linux-kernel - Re: [PATCH v4] x86/sgx: Free backing memory after faulting the enclave page

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Date:   Thu, 24 Feb 2022 09:14:05 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Jarkko Sakkinen <jarkko@...nel.org>, linux-sgx@...r.kernel.org
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>, stable@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Jethro Beekman <jethro@...tanix.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] x86/sgx: Free backing memory after faulting the
 enclave page

On 2/22/22 04:03, Jarkko Sakkinen wrote:
> +	if (pcmd_page_empty) {
> +		pgoff_t pcmd_off = encl->size + PAGE_SIZE /* SECS */ +
> +				   page_index * sizeof(struct sgx_pcmd);
> +
> +		sgx_encl_truncate_backing_page(encl, PFN_DOWN(pcmd_off));
> +	}
> +
>  	return ret;
>  }
>  
> @@ -583,7 +613,7 @@ static struct page *sgx_encl_get_backing_page(struct sgx_encl *encl,
>  static int sgx_encl_get_backing(struct sgx_encl *encl, unsigned long page_index,
>  				struct sgx_backing *backing)
>  {
> -	pgoff_t pcmd_index = PFN_DOWN(encl->size) + 1 + (page_index >> 5);
> +	pgoff_t pcmd_off = encl->size + PAGE_SIZE /* SECS */ + page_index * sizeof(struct sgx_pcmd);

Jarkko, I really don't like how this looks.  The '/* SECS */' thing is
pretty ugly and the comment in the middle of an arithmetic operation is
just really hard to read.

Then, there's the fact that this gem is copied-and-pasted.  Oh, and it
looks a wee bit over 80 columns.

I went to the trouble of writing a nice, fully-fleshed-out helper
function for this with a comment included:

> https://lore.kernel.org/all/8afec431-4dfc-d8df-152b-76cca0e17ccb@intel.com/

Was there a problem using that?  The change from the last version is:

* Sanitized the offset calculations.

Given that there have been multiple different calculations over the four
versions so far, which version was right?  v3 or v4?