-Images/uml.img
vec0:transport=tap,ifname=tap0,gro=1 mem=1024M rw
Core dump limits :
        soft - 0
        hard - NONE
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Checking advanced syscall emulation patch for ptrace...OK
Checking environment variables for a tempdir...none found
Checking if /dev/shm is on tmpfs...OK
Checking PROT_EXEC mmap in /dev/shm...OK
Adding 5906432 bytes to physical memory to account for exec-shield gap
kmsg_dump:
<5>Linux version 5.10.5 (buildd@x86-conova-01) (gcc (Debian 10.2.1-6)
10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1) #1 Mon Jan 11
20:40:53 UTC 2021
<6>Zone ranges:
<6>  Normal   [mem 0x0000000000000000-0x00000000a05a1fff]
<6>Movable zone start for each node
<6>Early memory node ranges
<6>  node   0: [mem 0x0000000000000000-0x00000000405a1fff]
<6>Initmem setup node 0 [mem 0x0000000000000000-0x00000000405a1fff]
<7>On node 0 totalpages: 263586
<7>  Normal zone: 4119 pages used for memmap
<7>  Normal zone: 0 pages reserved
<7>  Normal zone: 263586 pages, LIFO batch:63
<7>pcpu-alloc: s0 r0 d32768 u32768 alloc=1*32768
<7>pcpu-alloc: [0] 0 
<6>Built 1 zonelists, mobility grouping on.  Total pages: 259467
<5>Kernel command line: ubd0=/home/rrs/rrs-home/Libvirt-Images/uml.img
vec0:transport=tap,ifname=tap0,gro=1 mem=1024M rw root=98:0
<6>Dentry cache hash table entries: 131072 (order: 8, 1048576 bytes,
linear)
<6>Inode-cache hash table entries: 65536 (order: 7, 524288 bytes,
linear)
<6>mem auto-init: stack:off, heap alloc:off, heap free:off
<6>Memory: 1016464K/1054344K available (5830K kernel code, 1535K
rwdata, 1744K rodata, 191K init, 225K bss, 37880K reserved, 0K cma-
reserved)
<6>SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=1, Nodes=1
<6>NR_IRQS: 24
<6>clocksource: timer: mask: 0xffffffffffffffff max_cycles:
0x1cd42e205, max_idle_ns: 881590404426 ns
<6>Calibrating delay loop... 5731.94 BogoMIPS (lpj=28659712)
<6>pid_max: default: 32768 minimum: 301
<6>LSM: Security Framework initializing
<6>Yama: disabled by default; enable with sysctl kernel.yama.*
<6>SELinux:  Initializing.
<6>TOMOYO Linux initialized
<6>Mount-cache hash table entries: 2048 (order: 2, 16384 bytes, linear)
<6>Mountpoint-cache hash table entries: 2048 (order: 2, 16384 bytes,
linear)
<4>
<4>Modules linked in:
<6>Pid: 0, comm: swapper Not tainted 5.10.5
<6>RIP: 0033:[<00000000604d4201>]
<6>RSP: 00007ffca56a8890  EFLAGS: 00010206
<6>RAX: 0000000600000000 RBX: 0000000000000059 RCX: 00007ffca56a8000
<6>RDX: 0000000000000035 RSI: 0000000060b69a71 RDI: 0000000060d8ac3b
<6>RBP: 0000000000000000 R08: 0000000060b69a72 R09: 0000000060d8abe2
<6>R10: 0000000080000000 R11: 3d74696e695f676e R12: 0000000000000002
<6>R13: 0000000000000005 R14: 0000000000000000 R15: 0000000000000001
<0>Kernel panic - not syncing: Segfault with no mm
<4>CPU: 0 PID: 0 Comm: swapper Not tainted 5.10.5 #1
<4>Stack:
<4> 61335b50 8000000000000000 7fae69465908 7fae69465ae5
<4> 7fae698ae9e8 00000000 7ffca56a88d0 00000400
<4> 7fae6985bf20 7fae698ae9e8 00000000 00000000Call Trace:
<4> [<604d4fa3>] ? __printk_safe_enter+0x0/0x35
<4> [<604d154a>] ? arch_local_irq_save+0x0/0x22
<4> [<604d46f5>] ? vprintk_emit+0x9d/0x185
<4> [<604d49d3>] ? vprintk_deferred+0x1d/0x32
<4> [<60a26ee2>] ? printk_deferred+0x93/0x9b
<4> [<6088f79f>] ? bucket_table_alloc.isra.0+0x115/0x13d
<4> [<60a26e4f>] ? printk_deferred+0x0/0x9b
<4> [<6049cddb>] ? set_signals+0x0/0x38
<4> [<60589588>] ? arch_local_irq_save+0x0/0x22
<4> [<6055c928>] ? kvmalloc_node+0x56/0x96
<4> [<6058d3c0>] ? __kmalloc+0x1e2/0x1f9
<4> [<608e3d32>] ? ___ratelimit+0xd0/0xde
<4> [<6088f79f>] ? bucket_table_alloc.isra.0+0x115/0x13d
<4> [<60901485>] ? _warn_unseeded_randomness+0x60/0x8f
<4> [<6090295b>] ? get_random_u32+0x29/0x98
<4> [<6088f79f>] ? bucket_table_alloc.isra.0+0x115/0x13d
<4> [<6088f68a>] ? bucket_table_alloc.isra.0+0x0/0x13d
<4> [<6088ff7a>] ? rhashtable_init+0x175/0x1ca
<4> [<607ef317>] ? ipc_init_ids+0x4e/0x6f
<4> [<600153bd>] ? sem_init+0x17/0x45
<4> [<6049d0e5>] ? start_ptraced_child+0x0/0x180
<4> [<604a0ce0>] ? kernel_longjmp+0x0/0x20
<4> [<6049cc3d>] ? set_handler+0x123/0x15b
<4> [<6049c9ee>] ? hard_handler+0x0/0xcd
<4> [<604a0ce0>] ? kernel_longjmp+0x0/0x20
<4> [<6049c3a6>] ? openpty_cb+0x22/0x3b
<4> [<6049fb4b>] ? start_idle_thread+0x66/0x116
<4> [<60004613>] ? linux_main+0x2e7/0x2f9
<4> [<6049cc86>] ? change_sig+0x0/0x6a
<4> [<6000565e>] ? main+0x230/0x2dc
<4> [<60a256b0>] ? __libc_csu_init+0x0/0x60
<4> [<604827d0>] ? _start+0x0/0x30
<4> [<6000542e>] ? main+0x0/0x2dc
<4> [<604827d0>] ? _start+0x0/0x30
<4> [<604827d0>] ? _start+0x0/0x30
<4> [<604827fa>] ? _start+0x2a/0x30
<4> [<604827d0>] ? _start+0x0/0x30
Aborted (core dumped)
```
-- 
Ritesh Raj Sarraf | http://people.debian.org/~rrs
Debian - The Universal Operating System