Re: [PATCH] x86/alternatives: Fix alt_max_short macro to really be a max()

[Borislav Petkov]

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@xxxxxxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> ---
>  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.

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