Re: [Xen-devel] [PATCH v5 4/9] x86/traps: Enable all exception handler callbacks early

[Borislav Petkov <bp@xxxxxxxxx>]

On Sat, Apr 02, 2016 at 07:01:35AM -0700, Andy Lutomirski wrote:
> Now that early_fixup_exception has pt_regs, we can just call
> fixup_exception from it.  This will make fancy exception handlers
> work early.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> ---
>  arch/x86/mm/extable.c | 19 ++-----------------
>  1 file changed, 2 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 8997022abebc..50dfe438bd91 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -95,10 +95,6 @@ extern unsigned int early_recursion_flag;
>  /* Restricted version used during very early boot */
>  void __init early_fixup_exception(struct pt_regs *regs, int trapnr)
>  {
> -     const struct exception_table_entry *e;
> -     unsigned long new_ip;
> -     ex_handler_t handler;
> -
>       /* Ignore early NMIs. */
>       if (trapnr == X86_TRAP_NMI)
>               return;
> @@ -109,19 +105,8 @@ void __init early_fixup_exception(struct pt_regs *regs, 
> int trapnr)
>       if (regs->cs != __KERNEL_CS)
>               goto fail;
>  
> -     e = search_exception_tables(regs->ip);
> -     if (!e)
> -             goto fail;
> -
> -     new_ip  = ex_fixup_addr(e);
> -     handler = ex_fixup_handler(e);
> -
> -     /* special handling not supported during early boot */
> -     if (handler != ex_handler_default)
> -             goto fail;

Hold on, what happened to the uaccess handling not being supported
during early boot?

So before Tony changed it, the original code had:


 /* Restricted version used during very early boot */
 int __init early_fixup_exception(unsigned long *ip)
 {

...
-               if (fixup->fixup - fixup->insn >= 0x7ffffff0 - 4) {
-                       /* uaccess handling not supported during early boot */
-                       return 0;
-               }

I'm guessing that wasn't supported early, probably because some stuff
wasn't initialized yet. Our normal, late fixup is by doing:

        current_thread_info()->uaccess_err = 1;

and I'm assuming we can't do that early. current_thread_info is probably
not setup yet...

> -
> -     regs->ip = new_ip;
> -     return;
> +     if (fixup_exception(regs, trapnr))
> +             return;

So why can we do it now, all of a sudden?

/me is scratching head.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel