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Add scope to invertCompToken #3472

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May 14, 2021
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Add scope to invertCompToken #3472

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4 changes: 2 additions & 2 deletions src/core/internal/dassert.d
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Original file line number Diff line number Diff line change
Expand Up @@ -377,7 +377,7 @@ private string miniFormat(V)(const scope ref V v)
import core.atomic : atomicLoad;

// Inverts a comparison token for use in _d_assert_fail
private string invertCompToken(string comp) pure nothrow @nogc @safe
private string invertCompToken(scope string comp) pure nothrow @nogc @safe
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@aG0aep6G aG0aep6G May 14, 2021

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I'm not sure if this should compile. Later in the function, we have [comp]. I.e., comp is copied to the heap. Usually, that's not allowed for scope variables.

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@MoonlightSentinel MoonlightSentinel May 14, 2021 •
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I assume it works here because the array is assigned to a scope parameter and hence promoted to the stack.

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@dkorpel dkorpel May 14, 2021 •
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The [comp] is passed to parameter const scope string[] valB of combine, so the literal is allocated on the stack
Edit: that's what Florian said, didn't see his message when I posted

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@aG0aep6G aG0aep6G May 14, 2021

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I assume it works here because the array is assigned to a scope parameter and hence promoted to the stack.

That might be DMD's reasoning. But it's unsound. It actually doesn't matter if the array is on the heap or on the stack. A scope is being lost.

comp.ptr is a scope pointer. valB.ptr is also a scope pointer. valB[0].ptr is not a scope pointer, because scope is not transitive. So valB = [comp]; cannot be allowed. It's assigning a scope pointer to a non-scope pointer.

Reduced code demonstrating memory corruption:

string invertCompToken(scope string comp) @safe
{
    return combine([comp]); /* Should not compile. */
}
string combine(const scope string[] valB) @safe
{
    return valB[0];
}
string f() @safe
{
    immutable char[3] eq = "foo";
    return invertCompToken(eq);
}
void main() @safe
{
    import std.stdio;
    string s = f();
    writeln(s); /* prints garbage */
}

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@MoonlightSentinel MoonlightSentinel May 14, 2021 •
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Fair enough, that's just another oddity of scope not being transitive.
But there's no corruption here because combine doesn't escape the string. So it should be fine to wrap the call in a @trusted lambda to avoid future problems.

EDIT: The current solution is even better

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@dkorpel dkorpel May 14, 2021

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I put the operator in the middle argument of combine, side-stepping the issue.

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@dkorpel dkorpel May 14, 2021

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Btw nice catch @aG0aep6G! I think the relevant bugzilla issue is this:
https://issues.dlang.org/show_bug.cgi?id=20505

{
switch (comp)
{
Expand All @@ -402,7 +402,7 @@ private string invertCompToken(string comp) pure nothrow @nogc @safe
case "!in":
return "in";
default:
assert(0, combine(["Invalid comparison operator"], "-", [comp]));
assert(0, combine(["Invalid comparison operator '"], comp, ["'"]));
}
}

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