Re: m68k 54418 fails to execute user space

[Jean-Michel Hautbois <jeanmichel.hautbois@yoseli.org>]

Hi Michael,

On 26/06/2024 21:36, Michael Schmitz wrote:
> Jean-Michel,
> 
> On 27/06/24 01:28, Jean-Michel Hautbois wrote:
>> 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:
> 
> Please take a look at the comments at the start of 
> arch/m68k/mm/fault.c:do_page_fault(). The meaning of the bits in 
> error_code are explained there.
> 
> error_code != 0 is just one possible case out of the four that are 
> handled by do_page_fault(). It does not signify 'no error' - if there 
> hadn't been a page fault, do_page_fault() would not have been called.
> 
> You just forced a panic each time a write fault and/or a protection 
> fault happens. Write faults are absolutely expected to happen when 
> loading a library - ld.so needs to perform relocation after loading a 
> dynamic library, and that means writes to the GOT in the library's data 
> segment (PIC assumed).

I feel dumb -_- thanks for the explanation (I *knew* that page faults 
are expected, but as I couldn't not see when it works or not, I kept 
focus on this silly part).

> 
> 
>>  ./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).
> 0x41fb0000 seems to be init's page directory. The fault address is in 
> the range where I'd expect dynamic libraries to reside.

How can I check it is init's page directory ?

>>
>> 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.
> 
> The memory map ought to be accessible through sysrq - an alternative 
> would be to modify the ELF binfmt handler and dump the map once ld.so 
> has finished with relocations.

I can use sysrq but AFAIK I can show the memory info, not the memory map ?
Modifying the ELF binfmt handler might be doable...

JM

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