(POTENTIAL FIX) ExceptFrameCount in dynamic libraries compiled for Linux (x86-64) produces inconsistent results
## Summary
Dynamic libraries compiled for Linux produce inconsistent result in exception handling (ExceptFrameCount, for example).
Faced issue makes reliable exception backtrace production nearly impossible.
## System Information
- **Operating system:**
- Ubuntu 22.04.3 LTS
- Linux 6.1.31-gentoo #4 SMP PREEMPT_DYNAMIC Fri Aug 4 00:39:53 CEST 2023 x86_64 GNU/Linux
- **Processor architecture:** x86-64
- **Compiler version:**
- Free Pascal Compiler version 3.2.2+dfsg-9ubuntu1 [2022/04/11] for x86_64
- Free Pascal Compiler version 3.2.2 [2022/12/02] for x86_64
- Free Pascal Compiler version 3.3.1 [2024/02/09] for x86_64 (trunk)
- **Device:** Computer
## Steps to reproduce
Generally: Try to get exception frame count in dynamic library (just call ExceptFrameCount in except block print its result)
With example project:
1. Compile dynamic library example
2. Compile host program example
3. Run host program several times to see random values as an output (bash command may be used)
## Example Project
Minimal dynamic library code (libtest.pas)
```pascal
library testlib;
{$Mode ObjFPC}
uses
SysUtils;
procedure Test;
begin
raise Exception.Create('Testing exception');
end;
procedure A;
begin
Test;
end;
procedure B;
var FrameCnt: Integer;
begin
try
// Just raise an exception that passes several functions
A;
except
on E: Exception do begin
// And on exception it should just print the amount of frames in exception backtrace
FrameCnt := ExceptFrameCount;
WriteLn('Exception frame count = ', FrameCnt);
end;
end;
end;
exports
B;
begin
end.
```
And sample host program (invokelib.pas)
```pascal
program invokelib;
{$Mode DelphiUnicode}
uses
dynlibs,
SysUtils;
type
TProc = procedure;
var LibHandle: THandle;
Proc: TProc;
begin
// Load library
LibHandle := LoadLibrary('./libtestlib.so');
if LibHandle <= 32 then begin
WriteLn('Failed to load a library');
Exit;
end;
try
// Load procedure B
Proc := GetProcAddress(LibHandle, 'B');
if @Proc = nil then begin
WriteLn('Failed to get proc address');
Exit;
end;
// And call it
Proc();
finally
FreeLibrary(LibHandle);
end;
end.
```
## What is the current bug behavior?
Printed FrameCnt variable (that just stored the value of ExceptFrameCount) varies from 0 to 3 from run to run randomly.
## What is the expected (correct) behavior?
Printed FrameCnt should always have the same value of 3 in presented example, because it passes Test (ExceptAddr) -> A (Frame 0) -> B (Frame 1) -> HostMain (Frame 2)
## Relevant logs and/or screenshots
Running this command to test 20 runs of invokelib:
```bash
for i in {1..20}; do printf "Run #%d: " $i; ./invokelib; done
```
Produces next output (please note, that numbers might vary from run to run)
```
Run #1: Exception frame count = 3
Run #2: Exception frame count = 0
Run #3: Exception frame count = 3
Run #4: Exception frame count = 0
Run #5: Exception frame count = 3
Run #6: Exception frame count = 3
Run #7: Exception frame count = 3
Run #8: Exception frame count = 3
Run #9: Exception frame count = 3
Run #10: Exception frame count = 3
Run #11: Exception frame count = 3
Run #12: Exception frame count = 0
Run #13: Exception frame count = 0
Run #14: Exception frame count = 3
Run #15: Exception frame count = 3
Run #16: Exception frame count = 0
Run #17: Exception frame count = 2
Run #18: Exception frame count = 0
Run #19: Exception frame count = 3
Run #20: Exception frame count = 3
```
## Remarks
Logic presented in example projects works perfectly when compiled as windows DLL (at least win64 target)
## Potential Fix
Looking into `except.inc` (`PushExceptObject` routine), it has loop with next condition:
```pascal
while (framecount<RaiseMaxFrameCount) and (curr_frame > prev_frame) and
(curr_frame<StackTop) do
...
```
If part of condition `curr_frame < StackTop` is commented out (or otherwise made true, in my case I just inserted flag in system unit that is False by default, but I can set its value in dynamic library to see its effect; loop condition is changed like so: `while (framecount < RaiseMaxFrameCount) and (curr_frame > prev_frame) and (SYSTEM_FLAG_TO_SKIP_STACK_CHECK or (curr_frame < StackTop)) do ...`), then everything works as expected.
I am not familiar of why it can solve the issue and why if it is turned off - everything still works, but maybe it can be a solution - to just add the flag into system unit and allow programmer to decide to set it to False or to True depending on their situation?
issue
GitLab AI Context
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