[PATCH] x86/bug: Add printf() validation to HAVE_ARCH_BUG_FORMAT_ARGS WARNs

[Sean Christopherson <seanjc@google.com>]

Add explicit printf() validation for x86-64's newfangled WARN
implementation, as most (all?) compilers fail to detect basic formatting
issues without the annotation.  Lack of validation is especially
problematic for code that is 64-bit-only, as blatant goofs can easily go
unnoticed.

Cc: Yan Zhao <yan.y.zhao@intel.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/all/adc1IrD8uqWdaOKv@yzhao56-desk.sh.intel.com
Fixes: 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
Fixes: 11bb4944f014 ("x86/bug: Implement WARN_ONCE()")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---

This is *very* lightly tested.  Yan reported a bug against a commit in the
kvm-x86 tree (see the link) where I botched the formatting of a WARN_ONCE()
argument, but none of my builds (with W=1 and -Werror) detected the issue,
nor did any of the build bots (AFAIK).  I'm not entirely sure how Yan managed
to trigger the diagnostic, but it's easy to observe the lack of validation by
creating a malformed WARN/WARN_ONCE, and then toggling
HAVE_ARCH_BUG_FORMAT_ARGS.

Thankfully, it looks like my goof is the only one that has snuck in (and I
need to rebase that commit anyways).

 arch/x86/include/asm/bug.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index 80c1696d8d59..29b7dad4d5ef 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -153,6 +153,9 @@ struct arch_va_list {
 	struct sysv_va_list args;
 };
 extern void *__warn_args(struct arch_va_list *args, struct pt_regs *regs);
+static __always_inline __printf(1, 2) void __WARN_validate_printf(const char *fmt, ...) { }
+#else
+#define __WARN_validate_printf(fmt, ...)
 #endif /* __ASSEMBLER__ */
 
 #define __WARN_bug_entry(flags, format) ({				\
@@ -172,6 +175,7 @@ extern void *__warn_args(struct arch_va_list *args, struct pt_regs *regs);
 #define __WARN_print_arg(flags, format, arg...)				\
 do {									\
 	int __flags = (flags) | BUGFLAG_WARNING | BUGFLAG_ARGS ;	\
+	__WARN_validate_printf(format, ## arg);				\
 	static_call_mod(WARN_trap)(__WARN_bug_entry(__flags, format), ## arg); \
 	asm (""); /* inhibit tail-call optimization */			\
 } while (0)

base-commit: c9904c53ca958b5ebf5165dd1705c52f6afc2b2f
-- 
2.53.0.1213.gd9a14994de-goog