From: Kees Cook
> Sent: 14 December 2024 01:36
...
> In order to silence this false positive but keep deterministic
> compile-time warnings intact, hide the length variable from GCC with
> OPTIMIZE_HIDE_VAR() before calling the builtin memcpy.
...
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 0d99bf11d260..1eef0119671c 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -616,6 +616,12 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t
> size,
> return false;
> }
>
> +/*
> + * To work around what seems to be an optimizer bug, the macro arguments
> + * need to have const copies or the values end up changed by the time they
> + * reach fortify_warn_once(). See commit 6f7630b1b5bc ("fortify: Capture
> + * __bos() results in const temp vars") for more details.
> + */
> #define __fortify_memcpy_chk(p, q, size, p_size, q_size, \
> p_size_field, q_size_field, op) ({ \
> const size_t __fortify_size = (size_t)(size); \
> @@ -623,6 +629,8 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t
> size,
> const size_t __q_size = (q_size); \
> const size_t __p_size_field = (p_size_field); \
> const size_t __q_size_field = (q_size_field); \
> + /* Keep a mutable version of the size for the final copy. */ \
> + size_t __copy_size = __fortify_size; \
> fortify_warn_once(fortify_memcpy_chk(__fortify_size, __p_size, \
> __q_size, __p_size_field, \
> __q_size_field, FORTIFY_FUNC_ ##op), \
> @@ -630,7 +638,11 @@ __FORTIFY_INLINE bool fortify_memcpy_chk(__kernel_size_t
> size,
> __fortify_size, \
> "field \"" #p "\" at " FILE_LINE, \
> __p_size_field); \
> - __underlying_##op(p, q, __fortify_size); \
> + /* Hide only the run-time size from value range tracking to */ \
> + /* silence compile-time false positive bounds warnings. */ \
> + if (!__builtin_constant_p(__fortify_size)) \
> + OPTIMIZER_HIDE_VAR(__copy_size); \
I think you can make that:
if (!__builtin_constant_p(__copy_size)) \
OPTIMISER_HIDE_VAR(__copy_size) \
which is probably more readable.
David
> + __underlying_##op(p, q, __copy_size); \
> })
>
> /*
> --
> 2.34.1
>
-
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