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    SubjectRe: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max()
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    On 03/09/2018 04:07 PM, Andrew Morton wrote:
    > On Fri, 9 Mar 2018 12:05:36 -0800 Kees Cook <keescook@chromium.org> wrote:
    >
    >> When max() is used in stack array size calculations from literal values
    >> (e.g. "char foo[max(sizeof(struct1), sizeof(struct2))]", the compiler
    >> thinks this is a dynamic calculation due to the single-eval logic, which
    >> is not needed in the literal case. This change removes several accidental
    >> stack VLAs from an x86 allmodconfig build:
    >>
    >> $ diff -u before.txt after.txt | grep ^-
    >> -drivers/input/touchscreen/cyttsp4_core.c:871:2: warning: ISO C90 forbids variable length array ‘ids’ [-Wvla]
    >> -fs/btrfs/tree-checker.c:344:4: warning: ISO C90 forbids variable length array ‘namebuf’ [-Wvla]
    >> -lib/vsprintf.c:747:2: warning: ISO C90 forbids variable length array ‘sym’ [-Wvla]
    >> -net/ipv4/proc.c:403:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla]
    >> -net/ipv6/proc.c:198:2: warning: ISO C90 forbids variable length array ‘buff’ [-Wvla]
    >> -net/ipv6/proc.c:218:2: warning: ISO C90 forbids variable length array ‘buff64’ [-Wvla]
    >>
    >> Based on an earlier patch from Josh Poimboeuf.
    >
    > v1, v2 and v3 of this patch all fail with gcc-4.4.4:
    >
    > ./include/linux/jiffies.h: In function 'jiffies_delta_to_clock_t':
    > ./include/linux/jiffies.h:444: error: first argument to '__builtin_choose_expr' not a constant


    I'm seeing that problem with
    > gcc --version
    gcc (SUSE Linux) 4.8.5

    in mmotm.

    > That's with
    >
    > #define __max(t1, t2, x, y) \
    > __builtin_choose_expr(__builtin_constant_p(x) && \
    > __builtin_constant_p(y) && \
    > __builtin_types_compatible_p(t1, t2), \
    > (t1)(x) > (t2)(y) ? (t1)(x) : (t2)(y), \
    > __single_eval_max(t1, t2, \
    > __UNIQUE_ID(max1_), \
    > __UNIQUE_ID(max2_), \
    > x, y))
    > /**
    > * max - return maximum of two values of the same or compatible types
    > * @x: first value
    > * @y: second value
    > */
    > #define max(x, y) __max(typeof(x), typeof(y), x, y)
    >
    >
    > A brief poke failed to reveal a workaround - gcc-4.4.4 doesn't appear
    > to know that __builtin_constant_p(x) is a constant. Or something.
    >
    > Sigh. Wasn't there some talk about modernizing our toolchain
    > requirements?


    --
    ~Randy

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