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

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Date:	Wed, 23 Dec 2015 13:58:53 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Tony Luck <tony.luck@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>, Elliott@...tnic,
	Robert <elliott@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	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 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.

> I'll move the function and #defines as you suggest - we don't need
> new files for these.  Also will fix the assembly code.
> [In my defense that load immediate 0x8000000000000000 and 'or'
> was what gcc -O2 generates from a simple bit of C code to set
> bit 63 ... perhaps it is faster, or perhaps gcc is on drugs. In this
> case code compactness wins over possible speed difference].

Well, upon a second thought, the reason why gcc would use that huge
immediate could be because by using BTS, it clobbers the carry flag
in rFLAGS. And I guess we don't want that. Although any Jcc or other
conditional instructions touching rFLAGS following will overwrite that
bit so it won't really matter.

I've asked a gcc person, we'll see what interesting explanation comes
back.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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