linux-kernel - Re: [PATCH v2 1/2] kconfig: fix bad syntactic transformation in expr.c

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Date:	Tue, 07 Oct 2014 10:10:43 +0200
From:	Paul Bolle <pebolle@...cali.nl>
To:	Martin Walch <walch.martin@....de>
Cc:	yann.morin.1998@...e.fr, linux-kbuild@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] kconfig: fix bad syntactic transformation in
 expr.c

On Mon, 2014-09-22 at 19:13 +0200, Martin Walch wrote:
> expr_eliminate_dups2() in scripts/kconfig/expr.c applies two bad
> inference rules:
> 
> (FOO || BAR) && (!FOO && !BAR) -> n
> (FOO && BAR) || (!FOO || !BAR) -> y
> 
> They would be correct in propositional logic, but this is a three-valued
> logic, and here it is wrong in that it changes semantics. It becomes
> immediately visible when substituting FOO and BAR with m.

Well, not to me. When I apply the rules under "Menu dependencies" in
Documentation/kbuild/kconfig-language.txt, in embarrassingly small
steps, I get:
    (m || m) && (!m && !m) ->
    (1 || 1) && ((2 - 1) && (2 - 1)) ->
    (1 || 1) && (1 && 1) ->
    max(1, 1) && min(1, 1) ->
    1 && 1 ->
    min(1, 1) ->
    1 -> m

and
    (m && m) || (!m || !m) ->
    (1 && 1) || ((2 - 1) || (2 - 1)) ->
    (1 && 1) || (1 || 1) ->
    min(1, 1) || max(1, 1) ->
    1 || 1 ->
    max(1, 1) ->
    1 -> m

Did I do that right? Anyway, perhaps you'd like to add a line or two how
these two expressions actually evaluate when FOO and BAR are m.

> Fix it by removing expr_eliminate_dups2() and the functions that have no use
> anywhere else: expr_extract_eq_and(), expr_extract_eq_or(),
> and expr_extract_eq() from scripts/kconfig/expr.[ch]
> 
> Currently the bug is not triggered in mainline, so this patch does not modify
> the configuration space there.
> 
> As a side effect, this reduces code size in expr.c by roughly 10% and slightly
> improves startup time for all configuration frontends.
> 
> Signed-off-by: Martin Walch <walch.martin@....de>

Does Yann still care about Kconfig? Yann has been rather quiet for a
while now.


Paul Bolle

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