m68k 54418 fails to execute user space

[Jean-Michel Hautbois]

Hi Michael,

On 26/06/2024 03:56, Michael Schmitz wrote:
> Jean-Michel,
> 
> On 24/06/24 20:56, Jean-Michel Hautbois wrote:
>>
>> When I printk the do_page_fault first debug, I get for the first call 
>> to ls:
>> bash-5.2# ls
>> [   14.700000] do page fault:
>> [   14.700000] regs->sr=0x0, regs->pc=0x70069ee6, address=0x70069ee6, 
>> 0, (ptrval)
> 
> Page not present, read fault. Please disable obfuscation of kernel 
> pointer addresses by printk. Maybe also disable address space 
> randomization while debugging this.
> 
>> This call works almost fine (I still have the assert failed: 
>> folio->private != NULL issue).
>>
>> And when I call it a second time, I get:
>> bash-5.2# ls
>> [   19.820000] do page fault:
>> [   19.820000] regs->sr=0x0, regs->pc=0x6011d65a, address=0x700e2004, 
>> 2, (ptrval)
> 
> Page not present, write fault.
> 
> It would be helpful if you could get a dump of /proc/1/maps before the 
> execve() syscall in your helloworld init replacement. That might confirm 
> all these addresses are legit (assuming mappings survive across 
> execve(), that is), and what they correspond to.
> 
>>
>> The address corresponds to the defined zone ELF_ET_DYN_BASE as I set 
>> it to 0x70000000.
>>
>> regs->pc is not the same as the address. It might be unrelevant, but 
>> any help is appreciated to understand the process behind :-).
>>
>> I keep digging, and I am in the asm part which fears me a bit !
> 
> I don't see that you'd need to look at any asm code here.

I add a small test in do_page_fault, and in case of an error, it panics. 
The result follows:
  ./scripts/decode_stacktrace.sh vmlinux < /tmp/trace.log
[    3.857000] Run /bin/bash as init process
[    3.858000]   with arguments:
[    3.861000]     /bin/bash
[    3.862000]   with environment:
[    3.863000]     HOME=/
[    3.864000]     TERM=linux
[    4.242000] do page fault:
[    4.242000] regs->sr=0x2000, regs->pc=0x41366924, address=0x700b3364, 
2, 41fb0000
[    4.242000] Kernel panic - not syncing: page fault error
[    4.242000] CPU: 0 PID: 1 Comm: bash Not tainted 
6.10.0-rc5-g927da6cf01fe-dirty #25
[    4.242000] Stack from 4186dda8:
[    4.242000]         4186dda8 41423aa4 41423aa4 700b3300 00000001 
00000000 4136ee10 41423aa4
[    4.242000]         41366d7a 700b3364 700b3364 00000000 0000000d 
4186de60 41fb0000 41d51a60
[    4.242000]         41005696 41416a90 41416a4d 00002000 41366924 
700b3364 00000002 41fb0000
[    4.242000]         0000000a 700b3364 00000000 0000000d 00000012 
41d51a00 4186de60 41d51a60
[    4.242000]         41fb81c0 41d51a60 410052fe 4100529a 4186de60 
700b3364 00000002 00000000
[    4.242000]         700bc414 00000003 00008000 700ac000 41003660 
4186de60 00000000 00000000
[    4.242000] Call Trace: dump_stack (lib/dump_stack.c:124)
[    4.242000] panic (kernel/panic.c:266 kernel/panic.c:368)
[    4.242000] do_page_fault (arch/m68k/mm/fault.c:88 (discriminator 1))
[    4.242000] __clear_user (arch/m68k/lib/uaccess.c:108)
[    4.242000] buserr_c (arch/m68k/kernel/traps.c:725 
arch/m68k/kernel/traps.c:775)
[    4.242000] buserr_c (arch/m68k/kernel/traps.c:748 
arch/m68k/kernel/traps.c:775)
[    4.242000] buserr (arch/m68k/kernel/entry.S:116)
[    4.242000] ma_slots (lib/maple_tree.c:759)
[    4.242000] __clear_user (arch/m68k/lib/uaccess.c:108)
[    4.242000] elf_load (fs/binfmt_elf.c:125 (discriminator 1) 
fs/binfmt_elf.c:421 (discriminator 1))
[    4.242000] load_elf_binary (fs/binfmt_elf.c:1132)
[    4.242000] memset (arch/m68k/lib/memset.c:11)
[    4.242000] load_misc_binary (fs/binfmt_misc.c:97 
fs/binfmt_misc.c:146 fs/binfmt_misc.c:213)
[    4.242000] memset (arch/m68k/lib/memset.c:11)
[    4.242000] bprm_execve (fs/exec.c:1797 fs/exec.c:1839 fs/exec.c:1891 
fs/exec.c:1867)
[    4.242000] copy_strings_kernel (fs/exec.c:669)
[    4.242000] count_strings_kernel (fs/exec.c:473)
[    4.242000] kernel_execve (fs/exec.c:2058)
[    4.242000] __dynamic_pr_debug (lib/dynamic_debug.c:865)
[    4.242000] run_init_process (init/main.c:1389)
[    4.242000] _printk (kernel/printk/printk.c:2365)
[    4.242000] kernel_init (init/main.c:1508)
[    4.242000] kernel_init (init/main.c:1459)
[    4.242000] ret_from_kernel_thread (arch/m68k/kernel/entry.S:142)
[    4.242000]
[    4.242000] ---[ end Kernel panic - not syncing: page fault error ]---

Looks like a memory mapping failure, but why ?
My JTAG at this point dumps a list of 0s at 0x41fb0000 and my SDRAM 
starts at 0x40000000 and ends at 0x50000000 (256MB).

It looks like a TLB write miss which is obscure to me :-).

I tried to use the /proc but as expected it is not alive after mounting it.
Thanks,
JM


> Cheers,
> 
>      Michael
> 
>>
>> Thanks !
>> JM