Re: list of mandatory hooks in file system module implementation

On 2008-08-24 at 17:46:13 [+0200], Salvatore Benedetto <[email protected]> 
wrote:
> 
> while working on udf, from time to time I get panics on page fault
> with an eip = 0x0.
> This is clearly due to the fact that I forgot to add a certain hook,
> probably to the fs_vnode_ops
> structure.
> 
> I looked around in vfs.cpp for all the FS_CALL call used
> unconditionally (that is, without calling
> HAS_FS_CALL first), and I thought I implemented all of them (is
> remove_vnode necessary btw?),
> but it seems I'm still missing something. Here is the most important
> part of a panic I just got
> 
> udf_lookup: filename = VIDEO_TS found at vnodeId = 262144!
> vm_soft_fault: va 0x0 not covered by area in address space
> vm_page_fault: vm_soft_fault returned error 'Bad address' on fault at
> 0x0, ip 0x0, write 0, user 0, thread 0xc0
> PANIC: vm_page_fault: unhandled page fault in kernel space at 0x0, ip 0x0
> ....
> 12 9217bd58 (+ 124) 00000000
> 13 9217bdd4 (+  48) 8008ebe7   <kernel>:dir_open_entry_ref__FlxPCcb + 0x008b
> 14 9217be04 (+ 320) 80093b73   <kernel>:_user_open_dir_entry_ref + 0x006b
> 15 9217bf44 (+ 100) 800bcee2   <kernel>:pre_syscall_debug_done +
> 0x0002 (nearest)
> ...
> 
> I looked at dir_open_entry_ref in vfs.cpp but honestly I'm not sure
> which of the function
> is the one to blame.

Since the innermost function (i.e. the non-existing one at address 0) didn't 
set up a stack frame, the function that called it is missing from the stack 
trace. The return address is on the stack, though. You just have to 
dereference the stack pointer (esp) and lookup the symbol:

  ls *0x8...

Since several of the called functions are static and could be inlined, this 
might not necessarily be the function that actually called the NULL hook. A 
look into the objdump at that address can help you find out what hook was 
called exactly, though.

> By the way, why is the last entry call (line 12) blank?

That's the NULL pointer that has been called.

> Any help is appreciated.
> 
> Anyway, as the subject says, I'm wondering if there is a list of
> mandatory hooks a module
> should implement.

A while ago I wrote a bit of introduction that also includes a list of hooks 
needed for a minimal read-only FS. It's not up to date (written before the 
last bigger FS interface modifications) and might not be complete. It 
certainly doesn't harm to have a look, though:

  docs/user/drivers/fs_modules.dox

CU, Ingo

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