Re: [PATCH next 1/3] fortify: replace __compiletime_lessthan() with statically_true()

[Kees Cook <kees@kernel.org>]

On Mon, Mar 30, 2026 at 02:20:01PM +0100, david.laight.linux@gmail.com wrote:
> From: David Laight <david.laight.linux@gmail.com>
> 
> __compiletime_lessthan(a, b) is exactly the same as statically_true(a < b)
> Replace the former by the latter.
> 
> Signed-off-by: David Laight <david.laight.linux@gmail.com>

This one is a clear win: I didn't know about statically_true(). This
results in identical code.

I'll pick this one up.

-Kees

> ---
>  include/linux/fortify-string.h | 47 +++++++++++++++-------------------
>  1 file changed, 21 insertions(+), 26 deletions(-)
> 
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 171982e53c9a..214d237214d5 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -154,11 +154,6 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
>  #define POS0			__pass_object_size(0)
>  #endif
>  
> -#define __compiletime_lessthan(bounds, length)	(	\
> -	__builtin_constant_p((bounds) < (length)) &&	\
> -	(bounds) < (length)				\
> -)
> -
>  /**
>   * strncpy - Copy a string to memory with non-guaranteed NUL padding
>   *
> @@ -196,7 +191,7 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
>  {
>  	const size_t p_size = __member_size(p);
>  
> -	if (__compiletime_lessthan(p_size, size))
> +	if (statically_true(p_size < size))
>  		__write_overflow();
>  	if (p_size < size)
>  		fortify_panic(FORTIFY_FUNC_strncpy, FORTIFY_WRITE, p_size, size, p);
> @@ -287,14 +282,14 @@ __FORTIFY_INLINE ssize_t sized_strscpy(char * const POS p, const char * const PO
>  	 * If size can be known at compile time and is greater than
>  	 * p_size, generate a compile time write overflow error.
>  	 */
> -	if (__compiletime_lessthan(p_size, size))
> +	if (statically_true(p_size < size))
>  		__write_overflow();
>  
>  	/* Short-circuit for compile-time known-safe lengths. */
> -	if (__compiletime_lessthan(p_size, SIZE_MAX)) {
> +	if (statically_true(p_size < SIZE_MAX)) {
>  		len = __compiletime_strlen(q);
>  
> -		if (len < SIZE_MAX && __compiletime_lessthan(len, size)) {
> +		if (len < SIZE_MAX && statically_true(len < size)) {
>  			__underlying_memcpy(p, q, len + 1);
>  			return len;
>  		}
> @@ -469,12 +464,12 @@ __FORTIFY_INLINE bool fortify_memset_chk(__kernel_size_t size,
>  		 */
>  
>  		/* Error when size is larger than enclosing struct. */
> -		if (__compiletime_lessthan(p_size_field, p_size) &&
> -		    __compiletime_lessthan(p_size, size))
> +		if (statically_true(p_size_field < p_size) &&
> +		    statically_true(p_size < size))
>  			__write_overflow();
>  
>  		/* Warn when write size is larger than dest field. */
> -		if (__compiletime_lessthan(p_size_field, size))
> +		if (statically_true(p_size_field < size))
>  			__write_overflow_field(p_size_field, size);
>  	}
>  	/*
> @@ -557,15 +552,15 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
>  		 */
>  
>  		/* Error when size is larger than enclosing struct. */
> -		if (__compiletime_lessthan(p_size_field, p_size) &&
> -		    __compiletime_lessthan(p_size, size))
> +		if (statically_true(p_size_field < p_size) &&
> +		    statically_true(p_size < size))
>  			__write_overflow();
> -		if (__compiletime_lessthan(q_size_field, q_size) &&
> -		    __compiletime_lessthan(q_size, size))
> +		if (statically_true(q_size_field < q_size) &&
> +		    statically_true(q_size < size))
>  			__read_overflow2();
>  
>  		/* Warn when write size argument larger than dest field. */
> -		if (__compiletime_lessthan(p_size_field, size))
> +		if (statically_true(p_size_field < size))
>  			__write_overflow_field(p_size_field, size);
>  		/*
>  		 * Warn for source field over-read when building with W=1
> @@ -573,8 +568,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t size,
>  		 * the same time.
>  		 */
>  		if ((IS_ENABLED(KBUILD_EXTRA_WARN1) ||
> -		     __compiletime_lessthan(p_size_field, size)) &&
> -		    __compiletime_lessthan(q_size_field, size))
> +		     statically_true(p_size_field < size)) &&
> +		    statically_true(q_size_field < size))
>  			__read_overflow2_field(q_size_field, size);
>  	}
>  	/*
> @@ -699,7 +694,7 @@ __FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size)
>  {
>  	const size_t p_size = __struct_size(p);
>  
> -	if (__compiletime_lessthan(p_size, size))
> +	if (statically_true(p_size < size))
>  		__read_overflow();
>  	if (p_size < size)
>  		fortify_panic(FORTIFY_FUNC_memscan, FORTIFY_READ, p_size, size, NULL);
> @@ -713,9 +708,9 @@ int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t
>  	const size_t q_size = __struct_size(q);
>  
>  	if (__builtin_constant_p(size)) {
> -		if (__compiletime_lessthan(p_size, size))
> +		if (statically_true(p_size < size))
>  			__read_overflow();
> -		if (__compiletime_lessthan(q_size, size))
> +		if (statically_true(q_size < size))
>  			__read_overflow2();
>  	}
>  	if (p_size < size)
> @@ -730,7 +725,7 @@ void *memchr(const void * const POS0 p, int c, __kernel_size_t size)
>  {
>  	const size_t p_size = __struct_size(p);
>  
> -	if (__compiletime_lessthan(p_size, size))
> +	if (statically_true(p_size < size))
>  		__read_overflow();
>  	if (p_size < size)
>  		fortify_panic(FORTIFY_FUNC_memchr, FORTIFY_READ, p_size, size, NULL);
> @@ -742,7 +737,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
>  {
>  	const size_t p_size = __struct_size(p);
>  
> -	if (__compiletime_lessthan(p_size, size))
> +	if (statically_true(p_size < size))
>  		__read_overflow();
>  	if (p_size < size)
>  		fortify_panic(FORTIFY_FUNC_memchr_inv, FORTIFY_READ, p_size, size, NULL);
> @@ -755,7 +750,7 @@ __FORTIFY_INLINE void *kmemdup_noprof(const void * const POS0 p, size_t size, gf
>  {
>  	const size_t p_size = __struct_size(p);
>  
> -	if (__compiletime_lessthan(p_size, size))
> +	if (statically_true(p_size < size))
>  		__read_overflow();
>  	if (p_size < size)
>  		fortify_panic(FORTIFY_FUNC_kmemdup, FORTIFY_READ, p_size, size,
> @@ -793,7 +788,7 @@ char *strcpy(char * const POS p, const char * const POS q)
>  		return __underlying_strcpy(p, q);
>  	size = strlen(q) + 1;
>  	/* Compile-time check for const size overflow. */
> -	if (__compiletime_lessthan(p_size, size))
> +	if (statically_true(p_size < size))
>  		__write_overflow();
>  	/* Run-time check for dynamic size overflow. */
>  	if (p_size < size)
> -- 
> 2.39.5
> 

-- 
Kees Cook