On Fri, 9 Mar 2018 12:05:36 -0800 Kees Cook <keescook@xxxxxxxxxxxx> 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? -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
