Re: Oops in rpc_clnt_debugfs_register() from debugfs change

[Greg Kroah-Hartman]

On Tue, Feb 12, 2019 at 02:31:14PM +0000, David Howells wrote:
> I've bisected an oops that occurs in rpc_clnt_debugfs_register() trying to
> dereference a pointer with -EACCES in it.  This is the causing commit, though
> I suspect the bug is in sunrpc expecting to see NULL rather than an error.
> 
> ff9fb72bc07705c00795ca48631f7fffe24d2c6b is the first bad commit
> commit ff9fb72bc07705c00795ca48631f7fffe24d2c6b
> Author: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Date:   Wed Jan 23 11:28:14 2019 +0100
> 
>     debugfs: return error values, not NULL
>     
>     When an error happens, debugfs should return an error pointer value, not
>     NULL.  This will prevent the totally theoretical error where a debugfs
>     call fails due to lack of memory, returning NULL, and that dentry value
>     is then passed to another debugfs call, which would end up succeeding,
>     creating a file at the root of the debugfs tree, but would then be
>     impossible to remove (because you can not remove the directory NULL).
>     
>     So, to make everyone happy, always return errors, this makes the users
>     of debugfs much simpler (they do not have to ever check the return
>     value), and everyone can rest easy.
>     ...
> 
> The attached oops occurs during boot from the gssproxy process in
> rpc_clnt_debugfs_register().  The code at this point is:
> 
>    0xffffffff8195cbdd <+450>:   mov    0x50(%rax),%rcx   <--- oopsing
>    0xffffffff8195cbe1 <+454>:   mov    $0xffffffff821cc8ba,%rdx
>    0xffffffff8195cbe8 <+461>:   mov    $0x18,%esi
>    0xffffffff8195cbed <+466>:   lea    -0x30(%rbp),%rdi
>    0xffffffff8195cbf1 <+470>:   callq  0xffffffff819db773 <snprintf>
> 
> RAX is -EACCES.
> 
> Looking in the source:
> 
> 	len = snprintf(name, sizeof(name), "../../rpc_xprt/%s",
> 			xprt->debugfs->d_name.name);
> 
> I think xprt->debugfs is the value in RAX.
> 
> 	(gdb) p &((struct dentry *)0)->d_name.name
> 	$5 = (const unsigned char **) 0x50 <irq_stack_union+80>
> 
> which matches the offset on the oopsing MOV instruction.
> 
> This is with linus/master (aa0c38cf39de73bf7360a3da8f1707601261e518).

Ugh, yeah, I see the problem, sorry about that.

I wonder why the debugfs call is always failing, that's not good...

let me dig and see if I already have a patch for this...

greg k-h