[cDAC] Arm64Unwinder. An overflow in the arithmetic expression (ulong)spOffset of type ulong may occur

[AristidesASm]

problem line(s):

 if (spOffset < 0)
        {
        context.Sp -= (ulong)spOffset;
       }

Casting a negative int to a unsigned ulong will result is widening conversions rather than simply taking the absolute value
ex:

int negativeValue = -5;
(ulong)negativeValue; // this will be 18446744073709551611

but:

int negativeValue = -5;
ulong beginValue = 555;
beginValue -= (ulong)negativeValue;
Console.WriteLine(beginValue);
// beginValue will be 560 ( again not expected result)

Base on comment:

    /// Specifies a stack offset. Positive values are simply used
    /// as a base offset. Negative values assume a pre-decrement behavior:

, we expect subtraction. So to achieve the desired result without connecting additional libraries, we can use the approach in this commit

---

Found by Linux Verification Center (linuxtesting.org) with SVACE.

[dotnet-policy-service]

Tagging subscribers to this area: @steveisok, @tommcdon, @dotnet/dotnet-diag
See info in area-owners.md if you want to be subscribed.

[huoyaoyuan]

According to the native version:

runtime/src/coreclr/unwinder/arm64/unwinder.cpp

Lines 1378 to 1380 in 0d3df07

	if (SpOffset < 0) {
	ContextRecord->Sp -= SpOffset;
	}

-= negative value is the expected behavior. The cast is simply to satisfy the type.

[AristidesASm]

According to the native version:

runtime/src/coreclr/unwinder/arm64/unwinder.cpp

Lines 1378 to 1380 in 0d3df07

	if (SpOffset < 0) {
	ContextRecord->Sp -= SpOffset;
	}

-= negative value is the expected behavior. The cast is simply to satisfy the type.

logic is right, but here another problem:
cast negative signed to unsigned type don't get modulo value.
Please see explain in PR message

[huoyaoyuan]

cast negative signed to unsigned type don't get modulo value.

There's no intent to get the modulo value. -= spOffset is just a shorthand of += (-spOffset). In C#, integer overflow is well defined as wrapping.

Casting a negative int to a unsigned ulong will result is widening conversions rather than simply taking the absolute value

Nobody will ever expect to get the absolute value.

[huoyaoyuan]

int negativeValue = -5;
ulong beginValue = 555;
beginValue -= (ulong)negativeValue;
Console.WriteLine(beginValue);
// beginValue will be 560 ( again not expected result)

This is very expected behavior and well-defined in C#. 555 - (-5) is 555 + 5 is 560.

[AristidesASm]

int negativeValue = -5;
ulong beginValue = 555;
beginValue -= (ulong)negativeValue;
Console.WriteLine(beginValue);
// beginValue will be 560 ( again not expected result)

This is very expected behavior and well-defined in C#. 555 - (-5) is 555 + 5 is 560.

I see.
I got Mistake with understanding work logic. Thanks.