From 8ccf44142607b9db3850333018ebf4f57baf2c33 Mon Sep 17 00:00:00 2001
From: "Paul \"LeoNerd\" Evans" <leonerd@leonerd.org.uk>
Date: Thu, 10 Oct 2024 19:56:15 +0100
Subject: [PATCH] Set builtin:: globs to readonly so that attempted
 replacements of them do not break optimisations

---
 MANIFEST            |  1 +
 builtin.c           |  7 +++++++
 lib/builtin.pm      | 12 +++++++++++-
 pod/perldelta.pod   | 10 ++++++++++
 t/lib/croak/builtin |  6 ++++++
 5 files changed, 35 insertions(+), 1 deletion(-)
 create mode 100644 t/lib/croak/builtin

diff --git a/MANIFEST b/MANIFEST
index ac9456dd3c92..4bf60b748bd6 100644
--- a/MANIFEST
+++ b/MANIFEST
@@ -6080,6 +6080,7 @@ t/lib/common.pl				Helper for lib/{warnings,feature}.t
 t/lib/commonsense.t			See if configuration meets basic needs
 t/lib/Count.pm				Helper for t/op/method.t
 t/lib/croak.t				Test calls to Perl_croak() in the C source.
+t/lib/croak/builtin			Test croak calls from builtin.c
 t/lib/croak/class			Test croak calls from class.c
 t/lib/croak/gv				Test croak calls from gv.c
 t/lib/croak/mg				Test croak calls from mg.c
diff --git a/builtin.c b/builtin.c
index 34865ec9a4bb..f004a3275d79 100644
--- a/builtin.c
+++ b/builtin.c
@@ -804,6 +804,13 @@ Perl_boot_core_builtin(pTHX)
         if(builtin->checker) {
             cv_set_call_checker_flags(cv, builtin->checker, newSVuv(PTR2UV(builtin)), 0);
         }
+
+        /* Because of all these callcheckers and other optimisations, it would
+         * all break if we permitted runtime replacement of the functions,
+         * e.g. by glob tricks like `*builtin::reftype = sub { ... }`.
+         * Prevent modification of the GV so as to avoid this problem.
+         */
+        SvREADONLY_on((SV *)CvGV(cv));
     }
 
     newXS_flags("builtin::import", &XS_builtin_import, __FILE__, NULL, 0);
diff --git a/lib/builtin.pm b/lib/builtin.pm
index 0980884359b7..f11e6b0be93b 100644
--- a/lib/builtin.pm
+++ b/lib/builtin.pm
@@ -1,4 +1,4 @@
-package builtin 0.015;
+package builtin 0.016;
 
 use v5.40;
 
@@ -106,6 +106,16 @@ The following bundles currently exist:
     :5.40      true false weaken unweaken is_weak blessed refaddr reftype
                ceil floor is_tainted trim indexed
 
+=head2 Read-only Functions
+
+Various optimisations that apply to many functions in the L<builtin> package
+would be broken if the functions are ever replaced or changed, such as by
+assignment into glob references.  Because of this, the globs that contain
+them are set read-only since Perl version 5.41.5, preventing such replacement.
+
+    $ perl -e '*builtin::reftype = sub { "BOO" }'
+    Modification of a read-only value attempted at -e line 1.
+
 =head1 FUNCTIONS
 
 =head2 true
diff --git a/pod/perldelta.pod b/pod/perldelta.pod
index baa03b9396ea..7aa464665049 100644
--- a/pod/perldelta.pod
+++ b/pod/perldelta.pod
@@ -27,6 +27,16 @@ here, but most should go in the L</Performance Enhancements> section.
 
 [ List each enhancement as a =head2 entry ]
 
+=head2 Functions in the C<builtin::> package no longer permit replacement
+
+Various optimisations that apply to many functions in the L<builtin> package
+would be broken if the functions are ever replaced or changed, such as by
+assignment into glob references.  Because of this, the globs that contain
+them are now set read-only, preventing such replacement.
+
+    $ perl -e '*builtin::reftype = sub { "BOO" }'
+    Modification of a read-only value attempted at -e line 1.
+
 =head1 Security
 
 XXX Any security-related notices go here. In particular, any security
diff --git a/t/lib/croak/builtin b/t/lib/croak/builtin
new file mode 100644
index 000000000000..a1252b02cfe5
--- /dev/null
+++ b/t/lib/croak/builtin
@@ -0,0 +1,6 @@
+__END__
+########
+# Attempted replacement of builtin:: functions is not allowed as of Perl 5.41.5
+*builtin::reftype = sub { "BOO" };
+EXPECT
+Modification of a read-only value attempted at - line 2.