linux-kernel - Re: [PATCH] x86/mm: determine whether the fault address is canonical

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Date:   Fri, 4 Oct 2019 07:39:08 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Changbin Du <changbin.du@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/mm: determine whether the fault address is canonical

On 10/4/19 6:45 AM, Changbin Du wrote:
> +static inline bool is_canonical_addr(u64 addr)
> +{
> +#ifdef CONFIG_X86_64
> +	int shift = 64 - boot_cpu_data.x86_phys_bits;

I think you mean to check the virtual bits member, not "phys_bits".

BTW, I also prefer the IS_ENABLED(CONFIG_) checks to explicit #ifdefs.
Would one of those work in this case?

As for the error message:

>  {
> -	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault in user access. Non-canonical address?");
> +	WARN_ONCE(trapnr == X86_TRAP_GP, "General protection fault at %s address in user access.",
> +		  is_canonical_addr(fault_addr) ? "canonical" : "non-canonical");

I've always read that as "the GP might have been caused by a
non-canonical access".  The main nit I'd have with the change is that I
don't think all #GP's during user access functions which are given a
non-canonical address *necessarily* caused the #GP.

There are a billion ways you can get a #GP and I bet canonical
violations aren't the only way you can get one in a user copy function.