[PATCH v2 1/2] x86/bug: Add printf() validation to HAVE_ARCH_BUG_FORMAT_ARGS WARNs

[Sean Christopherson]

Add explicit printf() validation for x86-64's newfangled WARN
implementation, as most (all?) compilers fail to detect basic formatting
issues without the annotation.  E.g. even goofs like printing a u64 as a
string aren't detected:

  WARN_ONCE(1, "Bad message, %s", vcpu->arch.last_guest_tsc);

Lack of validation is especially problematic for code that is 64-bit-only,
as blatant goofs can easily go unnoticed, as they (somewhat ironically)
will only be noticed by CONFIG_BUG=n builds.

Cc: Yan Zhao <yan.y.zhao@xxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/adc1IrD8uqWdaOKv@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
Fixes: 11bb4944f014 ("x86/bug: Implement WARN_ONCE()")
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/include/asm/bug.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
index 80c1696d8d59..bf3c802654d1 100644
--- a/arch/x86/include/asm/bug.h
+++ b/arch/x86/include/asm/bug.h
@@ -153,6 +153,7 @@ 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, ...) { }
 #endif /* __ASSEMBLER__ */
 
 #define __WARN_bug_entry(flags, format) ({				\
@@ -172,6 +173,7 @@ 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)
-- 
2.54.0.545.g6539524ca2-goog