| Message ID | 20240129183411.3791340-5-keescook@chromium.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | overflow: Introduce wrapping helpers | expand |
On 29/01/2024 19.34, Kees Cook wrote: > This allows replacements of the idioms "var += offset" and "var -= offset" > with the inc_wrap() and dec_wrap() helpers respectively. They will avoid > wrap-around sanitizer instrumentation. > > Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk> > Cc: Mark Rutland <mark.rutland@arm.com> > Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org> > Cc: linux-hardening@vger.kernel.org > Signed-off-by: Kees Cook <keescook@chromium.org> > --- > include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++++ > 1 file changed, 32 insertions(+) > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > index 4f945e9e7881..080b18b84498 100644 > --- a/include/linux/overflow.h > +++ b/include/linux/overflow.h > @@ -138,6 +138,22 @@ static inline bool __must_check __must_check_overflow(bool overflow) > __sum; \ > }) > > +/** > + * add_wrap() - Intentionally perform a wrapping increment inc_wrap > + * @a: variable to be incremented > + * @b: amount to add > + * > + * Increments @a by @b with wrap-around. Returns the resulting > + * value of @a. Will not trip any wrap-around sanitizers. > + */ > +#define inc_wrap(var, offset) \ > + ({ \ > + if (check_add_overflow(var, offset, &var)) { \ > + /* do nothing */ \ > + } \ > + var; \ Hm. I wonder if multiple evaluations of var could be a problem. Obviously never if var is actually some automatic variable, nor if it is some simple foo->bar expression. But nothing really prevents var from being, say, foo[gimme_an_index()] or something similarly convoluted. Does the compiler generate ok code if one does typeof(var) *__pvar = &(var); if (check_add_overflow(*__pvar, offset, __pvar)) {} *__pvar; [in fact, does it even generate code, i.e. does it compile?] I dunno, maybe it's overkill to worry about. Rasmus
On Mon, Jan 29, 2024 at 09:16:36PM +0100, Rasmus Villemoes wrote: > On 29/01/2024 19.34, Kees Cook wrote: > > This allows replacements of the idioms "var += offset" and "var -= offset" > > with the inc_wrap() and dec_wrap() helpers respectively. They will avoid > > wrap-around sanitizer instrumentation. > > > > Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk> > > Cc: Mark Rutland <mark.rutland@arm.com> > > Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org> > > Cc: linux-hardening@vger.kernel.org > > Signed-off-by: Kees Cook <keescook@chromium.org> > > --- > > include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++++ > > 1 file changed, 32 insertions(+) > > > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > > index 4f945e9e7881..080b18b84498 100644 > > --- a/include/linux/overflow.h > > +++ b/include/linux/overflow.h > > @@ -138,6 +138,22 @@ static inline bool __must_check __must_check_overflow(bool overflow) > > __sum; \ > > }) > > > > +/** > > + * add_wrap() - Intentionally perform a wrapping increment > > inc_wrap Thanks, fixed. > > > + * @a: variable to be incremented > > + * @b: amount to add > > + * > > + * Increments @a by @b with wrap-around. Returns the resulting > > + * value of @a. Will not trip any wrap-around sanitizers. > > + */ > > +#define inc_wrap(var, offset) \ > > + ({ \ > > + if (check_add_overflow(var, offset, &var)) { \ > > + /* do nothing */ \ > > + } \ > > + var; \ > > Hm. I wonder if multiple evaluations of var could be a problem. I am normally defensive about this, but due to @a normally being an lvalue, I lacked the imagination to think of other side-effects, but you've set me straight below. > Obviously never if var is actually some automatic variable, nor if it is > some simple foo->bar expression. But nothing really prevents var from > being, say, foo[gimme_an_index()] or something similarly convoluted. > > Does the compiler generate ok code if one does > > typeof(var) *__pvar = &(var); > if (check_add_overflow(*__pvar, offset, __pvar)) {} > *__pvar; > > [in fact, does it even generate code, i.e. does it compile?] > > I dunno, maybe it's overkill to worry about. Yeah, an index-fetch is a great example that would get lost here. I've updated these to be defined in terms of add/sub_wrap() and to use your pointer typing method to avoid side-effects.
diff --git a/include/linux/overflow.h b/include/linux/overflow.h index 4f945e9e7881..080b18b84498 100644 --- a/include/linux/overflow.h +++ b/include/linux/overflow.h @@ -138,6 +138,22 @@ static inline bool __must_check __must_check_overflow(bool overflow) __sum; \ }) +/** + * add_wrap() - Intentionally perform a wrapping increment + * @a: variable to be incremented + * @b: amount to add + * + * Increments @a by @b with wrap-around. Returns the resulting + * value of @a. Will not trip any wrap-around sanitizers. + */ +#define inc_wrap(var, offset) \ + ({ \ + if (check_add_overflow(var, offset, &var)) { \ + /* do nothing */ \ + } \ + var; \ + }) + /** * check_sub_overflow() - Calculate subtraction with overflow checking * @a: minuend; value to subtract from @@ -169,6 +185,22 @@ static inline bool __must_check __must_check_overflow(bool overflow) __val; \ }) +/** + * dec_wrap() - Intentionally perform a wrapping decrement + * @a: variable to be decremented + * @b: amount to subtract + * + * Decrements @a by @b with wrap-around. Returns the resulting + * value of @a. Will not trip any wrap-around sanitizers. + */ +#define dec_wrap(var, offset) \ + ({ \ + if (check_sub_overflow(var, offset, &var)) { \ + /* do nothing */ \ + } \ + var; \ + }) + /** * check_mul_overflow() - Calculate multiplication with overflow checking * @a: first factor
This allows replacements of the idioms "var += offset" and "var -= offset" with the inc_wrap() and dec_wrap() helpers respectively. They will avoid wrap-around sanitizer instrumentation. Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk> Cc: Mark Rutland <mark.rutland@arm.com> Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org> Cc: linux-hardening@vger.kernel.org Signed-off-by: Kees Cook <keescook@chromium.org> --- include/linux/overflow.h | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+)