Messages in this thread | | | Date | Mon, 31 Jan 2022 15:26:37 -0600 | From | "Gustavo A. R. Silva" <> | Subject | Re: [PATCH] linux/const.h: Explain how __is_constexpr() works |
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On Mon, Jan 31, 2022 at 12:43:57PM -0800, Kees Cook wrote: > The __is_constexpr() macro is dark magic. Shed some light on it with > a comment to explain how and why it works. > > Cc: Jonathan Corbet <corbet@lwn.net> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Martin Uecker <Martin.Uecker@med.uni-goettingen.de> > Cc: Ingo Molnar <mingo@kernel.org> > Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com> > Cc: Rikard Falkeborn <rikard.falkeborn@gmail.com> > Cc: Arnd Bergmann <arnd@arndb.de> > Cc: linux-doc@vger.kernel.org > Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Thanks -- Gustavo
> --- > Jon, since this is pure comment, do you want to take it through the docs tree? > --- > include/linux/const.h | 24 ++++++++++++++++++++++++ > 1 file changed, 24 insertions(+) > > diff --git a/include/linux/const.h b/include/linux/const.h > index 435ddd72d2c4..7122d6a1f8ce 100644 > --- a/include/linux/const.h > +++ b/include/linux/const.h > @@ -7,6 +7,30 @@ > * 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() is 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 use of literal "8" is to avoid warnings about unaligned pointers; > + * these could otherwise just be "1"s. > + * - (long)(x) is used to avoid warnings about 64-bit types on 32-bit > + * architectures. > + * - The C standard defines an "integer constant expression" as different > + * from a "null pointer constant" (an integer constant 0 pointer). > + * - The conditional operator ("... ? ... : ...") returns the type of the > + * operand that isn't a null pointer constant. This behavior is the > + * central mechanism of the macro. > + * - If (x) is an integer constant expression, then the "* 0l" resolves it > + * into a null pointer constant, which forces the conditional operator > + * to return the type of the last operand: "(int *)". > + * - If (x) is not an integer constant expression, then the type of the > + * conditional operator is from the first operand: "(void *)". > + * - sizeof(int) == 4 and sizeof(void) == 1. > + * - The ultimate comparison to "sizeof(int)" chooses between either: > + * sizeof(*((int *) (8)) == sizeof(int) (x was a constant expression) > + * sizeof(*((void *)(8)) == sizeof(void) (x was not a constant expression) > */ > #define __is_constexpr(x) \ > (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) > -- > 2.30.2 >
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