linux-kernel - Re: [PATCH] x86: Reduce duplicated code in the x86

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:	Mon, 13 May 2013 19:30:50 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Joe Damato <ice799@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, mingo@...hat.com, hpa@...or.com,
	akpm@...ux-foundation.org
Subject: Re: [PATCH] x86: Reduce duplicated code in the x86_64 context switch
 path.


* Guenter Roeck <linux@...ck-us.net> wrote:

> On Sun, May 12, 2013 at 10:14:51PM -0700, Joe Damato wrote:
> > Signed-off-by: Joe Damato <ice799@...il.com>
> > ---
> >  arch/x86/include/asm/switch_to.h |   30 ++++++++++++++++++++++++++++++
> >  arch/x86/kernel/process_64.c     |   29 ++---------------------------
> >  2 files changed, 32 insertions(+), 27 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> > index 4ec45b3..a322cc6 100644
> > --- a/arch/x86/include/asm/switch_to.h
> > +++ b/arch/x86/include/asm/switch_to.h
> > @@ -124,6 +124,36 @@ do {									\
> >  	       __switch_canary_iparam					  \
> >  	     : "memory", "cc" __EXTRA_CLOBBER)
> >  
> > +#define loadsegment_fs(fs, index)					  \
> > +	loadsegment(fs, index)
> > +
> > +#define loadsegment_gs(gs, index)					  \
> > +	load_gs_index(index)
> > +
> > +#define switch_segment(prev, next, index, seg, msr)			       \
> > +	do {								       \
> > +		/*							       \
> > +		 * Segment register != 0 always requires a reload.  Also       \
> > +		 * reload when it has changed.  When prev process used 64bit   \
> > +		 * base always reload to avoid an information leak.	       \
> > +		 */							       \
> > +		if (unlikely(index | next->index | prev->seg)) {	       \
> > +			loadsegment_##seg(seg, next->index);		       \
> > +			/*						       \
> > +			 * Check if the user used a selector != 0; if yes      \
> > +			 *  clear 64bit base, since overloaded base is always  \
> > +			 *  mapped to the Null selector			       \
> > +			 */						       \
> > +			if (index)					       \
> > +			  prev->seg = 0;				       \
> > +		}							       \
> > +									       \
> > +		/* when next process has a 64bit base use it */		       \
> > +		if (next->seg)						       \
> > +		  wrmsrl(msr, next->seg);				       \
> > +		prev->index = index;					       \
> > +	} while (0)
> > +
> >  #endif /* CONFIG_X86_32 */
> >  
> For my part I'll never understand how code written as macros is supposed to
> improve anything. I always find it confusing and risky, as it is very easy
> to introduce side effects. Also, while it may reduce the source code size,
> it often results in increased object size.
> 
> My take: If you can not write it as inline function(s), don't bother.

Indeed.

Thanks,

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/