linux-kernel - Re: [4.4 STABLE BACKPORT] x86: paravirt: make native_save

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Date:   Sat, 21 Jul 2018 07:08:27 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: [4.4 STABLE BACKPORT] x86: paravirt: make native_save_fl extern
 inline

On Fri, Jul 20, 2018 at 03:36:41PM -0700, Nick Desaulniers wrote:
> native_save_fl() is marked static inline, but by using it as
> a function pointer in arch/x86/kernel/paravirt.c, it MUST be outlined.
> 
> paravirt's use of native_save_fl() also requires that no GPRs other than
> %rax are clobbered.
> 
> Compilers have different heuristics which they use to emit stack guard
> code, the emittance of which can break paravirt's callee saved assumption
> by clobbering %rcx.
> 
> Marking a function definition extern inline means that if this version
> cannot be inlined, then the out-of-line version will be preferred. By
> having the out-of-line version be implemented in assembly, it cannot be
> instrumented with a stack protector, which might violate custom calling
> conventions that code like paravirt rely on.
> 
> The semantics of extern inline has changed since gnu89. This means that
> folks using GCC versions >= 5.1 may see symbol redefinition errors at
> link time for subdirs that override KBUILD_CFLAGS (making the C standard
> used implicit) regardless of this patch. This has been cleaned up
> earlier in the patch set, but is left as a note in the commit message
> for future travelers.
> 
> Reports:
> https://lkml.org/lkml/2018/5/7/534
> https://github.com/ClangBuiltLinux/linux/issues/16
> 
> Discussion:
> https://bugs.llvm.org/show_bug.cgi?id=37512
> https://lkml.org/lkml/2018/5/24/1371
> 
> Thanks to the many folks that participated in the discussion.
> 
> Acked-by: Juergen Gross <jgross@...e.com>
> Debugged-by: Alistair Strachan <astrachan@...gle.com>
> Debugged-by: Matthias Kaehlcke <mka@...omium.org>
> Reported-by: Sedat Dilek <sedat.dilek@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Suggested-by: H. Peter Anvin <hpa@...or.com>
> Suggested-by: Tom Stellar <tstellar@...hat.com>
> Tested-by: Sedat Dilek <sedat.dilek@...il.com>
> ---
> Backport for 4.4. 4.4 is missing commit 784d5699eddc "x86: move exports to
> actual definitions" which doesn't apply cleanly, and not really worth
> backporting IMO. It's simpler to change this patch from upstream:
> + #include <asm-generic/export.h>
> rather than
> + #include <asm/export.h>

Yeah, that makes sense, thanks for the backport.  I'll queue it up after
the next round of stable kernels comes out in a few days.

greg k-h