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0 pointsjcranmer3y ago0 comments

> many things that are UB in C/C++ are UB because they are really hard to verify at compile time which makes them almost impossible to program around

The second half of the sentence doesn't follow from the first. Take everyone's favorite example, signed integer overflow: all you have to do to avoid UB on signed integer overflow is check for overflow before doing the operation (and C23 finally adds features to do that for you).

Taking a step back, the fundamental thing about UB is that it is very nearly always a bug in your code (and this includes especially integer overflow!). Even if you gave well-defined semantics to UB, the semantics you'd give would very rarely make the program not buggy. Complaining that we can't prove programs free of UB is tantamount to complaining that we can't prove programs free of bugs.

It actually turns out that UB is actually extremely helpful for tools that try to help programmers find bugs in their code. Since UB is automatically a bug, any tool that finds UB knows that it found a bug; if you give it well-defined semantics instead, it's a lot trickier to assert that it's a bug. In a real-world example, the infamous buffer overflow vulnerability Heartbleed stymied most (all?) static analyzers for the simple reason that, due to how OpenSSL did memory management, it wasn't actually undefined behavior by C's definition. Unsigned integer overflow also falls into this bucket--it's very hard to distinguish between intentional cases of unsigned integer overflow (e.g., hashing algorithms) from unintentional cases (e.g., calculating buffer sizes).

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9 comments · 3 top-level

the_why_of_y3y ago· 2 in thread

My complaint here is that it took C more than 30 years between defining signed integer overflow as UB and providing programmers with standard library facilities to check if a signed integer operation would result in overflow.

I much prefer Rust's approach to arithmetic, where overflow with plain arithmetic operators is defined as a bug, and panics on debug-enabled builds, plus special operations in the standard library like wrapping_add and saturating_add for the special cases where overflow is expected.

chongli3y ago

My complaint here is that it took C more than 30 years ... I much prefer Rust's approach

That's an odd complaint. Rust didn't spring forth fully formed from the ether, it stands on the shoulders of C (and other giants of PL history). 30 years ago you couldn't use Rust at all because it didn't exist.

The reason the committee doesn't just radically change C in all these nice ways to catch up to Rust is because it would be incompatible. Then you wouldn't have fixed C, you'd just have two languages: "old C", which all of the existing C code in the world is written in, and "new C", which nothing is written in. At that point why not just start over from scratch, like they did with Rust?

the_why_of_y3y ago

Interestingly, the first Ada standard in 1983 defined signed integer overflow to raise a CONSTRAINT_ERROR exception.

But apparently it lacked unsigned integers with modular arithmetic?

http://archive.adaic.com/standards/83lrm/html/lrm-11-01.html... http://archive.adaic.com/standards/83lrm/html/lrm-03-05.html

The 2012 version is a bit more readable, and has unsigned integers:

For a signed integer type, the exception Constraint_Error is raised by the execution of an operation that cannot deliver the correct result because it is outside the base range of the type. For any integer type, Constraint_Error is raised by the operators "/", "rem", and "mod" if the right operand is zero.

For a modular type, if the result of the execution of a predefined operator (see 4.5) is outside the base range of the type, the result is reduced modulo the modulus of the type to a value that is within the base range of the type.

http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-3-5-4.h...

c4mpute3y ago· 2 in thread

> all you have to do to avoid UB on signed integer overflow is check for overflow before doing the operation

All you have to do is add a check for overflow _that the compiler will not throw away because "UB won't happen"_. The very thing you want to avoid makes avoiding it very hard, and lots of bugs have resulted from compilers "optimizing" away such overflow checks.

chongli3y ago

This is covered in the article and numerous replies in this thread. Use <stdckdint.h>.

c4mpute3y ago

stdckdint.h is only available in C23. The problem has existed before that and lead to tons of exploits and bugs.

xigoi3y ago· 2 in thread

> all you have to do to avoid UB on signed integer overflow is check for overflow before doing the operation (and C23 finally adds features to do that for you).

…making your code practically unreadable, since you have to write ckd_add(ckd_add(ckd_mul(a,a),ckd_mul(ckd_mul(2,a),b)),ckd_mul(b,b)) instead of a * a + 2 * a * b + b * b.

chongli3y ago

That's not the correct syntax for the ckd_ operations. They take 3 operands, the first being a pointer to an integer where the result should be stored. And they return a bool, which you need to check in a conditional. If you're just going to throw out the bool and ignore the overflows, why bother with checked operations in the first place?

xigoi3y ago

Yeah, I realize that now. That's even worse. So you'll have to write something like

    int aa,twoa,twoab,bb,aaplustwoab,aaplustwoabplusbb;
    if (ckd_mul(a,a,&aa)) { return error; }
    if (ckd_mul(2,a,&twoa)) { return error; }
    // …
    if (ckd_add(aaplustwoab,bb,aaplustwoabplusbb)) { return error; }
    return aaplustwoabplusbb;

So ergonomic!

> If you're just going to throw out the bool and ignore the overflows, why bother with checked operations in the first place?

I'd expect the functions to return the result on success and crash on failure. Or better, raise an exception, but C doesn't have exceptions…

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9 comments · 3 top-level

the_why_of_y3y ago· 2 in thread

chongli3y ago

My complaint here is that it took C more than 30 years ... I much prefer Rust's approach

the_why_of_y3y ago

Interestingly, the first Ada standard in 1983 defined signed integer overflow to raise a CONSTRAINT_ERROR exception.

But apparently it lacked unsigned integers with modular arithmetic?

http://archive.adaic.com/standards/83lrm/html/lrm-11-01.html... http://archive.adaic.com/standards/83lrm/html/lrm-03-05.html

The 2012 version is a bit more readable, and has unsigned integers:

http://www.ada-auth.org/standards/rm12_w_tc1/html/RM-3-5-4.h...

c4mpute3y ago· 2 in thread

> all you have to do to avoid UB on signed integer overflow is check for overflow before doing the operation

chongli3y ago

This is covered in the article and numerous replies in this thread. Use <stdckdint.h>.

c4mpute3y ago

stdckdint.h is only available in C23. The problem has existed before that and lead to tons of exploits and bugs.

xigoi3y ago· 2 in thread

> all you have to do to avoid UB on signed integer overflow is check for overflow before doing the operation (and C23 finally adds features to do that for you).

…making your code practically unreadable, since you have to write ckd_add(ckd_add(ckd_mul(a,a),ckd_mul(ckd_mul(2,a),b)),ckd_mul(b,b)) instead of a * a + 2 * a * b + b * b.

chongli3y ago

xigoi3y ago

Yeah, I realize that now. That's even worse. So you'll have to write something like

    int aa,twoa,twoab,bb,aaplustwoab,aaplustwoabplusbb;
    if (ckd_mul(a,a,&aa)) { return error; }
    if (ckd_mul(2,a,&twoa)) { return error; }
    // …
    if (ckd_add(aaplustwoab,bb,aaplustwoabplusbb)) { return error; }
    return aaplustwoabplusbb;

So ergonomic!

> If you're just going to throw out the bool and ignore the overflows, why bother with checked operations in the first place?

I'd expect the functions to return the result on success and crash on failure. Or better, raise an exception, but C doesn't have exceptions…

1 more reply

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