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

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Message-ID: <20160402185227.GB2538@pd.tnic>
Date:	Sat, 2 Apr 2016 20:52:27 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	X86 ML <x86@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KVM list <kvm@...r.kernel.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	xen-devel <Xen-devel@...ts.xen.org>,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v5 4/9] x86/traps: Enable all exception handler callbacks
 early

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@...nel.org>
> ---
>  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.

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