Re: Seg fault occurs when running statically compiled binary from kernel using call_usermodehelper

[Ashish Sangwan <ashishsangwan2@gmail.com>]

Any heads up on this?

or could someone just advice what can we do to debug this?

The ret_from_fork currently looks like following:
/*
 * This is how we return from a fork.
 */
ENTRY(ret_from_fork)
        bl      schedule_tail
        cmp     r5, #0
        movne   r0, r4
        adrne   lr, BSYM(1f)
        movne   pc, r5
1:      get_thread_info tsk
        b       ret_slow_syscall
ENDPROC(ret_from_fork)

Is this a real issue? Because we are getting this just for static binaries.

On Mon, Jul 8, 2013 at 10:46 AM, Ashish Sangwan
<ashishsangwan2@gmail.com> wrote:
> Forget to mention, we are using ARM
>
> On Mon, Jul 8, 2013 at 10:45 AM, Ashish Sangwan
> <ashishsangwan2@gmail.com> wrote:
>> On kernel version 3.8.13, when we try to execute a statically compiled
>> binary from kernel, it is giving segfault:
>>
>> insmod /mnt/module2.ko
>> [   35.560000] sample.static: unhandled page fault (11) at 0x00000000,
>> code 0x80000007
>> [   36.440000] Pid: 257, comm:        sample.static
>> [   36.444000] CPU: 3    Tainted: G        W  O  (3.8.13+ #35)
>> [   36.448000] PC is at 0x0
>> [   36.452000] LR is at 0xa420
>> [   36.456000] pc : [<00000000>]    lr : [<0000a420>]    psr: 60000030
>> [   36.456000] sp : beb30d70  ip : 00000004  fp : 00000000
>> [   36.464000] r10: 0000a004  r9 : 0000a0a4  r8 : 00000001
>> [   36.472000] r7 : 00000001  r6 : 0008be5c  r5 : 00000000  r4 : 0008ce80
>> [   36.476000] r3 : 00000001  r2 : 00000001  r1 : 00000000  r0 : 00000000
>> [   36.484000] Flags: nZCv  IRQs on  FIQs on  Mode USER_32  ISA Thumb
>> Segment user
>> [   36.492000] Control: 10c53c7d  Table: 7b1f806a  DAC: 00000015
>> [   36.496000] Backtrace:
>> [   36.500000] [<c00171f8>] (dump_backtrace+0x0/0x11c) from
>> [<c03e8a54>] (dump_stack+0x20/0x24)
>> [   36.508000]  r6:e514dd80 r5:00000000 r4:e5175fb0 r3:271ae511
>> [   36.516000] [<c03e8a34>] (dump_stack+0x0/0x24) from [<c0014174>]
>> (show_regs+0x58/0x5c)
>> [   36.524000] [<c001411c>] (show_regs+0x0/0x5c) from [<c001794c>]
>> (show_info+0xe0/0x14c)
>> [   36.532000]  r4:00000000 r3:00000000
>> [   36.532000] [<c001786c>] (show_info+0x0/0x14c) from [<c001dfb4>]
>> (__do_user_fault+0x78/0xc8)
>> [   36.540000]  r7:80000007 r6:0000000b r5:00000000 r4:e514dd80
>> [   36.548000] [<c001df3c>] (__do_user_fault+0x0/0xc8) from
>> [<c03f4574>] (do_page_fault+0x360/0x3d4)
>> [   36.556000] [<c03f4214>] (do_page_fault+0x0/0x3d4) from
>> [<c0008470>] (do_PrefetchAbort+0x44/0xa8)
>> [   36.564000] [<c000842c>] (do_PrefetchAbort+0x0/0xa8) from
>> [<c03f2bbc>] (ret_from_exception+0x0/0x10)
>> [   36.572000] Exception stack(0xe5175fb0 to 0xe5175ff8)
>> [   36.960000] do_init_module: 'module2'->init suspiciously returned
>> 11, it should follow 0/-E convention
>> [   36.960000] do_init_module: loading module anyway...
>> [   36.964000] Backtrace:
>> [   36.968000] [<c00171f8>] (dump_backtrace+0x0/0x11c) from
>> [<c03e8a54>] (dump_stack+0x20/0x24)
>> [   36.972000]  r6:bf000090 r5:bf00009c r4:e51d7f48 r3:00000000
>> [   36.980000] [<c03e8a34>] (dump_stack+0x0/0x24) from [<c008ae88>]
>> (load_module+0x1a90/0x1eb4)
>> [   36.984000] [<c00893f8>] (load_module+0x0/0x1eb4) from [<c008b3b0>]
>> (sys_init_module+0xe8/0xf8)
