[PATCH] ubsan: Require GCC-8+ or Clang to use UBSAN

[Peter Zijlstra]

On Mon, Jan 04, 2021 at 04:13:17PM +0100, Peter Zijlstra wrote:
> On Tue, Dec 22, 2020 at 11:04:54PM -0600, Josh Poimboeuf wrote:
> > GCC 7 has a known bug where UBSAN ignores '-fwrapv' and generates false
> > signed-overflow-UB warnings.  The type mismatch between 'i' and
> > 'nr_segs' in copy_compat_iovec_from_user() is causing such a warning,
> > which also happens to violate uaccess rules:
> > 
> >   lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled
> > 
> > Fix it by making the variable types match.
> > 
> > This is similar to a previous commit:
> > 
> >   29da93fea3ea ("mm/uaccess: Use 'unsigned long' to placate UBSAN warnings on older GCC versions")
> 
> Maybe it's time we make UBSAN builds depend on GCC-8+ ?

---
Subject: ubsan: Require GCC-8+ or Clang to use UBSAN

Just like how we require GCC-8.2 for KASAN due to compiler bugs, require
a sane version of GCC for UBSAN.

Specifically, before GCC-8 UBSAN doesn't respect -fwrapv and thinks
signed arithmetic is buggered.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 lib/Kconfig.ubsan | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/lib/Kconfig.ubsan b/lib/Kconfig.ubsan
index 8b635fd75fe4..acc3df62460e 100644
--- a/lib/Kconfig.ubsan
+++ b/lib/Kconfig.ubsan
@@ -2,8 +2,13 @@
 config ARCH_HAS_UBSAN_SANITIZE_ALL
 	bool
 
+# UBSAN prior to GCC-8 gets -fwrapv wrong, we rely on that
+config UBSAN_SANE
+	def_bool !CC_IS_GCC || GCC_VERSION >= 80000
+
 menuconfig UBSAN
 	bool "Undefined behaviour sanity checker"
+	depends on UBSAN_SANE
 	help
 	  This option enables the Undefined Behaviour sanity checker.
 	  Compile-time instrumentation is used to detect various undefined