Messages in this thread | | | Date | Thu, 5 Oct 2017 09:38:09 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/alternatives: Fix alt_max_short macro to really be a max() |
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On Wed, Oct 04, 2017 at 08:08:12PM +0200, Mathias Krause wrote: > The alt_max_short() macro in asm/alternative.h does not work as > intended, leading to nasty bugs. E.g. alt_max_short("1", "3") > evaluates to 3, but alt_max_short("3", "1") evaluates to 1 -- not > exactly the maximum of 1 and 3. > > In fact, I had to learn it the hard way by crashing my kernel in not > so funny ways by attempting to make use of the ALTENATIVE_2 macro > with alternatives where the first one was larger than the second > one. > > According to [1] and commit dbe4058a6a44 ("x86/alternatives: Fix > ALTERNATIVE_2 padding generation properly") the right handed side > should read "-(-(a < b))" not "-(-(a - b))". Fix that, to make the > macro work as intended. > > While at it, fix up the comment regarding the additional "-", too. > It's not about gas' usage of s32 but brain dead logic of having a > "true" value of -1 for the < operator ... *sigh* > > Btw., the one in asm/alternative-asm.h is correct. And, apparently, > all current users of ALTERNATIVE_2() pass same sized alternatives, > avoiding to hit the bug. > > [1] http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax > > Fixes: dbe4058a6a44 ("x86/alternatives: Fix ALTERNATIVE_2 padding generation properly") > Signed-off-by: Mathias Krause <minipli@googlemail.com> > Cc: Borislav Petkov <bp@suse.de> > --- > arch/x86/include/asm/alternative.h | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h > index c096624137ae..7c553f48f163 100644 > --- a/arch/x86/include/asm/alternative.h > +++ b/arch/x86/include/asm/alternative.h > @@ -106,9 +106,9 @@ static inline int alternatives_text_reserved(void *start, void *end) > * max without conditionals. Idea adapted from: ^^^^^^^^^^^^^^^^^^^^^^^^^
You did read this part, right?
AFAIR, gas can't stomach conditionals but I don't remember the details anymore. Could be that -1 representation of "true". Let me add Micha to CC.
Anyway, how can I reproduce your observation? Code snippet and compiler pls.
Thx.
-- Regards/Gruss, Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
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