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Subject: Re: [RFC] saturate check_*_overflow() output?
To:     Kees Cook <keescook@chromium.org>
Cc:     Jason Gunthorpe <jgg@ziepe.ca>, Leon Romanovsky <leon@kernel.org>,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        Segher Boessenkool <segher@kernel.crashing.org>
References: <202008031118.36756FAD04@keescook>
 <f177a821-74a3-e868-81d3-55accfb5b161@rasmusvillemoes.dk>
 <202008041137.02D231B@keescook>
From:   Rasmus Villemoes <linux@rasmusvillemoes.dk>
Message-ID: <6d190601-68f1-c086-97ac-2ee1c08f5a34@rasmusvillemoes.dk>
Date:   Wed, 5 Aug 2020 13:38:58 +0200
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On 04/08/2020 21.23, Kees Cook wrote:
> On Tue, Aug 04, 2020 at 08:11:45AM +0200, Rasmus Villemoes wrote:

>> What we might do, to deal with the "caller fails to check the result",
>> is to add a
>>
>> static inline bool __must_check must_check_overflow(bool b) { return
>> unlikely(b); }
>>
>> and wrap all the final "did it overflow" results in that one - perhaps
>> also for the __builtin_* cases, I don't know if those are automatically
>> equipped with that attribute. [I also don't know if gcc propagates
>> likely/unlikely out to the caller, but it shouldn't hurt to have it
>> there and might improve code gen if it does.]
> 
> (What is the formal name for the ({ ...; return_value; }) C construct?)

https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

> Will that work as a macro return value? If so, that's extremely useful.

Yes and no. Just wrapping the last expression in the statement
expression with my must_check_overflow(), as in

@@ -67,17 +72,18 @@
        typeof(d) __d = (d);                    \
        (void) (&__a == &__b);                  \
        (void) (&__a == __d);                   \
-       __builtin_sub_overflow(__a, __b, __d);  \
+       must_check_overflow(__builtin_sub_overflow(__a, __b, __d));     \
 })

does not appear to work. For some reason, this can't be (ab)used to
overrule the __must_check this simply:

  - kstrtoint(a, b, c);
  + ({ kstrtoint(a, b, c); });

still gives a warning for kstrtoint(). But failing to use the result of
check_sub_overflow() as patched above does not give a warning.

I'm guessing gcc has some internal very early simplification that
replaces single-expression statement-exprs with just that expression,
and the warn-unused-result triggers later. But as soon as the
statement-expr becomes a little non-trivial (e.g. above), my guess is
that the whole thing gets assigned to some internal "variable"
representing the result, and that assignment then counts as a use of the
return value from must_check_overflow() - cc'ing Segher, as he usually
knows these details.

Anyway, we don't need to apply it to the last expression inside ({}), we
can just pass the whole ({}) to must_check_overflow() as in

-#define check_sub_overflow(a, b, d) ({         \
+#define check_sub_overflow(a, b, d) must_check_overflow(({             \
        typeof(a) __a = (a);                    \
        typeof(b) __b = (b);                    \
        typeof(d) __d = (d);                    \
        (void) (&__a == &__b);                  \
        (void) (&__a == __d);                   \
        __builtin_sub_overflow(__a, __b, __d);  \
-})
+}))

and that's even more natural for the fallback cases which would be

 #define check_sub_overflow(a, b, d)                                    \
+       must_check_overflow(                                            \
        __builtin_choose_expr(is_signed_type(typeof(a)),                \
                        __signed_sub_overflow(a, b, d),                 \
-                       __unsigned_sub_overflow(a, b, d))
+                       __unsigned_sub_overflow(a, b, d)))

(in all cases with some whitespace reflowing).

Rasmus