linux-kernel - Re: [PATCHV3 3/3] x86, ras: Add mcsafe_memcpy() function to recover from machine checks

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Message-ID: <CAPcyv4gXDHGgiqfve_fP1RLXBGfyWarjWgUU3QPMhnFn_BbshA@mail.gmail.com>
Date:	Wed, 23 Dec 2015 11:31:00 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Tony Luck <tony.luck@...il.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>, Elliott@...tnic,
	Robert <elliott@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-nvdimm <linux-nvdimm@...1.01.org>, X86-ML <x86@...nel.org>
Subject: Re: [PATCHV3 3/3] x86, ras: Add mcsafe_memcpy() function to recover
 from machine checks

On Wed, Dec 23, 2015 at 4:58 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Tue, Dec 22, 2015 at 11:38:07AM -0800, Tony Luck wrote:
>> I interpreted that comment as "stop playing with %rax in the fault
>> handler ... just change the IP to point the the .fixup location" ...
>> the target of the fixup being the "landing pad".
>>
>> Right now this function has only one set of fault fixups (for machine
>> checks). When I tackle copy_from_user() it will sprout a second
>> set for page faults, and then will look a bit more like Andy's dual
>> landing pad example.
>>
>> I still need an indicator to the caller which type of fault happened
>> since their actions will be different. So BIT(63) lives on ... but is
>> now set in the .fixup section rather than in the machine check
>> code.
>
> You mean this previous example of yours:
>
> int copy_from_user(void *to, void *from, unsigned long n)
> {
>         u64 ret = mcsafe_memcpy(to, from, n);
>
>         if (COPY_HAD_MCHECK(r)) {
>                 if (memory_failure(COPY_MCHECK_PADDR(ret) >> PAGE_SIZE, ...))
>                         force_sig(SIGBUS, current);
>                 return something;
>         } else
>                 return ret;
> }
>
> ?
>
> So what's wrong with mcsafe_memcpy() returning a proper retval which
> says what type of fault happened?
>
> I know, memcpy returns the ptr to @dest like a parrot but your version
> mcsafe_memcpy() will be different. It can even be called __mcsafe_memcpy
> and have a wrapper around it which fiddles out the proper retvals and
> returns @dest after all. It would still be cleaner this way IMHO.

We might leave this to the consumer.  It's already the case that
mcsafe_memcpy() is arch specific so I'm having to wrap its return
value into a generic value.  My current thinking is make
memcpy_from_pmem() return a pmem_cookie_t, and then have an arch
specific pmem_copy_error(pmem_cookit_t cookie) helper that interprets
the value.  This is similar to the situation we have with
dma_mapping_error().
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