From 0d756757877ecfb09fd2003cf887dfdf8067b702 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Fri, 14 Oct 2022 12:25:41 +0200
Subject: [PATCH 1/2] libmain: Add extraStackOverflowHandler

---
 src/libmain/shared.hh | 19 +++++++++++++++++++
 src/libmain/stack.cc  |  6 ++++++
 2 files changed, 25 insertions(+)

diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index 0cc56d47d..66d29a1f7 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -113,5 +113,24 @@ struct PrintFreed
 /* Install a SIGSEGV handler to detect stack overflows. */
 void detectStackOverflow();
 
+/* Pluggable behavior to run before _exit(1) in case of a stack overflow.
+
+   Default value: do nothing, return immediately.
+
+   This is called by the handler installed by detectStackOverflow().
+
+   This gives Nix library consumers a limit opportunity to report the error
+   condition.
+
+   NOTE: Use with diligence, because this runs in the signal handler, with very
+   limited stack space and a potentially a corrupted heap, all while the failed
+   thread is blocked indefinitely. All functions called must be reentrant. */
+extern std::function<void(siginfo_t * info, void * ctx)> stackOverflowHandler;
+
+/* The default, robust implementation of stackOverflowHandler.
+
+   Prints an error message directly to stderr using a syscall instead of the
+   logger. Exits the process immediately after. */
+void defaultStackOverflowHandler(siginfo_t * info, void * ctx);
 
 }
diff --git a/src/libmain/stack.cc b/src/libmain/stack.cc
index b0a4a4c5d..a6d10f738 100644
--- a/src/libmain/stack.cc
+++ b/src/libmain/stack.cc
@@ -1,4 +1,5 @@
 #include "error.hh"
+#include "shared.hh"
 
 #include <cstring>
 #include <cstddef>
@@ -31,6 +32,7 @@ static void sigsegvHandler(int signo, siginfo_t * info, void * ctx)
         if (diff < 4096) {
             char msg[] = "error: stack overflow (possible infinite recursion)\n";
             [[gnu::unused]] auto res = write(2, msg, strlen(msg));
+            nix::extraStackOverflowHandler(info, ctx);
             _exit(1); // maybe abort instead?
         }
     }
@@ -67,5 +69,9 @@ void detectStackOverflow()
 #endif
 }
 
+std::function<void(siginfo_t * info, void * ctx)> extraStackOverflowHandler(
+    [](siginfo_t * info, void * ctx) {
+    }
+);
 
 }

From ab4eb39386eab091f8682efa69e104f4ed74c1ca Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Fri, 14 Oct 2022 12:37:34 +0200
Subject: [PATCH 2/2] libmain: Make the entire stack overflow handler pluggable

---
 src/libmain/shared.hh |  7 ++++---
 src/libmain/stack.cc  | 16 ++++++++--------
 2 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/src/libmain/shared.hh b/src/libmain/shared.hh
index 66d29a1f7..3c37fd627 100644
--- a/src/libmain/shared.hh
+++ b/src/libmain/shared.hh
@@ -113,14 +113,15 @@ struct PrintFreed
 /* Install a SIGSEGV handler to detect stack overflows. */
 void detectStackOverflow();
 
-/* Pluggable behavior to run before _exit(1) in case of a stack overflow.
+/* Pluggable behavior to run in case of a stack overflow.
 
-   Default value: do nothing, return immediately.
+   Default value: defaultStackOverflowHandler.
 
    This is called by the handler installed by detectStackOverflow().
 
    This gives Nix library consumers a limit opportunity to report the error
-   condition.
+   condition. The handler should exit the process.
+   See defaultStackOverflowHandler() for a reference implementation.
 
    NOTE: Use with diligence, because this runs in the signal handler, with very
    limited stack space and a potentially a corrupted heap, all while the failed
diff --git a/src/libmain/stack.cc b/src/libmain/stack.cc
index a6d10f738..10f71c1dc 100644
--- a/src/libmain/stack.cc
+++ b/src/libmain/stack.cc
@@ -30,10 +30,7 @@ static void sigsegvHandler(int signo, siginfo_t * info, void * ctx)
         ptrdiff_t diff = (char *) info->si_addr - sp;
         if (diff < 0) diff = -diff;
         if (diff < 4096) {
-            char msg[] = "error: stack overflow (possible infinite recursion)\n";
-            [[gnu::unused]] auto res = write(2, msg, strlen(msg));
-            nix::extraStackOverflowHandler(info, ctx);
-            _exit(1); // maybe abort instead?
+            nix::stackOverflowHandler(info, ctx);
         }
     }
 
@@ -69,9 +66,12 @@ void detectStackOverflow()
 #endif
 }
 
-std::function<void(siginfo_t * info, void * ctx)> extraStackOverflowHandler(
-    [](siginfo_t * info, void * ctx) {
-    }
-);
+std::function<void(siginfo_t * info, void * ctx)> stackOverflowHandler(defaultStackOverflowHandler);
+
+void defaultStackOverflowHandler(siginfo_t * info, void * ctx) {
+    char msg[] = "error: stack overflow (possible infinite recursion)\n";
+    [[gnu::unused]] auto res = write(2, msg, strlen(msg));
+    _exit(1); // maybe abort instead?
+}
 
 }