linux-kernel - Re: [RFC PATCH 3/4] x86/traps: Attempt to fixup exceptions in vDSO before signaling

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Date:   Thu, 6 Dec 2018 10:22:32 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [RFC PATCH 3/4] x86/traps: Attempt to fixup exceptions in vDSO
 before signaling

On 12/5/18 3:20 PM, Sean Christopherson wrote:
> @@ -223,6 +224,10 @@ do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str,
>  	tsk->thread.error_code = error_code;
>  	tsk->thread.trap_nr = trapnr;
>  
> +	if (user_mode(regs) &&
> +	    fixup_vdso_exception(regs, trapnr, error_code, 0))
> +		return 0;
> +
>  	return -1;
>  }
>  
> @@ -563,6 +568,9 @@ do_general_protection(struct pt_regs *regs, long error_code)
>  	tsk->thread.error_code = error_code;
>  	tsk->thread.trap_nr = X86_TRAP_GP;
>  
> +	if (fixup_vdso_exception(regs, X86_TRAP_GP, error_code, 0))
> +		return;
> +
>  	show_signal(tsk, SIGSEGV, "", desc, regs, error_code);
>  
>  	force_sig(SIGSEGV, tsk);
> @@ -854,6 +862,9 @@ static void math_error(struct pt_regs *regs, int error_code, int trapnr)
>  	if (!si_code)
>  		return;
>  
> +	if (fixup_vdso_exception(regs, trapnr, error_code, 0))
> +		return;
> +
>  	force_sig_fault(SIGFPE, si_code,
>  			(void __user *)uprobe_get_trap_addr(regs), task);
>  }
> -- 

Needs commenting, please.

But, also, this seems really ad-hoc.  Probably, that's a result of our
signal generation being really ad-hoc itself.  But, if this claims
"Attempt to fixup exceptions in vDSO before signaling", how do we assure
ourselves that we hit all the ad-hoc signal generation cases?  How do we
know we didn't miss one or ten?

I want to hear more of the story of how you picked these sites and also
decided that this is a comprehensive-enough set of sites to patch.