[gcc r14-8819] libgcc: fix SEH C++ rethrow semantics [PR113337]

[gcc r14-8819] libgcc: fix SEH C++ rethrow semantics [PR113337]

[Jonathan Yong]

https://gcc.gnu.org/g:16774daa597f7633ae2f64efef20cad744b877b9

commit r14-8819-g16774daa597f7633ae2f64efef20cad744b877b9
Author: Matteo Italia <matteo@mitalia.net>
Date:   Wed Jan 17 12:51:44 2024 +0100

    libgcc: fix SEH C++ rethrow semantics [PR113337]
    
    SEH _Unwind_Resume_or_Rethrow invokes abort directly if
    _Unwind_RaiseException doesn't manage to find a handler for the rethrown
    exception; this is incorrect, as in this case std::terminate should be
    invoked, allowing an application-provided terminate handler to handle
    the situation instead of straight crashing the application through
    abort.
    
    The bug can be demonstrated with this simple test case:
    ===
    static void custom_terminate_handler() {
        fprintf(stderr, "custom_terminate_handler invoked\n");
        std::exit(1);
    }
    
    int main(int argc, char *argv[]) {
        std::set_terminate(&custom_terminate_handler);
        if (argc < 2) return 1;
        const char *mode = argv[1];
        fprintf(stderr, "%s\n", mode);
        if (strcmp(mode, "throw") == 0) {
            throw std::exception();
        } else if (strcmp(mode, "rethrow") == 0) {
            try {
                throw std::exception();
            } catch (...) {
                throw;
            }
        } else {
            return 1;
        }
        return 0;
    }
    ===
    
    On all gcc builds with non-SEH exceptions, this will print
    "custom_terminate_handler invoked" both if launched as ./a.out throw or
    as ./a.out rethrow, on SEH builds instead if will work as expected only
    with ./a.exe throw, but will crash with the "built-in" abort message
    with ./a.exe rethrow.
    
    This patch fixes the problem, forwarding back the error code to the
    caller (__cxa_rethrow), that calls std::terminate if
    _Unwind_Resume_or_Rethrow returns.
    
    The change makes the code path coherent with SEH _Unwind_RaiseException,
    and with the generic _Unwind_Resume_or_Rethrow from libgcc/unwind.inc
    (used for SjLj and Dw2 exception backend).
    
    libgcc/ChangeLog:
    
            PR libgcc/113337
    
            * unwind-seh.c (_Unwind_Resume_or_Rethrow): forward
            _Unwind_RaiseException return code back to caller instead of
            calling abort, allowing __cxa_rethrow to invoke std::terminate
            in case of uncaught rethrown exception

Diff:
---
 libgcc/unwind-seh.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libgcc/unwind-seh.c b/libgcc/unwind-seh.c
index 8ef0257b6163..f1b8f5a85199 100644
--- a/libgcc/unwind-seh.c
+++ b/libgcc/unwind-seh.c
@@ -395,9 +395,9 @@ _Unwind_Reason_Code
 _Unwind_Resume_or_Rethrow (struct _Unwind_Exception *exc)
 {
   if (exc->private_[0] == 0)
-    _Unwind_RaiseException (exc);
-  else
-    _Unwind_ForcedUnwind_Phase2 (exc);
+    return _Unwind_RaiseException (exc);
+
+  _Unwind_ForcedUnwind_Phase2 (exc);
   abort ();
 }