[mono][debugger] fix exception while decoding value that has a byref field

[thaystg]

To reproduce:

Run();
static void Run()
{
    Invoke(new string[] { "TEST" });
}
static void Invoke(object[] parameters)
{
    CheckArguments(parameters);
}
static void CheckArguments(ReadOnlySpan<object> parameters)
{
    System.Diagnostics.Debugger.Break();
}

Attach debugger and try to inspect parameters parameter, mono runtime will crash.

While decoding a object / valuetype that has a byref field, it was dereferencing the pointer twice when it was trying to use it, and then the address was invalid.

@lambdageek do I need to free the new memory that I allocated or it will be freed when the object is freed?

On wasm we don't have the exception because we don't try to access the synchronisation field that was the one that become invalid.
But while trying to create a test for wasm I found another issue related and it's fixed here.

Also created a issue to support Format Specifiers in C# while debugging on wasm:
#75773

[ghost]

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Issue Details

---

To reproduce:

Run();
static void Run()
{
    Invoke(new string[] { "TEST" });
}
static void Invoke(object[] parameters)
{
    CheckArguments(parameters);
}
static void CheckArguments(ReadOnlySpan<object> parameters)
{
    System.Diagnostics.Debugger.Break();
}

Attach debugger and try to inspect parameters parameter, mono runtime will crash.

While decoding a object / valuetype that has a byref field, it was dereferencing the pointer twice when it was trying to use it, and then the address was invalid.

@lambdageek do I need to free the new memory that I allocated or it will be freed when the object is freed?

On wasm we don't have the exception because we don't try to access the synchronisation field that was the one that become invalid.
But while trying to create a test for wasm I found another issue related and it's fixed here.

Also created a issue to support Format Specifiers in C# while debugging on wasm:
#75773

Author:	thaystg
Assignees:	-
Labels:	area-Debugger-mono
Milestone:	-

[lewing]

Minor feedback

[lewing]

maybe turn all these into a regex (and or use ordinal)

[thaystg]

/backport to release/7.0

[github-actions]

Started backporting to release/7.0: https://github.com/dotnet/runtime/actions/runs/3114596965

[lambdageek]

The current fix is kind of a stopgap because it leaks memory. The real issue is that debugger method invokes dont' handle ref fields in ref structs (which may appear as this or as arguments).

Thays is working on another approach where we precalculate the extra space needed for values that the ref fields can point at and then use g_alloca to grab the needed temporary space before calling decode_value (which will use the passed-in extra buffer). Depending on how complex the fix becomes, we shoudl consider it for servicing net7. /cc @lewing