The following commit has been merged into the x86/misc branch of tip:

Commit-ID:     40c4b47f41b95dff743c841536cb64014e65ef0c
Gitweb:        https://git.kernel.org/tip/40c4b47f41b95dff743c841536cb64014e65ef0c
Author:        Sean Christopherson <seanjc@google.com>
AuthorDate:    Thu, 23 Apr 2026 07:54:17 -07:00
Committer:     Dave Hansen <dave.hansen@linux.intel.com>
CommitterDate: Mon, 27 Apr 2026 12:02:40 -07:00

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

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);

32-bit x86 doesn't support HAVE_ARCH_BUG_FORMAT_ARGS and uses generic
implementations that provide printf() validation. This means there's
now a big blind spot is code that is strictly x86-64. Inconveniently,
new features are also frequently x86-64-only.

Fix the blind 64-bit blind spot.

[ dhansen: changelog tweaks to flesh out the 64-bit-only details ]

Fixes: 5b472b6e5bd9 ("x86_64/bug: Implement __WARN_printf()")
Fixes: 11bb4944f014 ("x86/bug: Implement WARN_ONCE()")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/adc1IrD8uqWdaOKv@yzhao56-desk.sh.intel.com
Link: https://patch.msgid.link/20260423145419.459988-2-seanjc@google.com
---
 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 80c1696..bf3c802 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)

On Mon, Apr 27, 2026, tip-bot2 for Sean Christopherson wrote:
> The following commit has been merged into the x86/misc branch of tip:
> 
> Commit-ID:     40c4b47f41b95dff743c841536cb64014e65ef0c
> Gitweb:        https://git.kernel.org/tip/40c4b47f41b95dff743c841536cb64014e65ef0c
> Author:        Sean Christopherson <seanjc@google.com>
> AuthorDate:    Thu, 23 Apr 2026 07:54:17 -07:00
> Committer:     Dave Hansen <dave.hansen@linux.intel.com>
> CommitterDate: Mon, 27 Apr 2026 12:02:40 -07:00
> 
> x86/bug: Add printf() validation to HAVE_ARCH_BUG_FORMAT_ARGS WARNs
> 
> 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);
> 
> 32-bit x86 doesn't support HAVE_ARCH_BUG_FORMAT_ARGS and uses generic
> implementations that provide printf() validation. This means there's
> now a big blind spot is code that is strictly x86-64. Inconveniently,
> new features are also frequently x86-64-only.
> 
> Fix the blind 64-bit blind spot.
> 
> [ dhansen: changelog tweaks to flesh out the 64-bit-only details ]

Much better than what I wrote, thanks Dave!