[Crash-utility] invalid regs display in bt

[Dave Anderson]

Richard J Moore wrote:
> 
> crash-utility-bounces at redhat.com wrote on 27/09/2007 15:45:21:
> 
>  > Richard J Moore wrote:
>  >
>  > > On looking at the code in entry.S at page_fault and the other 
> exception
>  > > entry points I see no attempt to save regs to create a pt_regs struct.
>  > > The fact that do_page_fault takes pt_regs as the first arg is a 
> hack to
>  > > get at CS:EIP and SS:ESP at the time of exception.
>  >
>  > KPROBE_ENTRY(page_fault)
>  >    RING0_EC_FRAME
>  >    pushl $do_page_fault
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    ALIGN
>  > error_code:
>  >    /* the function address is in %fs's slot on the stack */
>  >    pushl %es
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    /*CFI_REL_OFFSET es, 0*/
>  >    pushl %ds
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    /*CFI_REL_OFFSET ds, 0*/
>  >    pushl %eax
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    CFI_REL_OFFSET eax, 0
>  >    pushl %ebp
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    CFI_REL_OFFSET ebp, 0
>  >    pushl %edi
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    CFI_REL_OFFSET edi, 0
>  >    pushl %esi
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    CFI_REL_OFFSET esi, 0
>  >    pushl %edx
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    CFI_REL_OFFSET edx, 0
>  >    pushl %ecx
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    CFI_REL_OFFSET ecx, 0
>  >    pushl %ebx
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    CFI_REL_OFFSET ebx, 0
>  >    cld
>  >    pushl %fs
>  >    CFI_ADJUST_CFA_OFFSET 4
>  >    /*CFI_REL_OFFSET fs, 0*/
>  >    movl $(__KERNEL_PERCPU), %ecx
>  >    movl %ecx, %fs
>  >    UNWIND_ESPFIX_STACK
>  >    popl %ecx
>  >    CFI_ADJUST_CFA_OFFSET -4
>  >    /*CFI_REGISTER es, ecx*/
>  >    movl PT_FS(%esp), %edi      # get the function address
>  >    movl PT_ORIG_EAX(%esp), %edx   # get the error code
>  >    movl $-1, PT_ORIG_EAX(%esp)   # no syscall to restart
>  >    mov  %ecx, PT_FS(%esp)
>  >    /*CFI_REL_OFFSET fs, ES*/
>  >    movl $(__USER_DS), %ecx
>  >    movl %ecx, %ds
>  >    movl %ecx, %es
>  >    movl %esp,%eax         # pt_regs pointer
>  >    call *%edi
>  >    jmp ret_from_exception
>  >    CFI_ENDPROC
>  > KPROBE_END(page_fault)
>  >
> 
> Dave, it looks like error_code: has been change relatively recently. My 
> source starts:
> error_code:
>         pushl %ds
>         pushl %eax
>         xorl  %eax,%eax
>         pushl %ebp
>        
> and so on. Clearly not a valid pt_regs struct.
> The source I'm working with is from FC5. On your version of the kernel 
> only the ERR, SS and ESP would be invalid.
> What tree is yours taken from?
> 

My example was from upstream 2.6.22-5.  RHEL5 (2.6.18-based)
is slightly different, as the "error_code:" chunk is located
in the "divide_error" entry point, but like 2.6.22-5, all of
the other exceptions jmp to it.

But even your code is creating the remainder of the pt_regs
after the essential registers laid down by the hardware
exception mechanism, by pushing the remaining registers,
"upwards" towards the beginning of the structure:

struct pt_regs {
         long ebx;
         long ecx;
         long edx;
         long esi;
         long edi;
         long ebp;
         long eax;
         int  xds;
         int  xes;
         long orig_eax;
         long eip;
         int  xcs;
         long eflags;
         long esp;
         int  xss;
};

Right?

Dave