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| author | Kees Cook <keescook@chromium.org> | 2024-02-29 20:44:37 -0800 |
|---|---|---|
| committer | Kees Cook <keescook@chromium.org> | 2024-03-01 16:03:32 -0800 |
| commit | c3b9a398fb0dae67f91e7ae4bb492e04ac2c80c0 (patch) | |
| tree | 09538b011ddcd10f646764778314c94b4f5f0463 /include/linux/compiler.h | |
| parent | bd1ebf2467f9c5d157bec7b025e83f8ffdae1318 (diff) | |
| download | lwn-c3b9a398fb0dae67f91e7ae4bb492e04ac2c80c0.tar.gz lwn-c3b9a398fb0dae67f91e7ae4bb492e04ac2c80c0.zip | |
compiler.h: Explain how __is_constexpr() works
The __is_constexpr() macro is dark magic. Shed some light on it with
a comment to explain how and why it works.
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://lore.kernel.org/r/20240301044428.work.411-kees@kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Diffstat (limited to 'include/linux/compiler.h')
| -rw-r--r-- | include/linux/compiler.h | 39 |
1 files changed, 39 insertions, 0 deletions
diff --git a/include/linux/compiler.h b/include/linux/compiler.h index bb1339c7057b..b688ad992127 100644 --- a/include/linux/compiler.h +++ b/include/linux/compiler.h @@ -231,6 +231,45 @@ static inline void *offset_to_ptr(const int *off) * This returns a constant expression while determining if an argument is * a constant expression, most importantly without evaluating the argument. * Glory to Martin Uecker <Martin.Uecker@med.uni-goettingen.de> + * + * Details: + * - sizeof() return an integer constant expression, and does not evaluate + * the value of its operand; it only examines the type of its operand. + * - The results of comparing two integer constant expressions is also + * an integer constant expression. + * - The first literal "8" isn't important. It could be any literal value. + * - The second literal "8" is to avoid warnings about unaligned pointers; + * this could otherwise just be "1". + * - (long)(x) is used to avoid warnings about 64-bit types on 32-bit + * architectures. + * - The C Standard defines "null pointer constant", "(void *)0", as + * distinct from other void pointers. + * - If (x) is an integer constant expression, then the "* 0l" resolves + * it into an integer constant expression of value 0. Since it is cast to + * "void *", this makes the second operand a null pointer constant. + * - If (x) is not an integer constant expression, then the second operand + * resolves to a void pointer (but not a null pointer constant: the value + * is not an integer constant 0). + * - The conditional operator's third operand, "(int *)8", is an object + * pointer (to type "int"). + * - The behavior (including the return type) of the conditional operator + * ("operand1 ? operand2 : operand3") depends on the kind of expressions + * given for the second and third operands. This is the central mechanism + * of the macro: + * - When one operand is a null pointer constant (i.e. when x is an integer + * constant expression) and the other is an object pointer (i.e. our + * third operand), the conditional operator returns the type of the + * object pointer operand (i.e. "int *). Here, within the sizeof(), we + * would then get: + * sizeof(*((int *)(...)) == sizeof(int) == 4 + * - When one operand is a void pointer (i.e. when x is not an integer + * constant expression) and the other is an object pointer (i.e. our + * third operand), the conditional operator returns a "void *" type. + * Here, within the sizeof(), we would then get: + * sizeof(*((void *)(...)) == sizeof(void) == 1 + * - The equality comparison to "sizeof(int)" therefore depends on (x): + * sizeof(int) == sizeof(int) (x) was a constant expression + * sizeof(int) != sizeof(void) (x) was not a constant expression */ #define __is_constexpr(x) \ (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) |