In the quest to remove all stack VLAs from the kernel[1], this introduces a new "simple max" macro, and changes the "sym" array size calculation to use it. The value is actually a fixed size, but since the max() macro uses some extensive tricks for safety, it ends up looking like a variable size to the compiler.
[1] https://lkml.org/lkml/2018/3/7/621 Signed-off-by: Kees Cook <keesc...@chromium.org> --- include/linux/kernel.h | 11 +++++++++++ lib/vsprintf.c | 4 ++-- 2 files changed, 13 insertions(+), 2 deletions(-) diff --git a/include/linux/kernel.h b/include/linux/kernel.h index 3fd291503576..1da554e9997f 100644 --- a/include/linux/kernel.h +++ b/include/linux/kernel.h @@ -820,6 +820,17 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { } x, y) /** + * SIMPLE_MAX - return maximum of two values without any type checking + * @x: first value + * @y: second value + * + * This should only be used in stack array sizes, since the type-checking + * from max() confuses the compiler into thinking a VLA is being used. + */ +#define SIMPLE_MAX(x, y) ((size_t)(x) > (size_t)(y) ? (size_t)(x) \ + : (size_t)(y)) + +/** * min3 - return minimum of three values * @x: first value * @y: second value diff --git a/lib/vsprintf.c b/lib/vsprintf.c index d7a708f82559..50cce36e1cdc 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -744,8 +744,8 @@ char *resource_string(char *buf, char *end, struct resource *res, #define FLAG_BUF_SIZE (2 * sizeof(res->flags)) #define DECODED_BUF_SIZE sizeof("[mem - 64bit pref window disabled]") #define RAW_BUF_SIZE sizeof("[mem - flags 0x]") - char sym[max(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE, - 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)]; + char sym[SIMPLE_MAX(2*RSRC_BUF_SIZE + DECODED_BUF_SIZE, + 2*RSRC_BUF_SIZE + FLAG_BUF_SIZE + RAW_BUF_SIZE)]; char *p = sym, *pend = sym + sizeof(sym); int decode = (fmt[0] == 'R') ? 1 : 0; -- 2.7.4
