[PATCH 1/2] meson: detect broken clang 17 with -fzero-call-used-regs

["Daniel P. Berrangé" <berrange@redhat.com>]

Clang 17 will segv if given  -fzero-call-used-regs and optimization
is enabled. Since upstream hasn't triaged the bug, distros are
increasingly shipping with broken Clang.

https://github.com/llvm/llvm-project/issues/75168
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=277474
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 meson.build | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/meson.build b/meson.build
index 0ef1654e86..762798f2ee 100644
--- a/meson.build
+++ b/meson.build
@@ -555,17 +555,24 @@ endif
 # Check further flags that make QEMU more robust against malicious parties
 
 hardening_flags = [
-    # Zero out registers used during a function call
-    # upon its return. This makes it harder to assemble
-    # ROP gadgets into something usable
-    '-fzero-call-used-regs=used-gpr',
-
     # Initialize all stack variables to zero. This makes
     # it harder to take advantage of uninitialized stack
     # data to drive exploits
     '-ftrivial-auto-var-init=zero',
 ]
 
+# Zero out registers used during a function call
+# upon its return. This makes it harder to assemble
+# ROP gadgets into something usable
+#
+# NB: CLang 17 is broken and SEGVs
+# https://github.com/llvm/llvm-project/issues/75168
+if cc.compiles('extern struct { void (*cb)(void); } s; void f(void) { s.cb(); }',
+               name: '-fzero-call-used-regs=used-gpr',
+               args: ['-O2', '-fzero-call-used-regs=used-gpr'])
+    hardening_flags += '-fzero-call-used-regs=used-gpr'
+endif
+
 qemu_common_flags += cc.get_supported_arguments(hardening_flags)
 
 add_global_arguments(qemu_common_flags, native: false, language: all_languages)
-- 
2.43.0