executor: introduce __no_stack_protector and use it for guest code

When compiling the executor in syz-env-old, -fstack-protector may
kick in and introduce global accesses that tools/check-syzos.sh reports.

To prevent this, introduce the __no_stack_protector macro attribute that
disable stack protection for the function in question, and use it for
guest code.

While at it, factor out some common definitions into common_kvm_syzos.h

1 files changed, 33 insertions, 0 deletions

diff --git a/executor/common_kvm_syzos.h b/executor/common_kvm_syzos.h
new file mode 100644
index 000000000..a635d517b
--- /dev/null
+++ b/executor/common_kvm_syzos.h
@@ -0,0 +1,33 @@
+// Copyright 2025 syzkaller project authors. All rights reserved.
+// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.
+
+// Common SYZOS definitions.
+
+// Prevent function inlining. This attribute is applied to every guest_handle_* function,
+// making sure they remain small so that the compiler does not attempt to be too clever
+// (e.g. generate switch tables).
+#define noinline __attribute__((noinline))
+
+// __no_stack_protector disables -fstack-protector which may introduce unwanted global accesses.
+// TODO(glider): once syz-env-old migrates to GCC>11 we can just use
+// __attribute__((no_stack_protector)).
+#if defined(__clang__)
+// Clang supports the no_stack_protector attribute.
+#define __no_stack_protector __attribute__((no_stack_protector))
+#elif defined(__GNUC__)
+// The no_stack_protector attribute was introduced in GCC 11.1.
+#if __GNUC__ > 11
+#define __no_stack_protector __attribute__((no_stack_protector))
+#else
+// Fallback to the optimize attribute for older GCC versions.
+#define __no_stack_protector __attribute__((__optimize__("-fno-stack-protector")))
+#endif
+#else
+#define __no_stack_protector
+#endif
+
+// Host will map the code in this section into the guest address space.
+#define GUEST_CODE __attribute__((section("guest"))) __no_stack_protector
+
+// Start/end of the guest section.
+extern char *__start_guest, *__stop_guest;