UML kernel panic on initialization

[subashab at codeaurora.org]

Hi

I am seeing a panic on init when using Linux 5.13.1.
Following is the panic call stack-

Run /sbin/init as init process
init[1]: segfault at 0 ip 0000000040000af0 sp 0000007fbfb0be40 error 0 
in ld-2.13.so[40000000+20000]
Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
CPU: 0 PID: 1 Comm: init Not tainted 5.13.1 #10
Stack:
  60462043 60462043 62037bc0 6202a000
  60537f9b 60462043 62045b01 6004191f
  62037bd0 6046f12c 62037cf0 60461d67
Call Trace:
  [<60462043>] ? printk+0x0/0x94
  [<600251ee>] show_stack+0x101/0x14e
  [<60462043>] ? printk+0x0/0x94
  [<60462043>] ? printk+0x0/0x94
  [<60462043>] ? printk+0x0/0x94
  [<6004191f>] ? exit_mm+0x0/0x2b0
  [<6046f12c>] dump_stack+0x34/0x36
  [<60461d67>] panic+0x18c/0x3d3
  [<60037322>] ? set_signals+0x28/0x40
  [<60461bdb>] ? panic+0x0/0x3d3
  [<60098bba>] ? acct_collect+0x0/0x1ff
  [<60037322>] ? set_signals+0x28/0x40
  [<6047651b>] ? _raw_spin_unlock_irq+0x1c/0x1e
  [<600372fa>] ? set_signals+0x0/0x40
  [<60037322>] ? set_signals+0x28/0x40
  [<6004aba1>] ? percpu_up_read.constprop.46+0x74/0x79
  [<6004ab2d>] ? percpu_up_read.constprop.46+0x0/0x79
  [<6004d59e>] ? exit_signals+0xa2/0x180
  [<6004191f>] ? exit_mm+0x0/0x2b0
  [<6004259c>] do_exit+0x27c/0x99a
  [<60043c8f>] do_group_exit+0x94/0x127
  [<6004e8e0>] get_signal+0x8cf/0x8fe
  [<60476406>] ? _raw_spin_lock_irq+0x0/0x2a
  [<6004dc20>] ? force_sig_info_to_task+0xd7/0xe9
  [<60024f9c>] do_signal+0x21/0x109
  [<6004e00f>] ? force_sig_fault+0x21/0x23
  [<600241ba>] interrupt_end+0x5c/0xc7
  [<6003a18c>] userspace+0x546/0x554
  [<6002415e>] ? interrupt_end+0x0/0xc7
  [<6003c6d5>] ? get_fp_registers+0x0/0x12
  [<60462043>] ? printk+0x0/0x94
  [<600223b8>] ? run_init_process+0xc2/0xca
  [<600223c0>] ? try_to_run_init_process+0x0/0x44
  [<600223d6>] ? try_to_run_init_process+0x16/0x44
  [<600223c0>] ? try_to_run_init_process+0x0/0x44
  [<60023ec6>] new_thread_handler+0xb0/0xb2
Aborted (core dumped)

After doing a git bisect, the first bad commit is listed as-

commit 84b2789d61156db0224724806b20110c0d34b07c
Author: Johannes Berg <johannes.berg at intel.com>
Date:   Wed Jan 13 22:09:42 2021 +0100

     um: separate child and parent errors in clone stub

     If the two are mixed up, then it looks as though the parent
     returned an error if the child failed (before) the mmap(),
     and then the resulting process never gets killed. Fix this
     by splitting the child and parent errors, reporting and
     using them appropriately.

     Signed-off-by: Johannes Berg <johannes.berg at intel.com>
     Signed-off-by: Richard Weinberger <richard at nod.at>

I have reverted the following patches (to allow for a clean revert)
and I don't see the panic anymore -

bfc58e2b98e99737409cd9f4d86a79677c5b887c
dc01a3b9db43abf95b801c9694980777a329e303
9f0b4807a44ff81cf59421c8a86641efec586610
84b2789d61156db0224724806b20110c0d34b07c

Let me know if more information is needed to debug this.

The startup arguments I have been using are the following and is working 
upto Linux 5.11 -

./linux ubda=<full_path_to_rootfs> mem=512M 
eth0=daemon,00:18:8b:75:1d:e1

UML is compiled using the following arguments -

make -j32 linux ARCH=um SUBARCH=x86_64 CROSS_COMPILE=