OSDev.org

Posted: **Fri Apr 03, 2009 1:48 pm**

Hi,

I've a really strange invalid page fault. It happens when I try to start one of the applications inside my OS. What makes it stranger is that it happens only 1 time from 15-20 tries, so it's happening randomly...

The information I got my page fault handler is the following:

Code: Select all

Invalid page fault at 0x4000b160 (no region for address)
Error code: 4                                                         
EAX=0 EBX=0 ECX=0 EDX=0                                               
ESI=0 EDI=0                                                           
EBP=0                                                                 
CS:EIP=1b:40000150                                                    
SS:ESP=23:c0007fae                                                    
Process: taskbar thread: main                                         
Memory context dump:                                                  
  region count: 3                                                     
  region #0                                                           
    id: 70 name: ro                                                   
    start: 40000000 size: 20480                                       
    flags: 1 alloc method: 1                                          
  region #1                                                           
    id: 71 name: rw                                                   
    start: 40005000 size: 8192                                        
    flags: 3 alloc method: 1
  region #2
    id: 72 name: stack
    start: c0000000 size: 32768
    flags: 13 alloc method: 2
Data at EIP:
   e8 3b 35 0 0 eb fe 90 90 90 90 90 90 90 90 90

The informations I posted above are from the stack frame that the page fault handler pushed to the stack.

As you can see the fault was caused because the thread tried to acces the data at address: 0x4000b160. The memory content at the EIP location is also dumped and it is the same as I see in the objdump output. The real instruction there is the following:

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40000150 <_start>:
40000150:       e8 3b 35 00 00          call   40003690 <__libc_start_main>

The strange thing is that this is a call instruction. The stack pointer is valid and that address is mapped as you can see from the memory context dump.

Now my question is the following: what the hell caused the page fault?

Thanks,
giszo

Posted: **Fri Apr 03, 2009 3:22 pm**

Hi,

From the error code, the PFE happened due to an attempted write to a read only page (this ties in with your context dump). Oddly, this was an attempted data write, not an instruction fetch. This would make sense if you use lazy loading for your processes and something is trying to load executable code to that region, perhaps?

Cheers,
Adam

Posted: **Sat Apr 04, 2009 12:11 am**

AJ wrote:Hi,

From the error code, the PFE happened due to an attempted write to a read only page (this ties in with your context dump). Oddly, this was an attempted data write, not an instruction fetch. This would make sense if you use lazy loading for your processes and something is trying to load executable code to that region, perhaps?

Cheers,
Adam

I'm using lazy binary loading, yes. But there are two things that makes me think it's not a problem in the lazy loader stuff:

The fault address is not page aligned. If it was caused by the lazy loader, then it would happen at the beginning of a page and not in the middle.
In the case of an error in the lazy loader the EIP value should be from the kernel somewhere, or not?

giszo

Posted: **Sun Apr 05, 2009 2:23 pm**

I think I found the problem

The problem was that I didn't save the value of the CR2 register during a context switch. So if the context switch happened between the PF handler start point and the point where I get the value of CR2, it caused a problem. When that thread that was interrupted got time to run again and read a bad CR2 value from the previous thread just crashed simply with that invalid page fault.

giszo

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Invalid page fault, caused by what?

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Re: Invalid page fault, caused by what?