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Message-ID: <202505301037.D816A49@keescook>
Date: Fri, 30 May 2025 10:48:47 -0700
From: Kees Cook <kees@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Alessandro Carminati <acarmina@...hat.com>,
linux-kselftest@...r.kernel.org,
Dan Carpenter <dan.carpenter@...aro.org>,
Daniel Diaz <daniel.diaz@...aro.org>,
David Gow <davidgow@...gle.com>,
Arthur Grillo <arthurgrillo@...eup.net>,
Brendan Higgins <brendan.higgins@...ux.dev>,
Naresh Kamboju <naresh.kamboju@...aro.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Maxime Ripard <mripard@...nel.org>,
Ville Syrjala <ville.syrjala@...ux.intel.com>,
Daniel Vetter <daniel@...ll.ch>, Guenter Roeck <linux@...ck-us.net>,
Alessandro Carminati <alessandro.carminati@...il.com>,
Jani Nikula <jani.nikula@...el.com>,
Jeff Johnson <jeff.johnson@....qualcomm.com>,
Josh Poimboeuf <jpoimboe@...nel.org>,
Shuah Khan <skhan@...uxfoundation.org>,
Linux Kernel Functional Testing <lkft@...aro.org>,
dri-devel@...ts.freedesktop.org, kunit-dev@...glegroups.com,
linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v5 1/5] bug/kunit: Core support for suppressing warning
backtraces
On Fri, May 30, 2025 at 04:01:40PM +0200, Peter Zijlstra wrote:
> I'm not really concerned with performance here, but more with the size
> of the code emitted by WARN_ONCE(). There are a *ton* of WARN sites,
> while only one report_bug() and printk().
>
> The really offensive thing is that this is for a feature most nobody
> will ever need :/
Well, it won't be enabled often -- this reminds me of ftrace: it needs
to work, but it'll be off most of the time.
> The below results in:
>
> 03dc 7ac: 48 c7 c0 00 00 00 00 mov $0x0,%rax 7af: R_X86_64_32S .rodata.str1.1+0x223
> 03e3 7b3: ba 2a 00 00 00 mov $0x2a,%edx
> 03e8 7b8: 48 0f b9 d0 ud1 %rax,%rdx
>
> And it even works :-)
>
> Hmm... I should try and stick the format string into the __bug_table,
> its const after all. Then I can get 2 arguments covered.
I like the patch! Can you add a _little_ documentation, though? e.g.
explaining that BUG_WARN ... BUG_WARN_END is for format string args,
etc.
But yes, I *love* that this moves the printk into the handler! Like you,
I have been bothered by this for a long time and could not find a good
way to do it. This is nice.
>
> ---
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index f0e9acf72547..88b305d49f35 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -5,6 +5,7 @@
> #include <linux/stringify.h>
> #include <linux/instrumentation.h>
> #include <linux/objtool.h>
> +#include <linux/args.h>
>
> /*
> * Despite that some emulators terminate on UD2, we use it for WARN().
> @@ -28,50 +29,44 @@
> #define BUG_UD1_UBSAN 0xfffc
> #define BUG_EA 0xffea
> #define BUG_LOCK 0xfff0
> +#define BUG_WARN 0xfe00
> +#define BUG_WARN_END 0xfeff
>
> #ifdef CONFIG_GENERIC_BUG
>
> #ifdef CONFIG_X86_32
> -# define __BUG_REL(val) ".long " __stringify(val)
> +#define ASM_BUG_REL(val) .long val
> #else
> -# define __BUG_REL(val) ".long " __stringify(val) " - ."
> +#define ASM_BUG_REL(val) .long val - .
