linux-kernel - Re: [PATCH v2 4/9] x86, memcpy_mcsafe: add write-protection-fault handling

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Date:   Thu, 3 May 2018 08:29:46 +0300
From:   Mika Penttilä <mika.penttila@...tfour.com>
To:     Dan Williams <dan.j.williams@...el.com>, linux-nvdimm@...ts.01.org
Cc:     x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Tony Luck <tony.luck@...el.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>, hch@....de,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 4/9] x86, memcpy_mcsafe: add write-protection-fault
 handling

On 05/03/2018 07:59 AM, Dan Williams wrote:
> In preparation for using memcpy_mcsafe() to handle user copies it needs
> to be to handle write-protection faults while writing user pages. Add
> MMU-fault handlers alongside the machine-check exception handlers.
> 
> Note that the machine check fault exception handling makes assumptions
> about source buffer alignment and poison alignment. In the write fault
> case, given the destination buffer is arbitrarily aligned, it needs a
> separate / additional fault handling approach. The mcsafe_handle_tail()
> helper is reused. The @limit argument is set to @len since there is no
> safety concern about retriggering an MMU fault, and this simplifies the
> assembly.
> 

> diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
> index 75d3776123cc..9787f5ee0cf9 100644
> --- a/arch/x86/lib/usercopy_64.c
> +++ b/arch/x86/lib/usercopy_64.c
> @@ -75,6 +75,23 @@ copy_user_handle_tail(char *to, char *from, unsigned len)
>  	return len;
>  }
>  
> +/*
> + * Similar to copy_user_handle_tail, probe for the write fault point,
> + * but reuse __memcpy_mcsafe in case a new read error is encountered.
> + * clac() is handled in _copy_to_iter_mcsafe().
> + */
> +__visible unsigned long
> +mcsafe_handle_tail(char *to, char *from, unsigned len)
> +{
> +	for (; len; --len, to++) {
> +		unsigned long rem = memcpy_mcsafe(to, from, 1);
> +


Hmm why not 
	for (; len; --len, from++, to++)



> +		if (rem)
> +			break;
> +	}
> +	return len;
> +}


--Mika