Re: m68k 54418 fails to execute user space

[Greg Ungerer <gerg@linux-m68k.org>]

Hi JM,

On 27/6/24 22:36, Jean-Michel Hautbois wrote:
> 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).
>>
>>
>>>  ./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.
>>>
>>> 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 added a dump in the binfmt_elf file:
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index a43897b03ce9..395f556f3a90 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -816,6 +816,63 @@ static int parse_elf_properties(struct file *f, const struct elf_phdr *phdr,
>          return ret == -ENOENT ? 0 : ret;
>   }
> 
> +static int dump_memory_map(struct task_struct *task)
> +{
> +    struct mm_struct *mm = task->mm;
> +    struct vm_area_struct *vma;
> +       MA_STATE(mas, &mm->mm_mt, 0, -1);
> +    struct file *file;
> +    struct path *path;
> +    char *buf;
> +    char *pathname;
> +
> +    // Acquire the read lock for mmap_lock
> +    down_read(&mm->mmap_lock);
> +       mas_lock(&mas);
> +    for (vma = mas_find(&mas, ULONG_MAX); vma; vma = mas_find(&mas, ULONG_MAX)) {
> +        if (vma->vm_file) {
> +            buf = (char *)__get_free_page(GFP_KERNEL);
> +            if (!buf) {
> +                continue; // Handle memory allocation failure
> +            }
> +
> +            file = vma->vm_file;
> +            path = &file->f_path;
> +            pathname = d_path(path, buf, PAGE_SIZE);
> +            if (IS_ERR(pathname)) {
> +                pathname = NULL;
> +            }
> +
> +            pr_info("%lx-%lx %c%c%c%c %08lx %02x:%02x %lu %s\n",
> +                vma->vm_start, vma->vm_end,
> +                vma->vm_flags & VM_READ ? 'r' : '-',
> +                vma->vm_flags & VM_WRITE ? 'w' : '-',
> +                vma->vm_flags & VM_EXEC ? 'x' : '-',
> +                vma->vm_flags & VM_MAYSHARE ? 's' : 'p',
> +                vma->vm_pgoff << PAGE_SHIFT,
> +                MAJOR(file->f_inode->i_rdev),
> +                MINOR(file->f_inode->i_rdev),
> +                file->f_inode->i_ino,
> +                pathname ? pathname : "");
> +
> +            free_page((unsigned long)buf);
> +        } else {
> +            pr_info("%lx-%lx %c%c%c%c %08lx 00:00 0\n",
> +                vma->vm_start, vma->vm_end,
> +                vma->vm_flags & VM_READ ? 'r' : '-',
> +                vma->vm_flags & VM_WRITE ? 'w' : '-',
> +                vma->vm_flags & VM_EXEC ? 'x' : '-',
> +                vma->vm_flags & VM_MAYSHARE ? 's' : 'p',
> +                vma->vm_pgoff << PAGE_SHIFT);
> +        }
> +    }
> +       mas_unlock(&mas);
> +    // Release the read lock for mmap_lock
> +    up_read(&mm->mmap_lock);
> +
> +    return 0;
> +}
> +
>   static int load_elf_binary(struct linux_binprm *bprm)
>   {
>          struct file *interpreter = NULL; /* to shut gcc up */
> @@ -1299,6 +1356,9 @@ static int load_elf_binary(struct linux_binprm *bprm)
> 
>          finalize_exec(bprm);
>          START_THREAD(elf_ex, regs, elf_entry, bprm->p);
> +       if (current->pid == 1) {  // Check if this is the init process
> +            dump_memory_map(current);
> +    }
>          retval = 0;
>   out:
>          return retval;
> 
> I think it is quick and dirty, but seems to do the trick.
> I then get in my console:
> [    4.265000] 60000000-6001e000 r-xp 00000000 00:00 178 /lib/ld.so.1
> [    4.266000] 6001e000-60022000 rw-p 0001c000 00:00 178 /lib/ld.so.1
> [    4.267000] 70000000-700ac000 r-xp 00000000 00:00 27 /bin/bash
> [    4.268000] 700ac000-700b4000 rw-p 000ac000 00:00 27 /bin/bash
> [    4.269000] 700b4000-700be000 rwxp 700b4000 00:00 0
> [    4.270000] bfe7a000-bfe9c000 rw-p bffde000 00:00 0
> 
> But nothing rings a bell at this level for me...
> Thanks !

Here is the same dump trace generated on my newly resurrected M5475EVB for comparison:

[snip]
Freeing unused kernel image (initmem) memory: 80K
This architecture does not have kernel memory protection.
Run /sbin/init as init process
Run /etc/init as init process
Run /bin/init as init process
process '/bin/init' started with executable stack
60000000-60008000 r-xp 00000000 00:00 550544 /lib/ld-uClibc-0.9.33.2.so
60008000-6000c000 rw-p 00006000 00:00 550544 /lib/ld-uClibc-0.9.33.2.so
80000000-80004000 r-xp 00000000 00:00 1882624 /bin/init
80004000-80008000 rw-p 00002000 00:00 1882624 /bin/init
bfc9a000-bfcbc000 rwxp bffde000 00:00 0
Welcome to
...

Execution otherwise continues as normal to a shell after this.

Regards
Greg