> #endif
>
> #ifdef CONFIG_DEBUG_BUGVERBOSE
> +#define ASM_BUGTABLE_VERBOSE(file, line) \
> + ASM_BUG_REL(file) ; \
> + .word line
> +#define ASM_BUGTABLE_VERBOSE_SIZE 6
> +#else
> +#define ASM_BUGTABLE_VERBOSE(file, line)
> +#define ASM_BUGTABLE_VERBOSE_SIZE 0
> +#endif
>
> -#define _BUG_FLAGS(ins, flags, extra) \
> -do { \
> - asm_inline volatile("1:\t" ins "\n" \
> - ".pushsection __bug_table,\"aw\"\n" \
> - "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n" \
> - "\t" __BUG_REL(%c0) "\t# bug_entry::file\n" \
> - "\t.word %c1" "\t# bug_entry::line\n" \
> - "\t.word %c2" "\t# bug_entry::flags\n" \
> - "\t.org 2b+%c3\n" \
> - ".popsection\n" \
> - extra \
> - : : "i" (__FILE__), "i" (__LINE__), \
> - "i" (flags), \
> - "i" (sizeof(struct bug_entry))); \
> -} while (0)
> -
> -#else /* !CONFIG_DEBUG_BUGVERBOSE */
> +#define ASM_BUGTABLE_FLAGS(at, file, line, flags) \
> + .pushsection __bug_table, "aw" ; \
> + 123: ASM_BUG_REL(at) ; \
> + ASM_BUGTABLE_VERBOSE(file, line) ; \
> + .word flags ; \
> + .org 123b + 6 + ASM_BUGTABLE_VERBOSE_SIZE ; \
> + .popsection
>
> -#define _BUG_FLAGS(ins, flags, extra) \
> +#define _BUG_FLAGS(ins, flags, extra, extra_args...) \
> do { \
> asm_inline volatile("1:\t" ins "\n" \
> - ".pushsection __bug_table,\"aw\"\n" \
> - "2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n" \
> - "\t.word %c0" "\t# bug_entry::flags\n" \
> - "\t.org 2b+%c1\n" \
> - ".popsection\n" \
> - extra \
> - : : "i" (flags), \
> - "i" (sizeof(struct bug_entry))); \
> + __stringify(ASM_BUGTABLE_FLAGS(1b, %c0, %c1, %c2)) "\n" \
> + extra \
> + : : "i" (__FILE__), "i" (__LINE__), \
> + "i" (flags), ## extra_args); \
> } while (0)
>
> -#endif /* CONFIG_DEBUG_BUGVERBOSE */
> -
> #else
>
> #define _BUG_FLAGS(ins, flags, extra) asm volatile(ins)
> @@ -100,6 +95,40 @@ do { \
> instrumentation_end(); \
> } while (0)
>
> +#define __WARN_printf_1(taint, format) \
> +do { \
> + __auto_type __flags = BUGFLAG_WARNING | BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint); \
> + unsigned long dummy = 0; \
> + instrumentation_begin(); \
> + asm_inline volatile("1: ud1 %[fmt], %[arg]\n" \
> + __stringify(ASM_BUGTABLE_FLAGS(1b, %c0, %c1, %c2)) "\n" \
> + : : "i" (__FILE__), "i" (__LINE__), \
> + "i" (__flags), [fmt] "r" (format), [arg] "r" (dummy)); \
> + instrumentation_end(); \
> +} while (0)
> +
> +#define __WARN_printf_2(taint, format, _arg) \
> +do { \
> + __auto_type __flags = BUGFLAG_WARNING | BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint); \
> + instrumentation_begin(); \
> + asm_inline volatile("1: ud1 %[fmt], %[arg]\n" \
> + __stringify(ASM_BUGTABLE_FLAGS(1b, %c0, %c1, %c2)) "\n" \
> + : : "i" (__FILE__), "i" (__LINE__), \
> + "i" (__flags), [fmt] "r" (format), [arg] "r" ((unsigned long)(_arg))); \
> + instrumentation_end(); \
> +} while (0)
> +
> +#define __WARN_printf_n(taint, fmt, arg...) do { \
> + instrumentation_begin(); \
> + __warn_printk(fmt, arg); \
> + __WARN_FLAGS(BUGFLAG_NO_CUT_HERE | BUGFLAG_TAINT(taint));\
> + instrumentation_end(); \
> + } while (0)
> +
> +#define WARN_ARGS(X...) __COUNT_ARGS(, ##X, n, n, n, n, n, n, n, n, n, n, n, n, n, 2, 1, 0)
> +
> +#define __WARN_printf(taint, arg...) CONCATENATE(__WARN_printf_, WARN_ARGS(arg))(taint, arg)
This needs docs too. I think this is collapsing 1 and 2 argument WARNs
into the ud1, and anything larger is explicitly calling __warn_printk +
__WARN_FLAGS? If only 1 and 2 args can be collapsed, why reserve 0xfe00
through 0xfeff? I feel like I'm missing something here...
> +
> #include <asm-generic/bug.h>
>
> #endif /* _ASM_X86_BUG_H */
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index 94c0236963c6..b7f69f4addf4 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -81,18 +81,6 @@
>
> DECLARE_BITMAP(system_vectors, NR_VECTORS);
>
> -__always_inline int is_valid_bugaddr(unsigned long addr)
> -{
> - if (addr < TASK_SIZE_MAX)
> - return 0;
> -
> - /*
> - * We got #UD, if the text isn't readable we'd have gotten
> - * a different exception.
> - */
> - return *(unsigned short *)addr == INSN_UD2;
> -}
> -
> /*
> * Check for UD1 or UD2, accounting for Address Size Override Prefixes.
