linux-kernel - Re: [PATCH] kconfig: fix comparison to constant symbols, 'm', 'n'

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Date: Sun, 19 May 2024 18:34:40 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: linux-kbuild@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kconfig: fix comparison to constant symbols, 'm', 'n'

On Sun, May 19, 2024 at 6:22 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> Currently, comparisons to 'm' or 'n' result in incorrect output.
>
> [Test Code]
>
>     config MODULES
>             def_bool y
>             modules
>
>     config A
>             def_tristate m
>
>     config B
>             def_bool A > n
>
> CONFIG_B is actually unset, while the expectation is CONFIG_B=y.
>
> The reason for the issue is because Kconfig compares the tristate values
> as strings.
>
> Currently, the .type fields of the constant symbols, 'y', 'm', and 'n'
> are unspecified, i.e., S_UNKNOWN.
>
> When expr_calc_value() evaluates 'A > n', it checks the types of 'A' and
> 'n' to determine how to compare them.
>
> The left-hand side, 'A', is a tristate symbol with a value of 'm', which
> corresponds to a numeric value of 1. (Internally, 'y', 'm', and 'n' are
> represented as 2, 1, and 0, respectively.)
>
> The right-hand side, 'n', has an unknown type, so it is treated as the
> string "n" during the comparison.
>
> expr_calc_value() compares two values numerically only when both can
> have numeric values. Otherwise, they are compared as strings.
>
>     symbol    numeric value    ASCII code
>     -------------------------------------
>       y           2             0x79
>       m           1             0x6d
>       n           0             0x6e
>
> 'm' is greater than 'n' if compared numerically (since 1 is greater
> than 0), but small than 'n' if compared as strings (since the ASCII
> code 0x6d is smaller than 0x6e).
>
> Specifying .type=S_TRISTATE for symbol_{yes,mod,no} fixes the above
> test code.
>
> However, this would cause a regression to the following test code.
>
> [Test Code 2]
>
>     config MODULES
>             def_bool n
>             modules
>
>     config A
>             def_tristate n
>
>     config B
>             def_bool A = m
>
> You would get CONFIG_B=y, while CONFIG_B should not be set.
>
> The reason is because sym_get_string_value() turns 'm' into 'n' when
> the module feature is disabled. Consequently, expr_calc_value() returns
> the result of 'A = n' instead of 'A = m'. This oddity has been hidden
> because the type of 'm' was previously S_UNKNOWN instead of S_TRISTATE.
>
> sym_get_string_value() should not tweak the string because the tristate
> value has already been correctly calculated. There is no reason to
> return the string "n" where its tristate value is mod.
>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---


Fixes: 31847b67bec0 ("kconfig: allow use of relations other than (in)equality")


-- 
Best Regards
Masahiro Yamada