>> [   36.988000] [<c008b2c8>] (sys_init_module+0x0/0xf8) from
>> [<c0012dc0>] (ret_fast_syscall+0x0/0x48)
>> [   36.992000]  r6:bea51a14 r5:bea51b7c r4:00006ee4
>> [   36.996000] module2 mod ld
>>
>> module2.c =>
>> #include <linux/module.h>
>> #include <linux/init.h>
>> #include <linux/kmod.h>
>> MODULE_LICENSE( "GPL" );
>> static int test( void )
>> {
>>   char *argv[] = { "/mnt/sample.static", NULL };
>>   static char *envp[] = {
>>         "HOME=/",
>>         "TERM=linux",
>>         "PATH=/sbin:/bin:/usr/sbin:/usr/bin", NULL };
>>   return call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC );
>> }
>> static int __init mod_entry_func( void )
>> {
>>   return test();
>> }
>>
>> static void __exit mod_exit_func( void )
>> {
>>   return;
>> }
>> module_init( mod_entry_func );
>> module_exit( mod_exit_func );
>>
>>
>> sample.static is a simple "Hello_world" program.
>>
>> However, there is no problem in executing dynamically compiled binaries.
>>
>> When we revert commit 9fff2fa0db911b0b75ec1f9bec72460c0a676ef5 (arm:
>> switch to saner kernel_execve() semantics), there is no problem.
>>
>> OR
>>
>> for kernel 3.8.13 (which is just a plain revert when arch specific
>> kernel_execve was present), then also no problem =>
>>
>> diff --git a/kernel/kmod.c b/kernel/kmod.c
>> index 0023a87..9cf6e15 100644
>> --- a/kernel/kmod.c
>> +++ b/kernel/kmod.c
>> @@ -184,6 +184,10 @@ static int ____call_usermodehelper(void *data)
>>         struct subprocess_info *sub_info = data;
>>         struct cred *new;
>>         int retval;
>> +#ifdef ARM
>> +       struct pt_regs regs;
>> +       struct pt_regs *curr_ptr;
>> +#endif
>>         spin_lock_irq(&current->sighand->siglock);
>>         flush_signal_handlers(current, 1);
>> @@ -222,6 +226,36 @@ static int ____call_usermodehelper(void *data)
>>         retval = do_execve(sub_info->path,
>>                            (const char __user *const __user *)sub_info->argv,
>>                            (const char __user *const __user *)sub_info->envp);
>> +
>> +#ifdef ARM
>> +       if (retval)
>> +               goto fail;
>> +       curr_ptr = current_pt_regs();
>> +       memcpy(&regs, curr_ptr, sizeof(struct pt_regs));
>> +       /*
>> +        * Save argc to the register structure for userspace.
>> +        */
>> +       regs.ARM_r0 = retval;
>> +
>> +       /*
>> +        * We were successful.  We won't be returning to our caller, but
>> +        * instead to user space by manipulating the kernel stack.
>> +        */
>> +       asm(    "add    r0, %0, %1\n\t"
>> +               "mov    r1, %2\n\t"
>> +               "mov    r2, %3\n\t"
>> +               "bl     memmove\n\t"    /* copy regs to top of stack */
>> +               "mov    r8, #0\n\t"     /* not a syscall */
>> +               "mov    r9, %0\n\t"     /* thread structure */
>> +               "mov    sp, r0\n\t"     /* reposition stack pointer */
>> +               "b      ret_to_user"
>> +               :
>> +               : "r" (current_thread_info()),
>> +                 "Ir" (THREAD_START_SP - sizeof(regs)),
>> +                 "r" (&regs),
>> +                 "Ir" (sizeof(regs))
>> +               : "r0", "r1", "r2", "r3", "r8", "r9", "ip", "lr", "memory");
>> +#endif
>>         if (!retval)
>>                 return 0;
>> --
>>
>> Please suggest a proper solution.
>>
>> Regards,
>> Ashish