> * If it's a UD1, further decode to determine its use:
> @@ -102,25 +90,37 @@ __always_inline int is_valid_bugaddr(unsigned long addr)
> * UBSan{0}: 67 0f b9 00 ud1 (%eax),%eax
> * UBSan{10}: 67 0f b9 40 10 ud1 0x10(%eax),%eax
> * static_call: 0f b9 cc ud1 %esp,%ecx
> + * WARN_printf: ud1 %reg,%reg
> *
> - * Notably UBSAN uses EAX, static_call uses ECX.
> + * Notably UBSAN uses (%eax), static_call uses %esp,%ecx
> */
> __always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
> {
> unsigned long start = addr;
> + u8 v, rex = 0, reg, rm;
> bool lock = false;
> - u8 v;
> + int type = BUG_UD1;
>
> if (addr < TASK_SIZE_MAX)
> return BUG_NONE;
>
> - v = *(u8 *)(addr++);
> - if (v == INSN_ASOP)
> + for (;;) {
> v = *(u8 *)(addr++);
>
> - if (v == INSN_LOCK) {
> - lock = true;
> - v = *(u8 *)(addr++);
> + if (v == INSN_ASOP)
> + continue;
> +
> + if (v == INSN_LOCK) {
> + lock = true;
> + continue;
> + }
> +
> + if ((v & 0xf0) == 0x40) {
> + rex = v;
> + continue;
> + }
> +
> + break;
> }
>
> switch (v) {
> @@ -156,9 +156,13 @@ __always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
> if (X86_MODRM_MOD(v) != 3 && X86_MODRM_RM(v) == 4)
> addr++; /* SIB */
>
> + reg = X86_MODRM_REG(v) + 8*!!X86_REX_R(rex);
> + rm = X86_MODRM_RM(v) + 8*!!X86_REX_B(rex);
> +
> /* Decode immediate, if present */
> switch (X86_MODRM_MOD(v)) {
> - case 0: if (X86_MODRM_RM(v) == 5)
> + case 0: *imm = 0;
> + if (X86_MODRM_RM(v) == 5)
> addr += 4; /* RIP + disp32 */
> break;
>
> @@ -170,18 +174,37 @@ __always_inline int decode_bug(unsigned long addr, s32 *imm, int *len)
> addr += 4;
> break;
>
> - case 3: break;
> + case 3: if (rm != 4) /* %esp */
> + type = BUG_WARN | (rm << 4) | reg;
> + break;
> }
>
> /* record instruction length */
> *len = addr - start;
>
> - if (X86_MODRM_REG(v) == 0) /* EAX */
> + if (!rm && X86_MODRM_MOD(v) != 3) /* (%eax) */
> return BUG_UD1_UBSAN;
>
> - return BUG_UD1;
> + return type;
> }
>
> +int is_valid_bugaddr(unsigned long addr)
> +{
> + int ud_type, ud_len;
> + u32 ud_imm;
> +
> + if (addr < TASK_SIZE_MAX)
> + return 0;
> +
> + /*
> + * We got #UD, if the text isn't readable we'd have gotten
> + * a different exception.
> + */
> + ud_type = decode_bug(addr, &ud_imm, &ud_len);
> +
> + return ud_type == BUG_UD2 ||
> + (ud_type >= BUG_WARN && ud_type <= BUG_WARN_END);
> +}
>
> static nokprobe_inline int
> do_trap_no_signal(struct task_struct *tsk, int trapnr, const char *str,
> @@ -305,6 +328,14 @@ static inline void handle_invalid_op(struct pt_regs *regs)
> ILL_ILLOPN, error_get_trap_addr(regs));
> }
>
> +static inline unsigned long pt_regs_val(struct pt_regs *regs, int nr)
> +{
> + int offset = pt_regs_offset(regs, nr);
> + if (WARN_ON_ONCE(offset < -0))
> + return 0;
> + return *((unsigned long *)((void *)regs + offset));
> +}
> +
> static noinstr bool handle_bug(struct pt_regs *regs)
> {
> unsigned long addr = regs->ip;
> @@ -334,6 +365,14 @@ static noinstr bool handle_bug(struct pt_regs *regs)
> raw_local_irq_enable();
>
> switch (ud_type) {
> + case BUG_WARN ... BUG_WARN_END:
> + int ud_reg = ud_type & 0xf;
> + int ud_rm = (ud_type >> 4) & 0xf;
> +
> + __warn_printk((const char *)(pt_regs_val(regs, ud_rm)),
> + pt_regs_val(regs, ud_reg));
> + fallthrough;
Yay, internal printk. :):)
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 62b3416f5e43..564513f605ac 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8703,6 +8703,8 @@ void __init sched_init(void)
> preempt_dynamic_init();
>
> scheduler_running = 1;
> +
> + WARN(true, "Ultimate answer: %d\n", 42);
> }
>
> #ifdef CONFIG_DEBUG_ATOMIC_SLEEP
If any code would emit The Answer, it would be the scheduler. :)
--
Kees Cook
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