Messages in this thread | | | Date | Fri, 9 Mar 2018 16:07:19 -0800 | From | Andrew Morton <> | Subject | Re: [PATCH v3] kernel.h: Skip single-eval logic on literals in min()/max() |
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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
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?
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