From 97c358198e73c4d3feff561ebf813e84cfb1ef5a Mon Sep 17 00:00:00 2001 From: Mete Durlu Date: Mon, 31 Aug 2026 11:57:15 +0200 Subject: panic: introduce arch_do_panic Patch series "Introduce arch_do_panic", v6. Replace architecture-specific ifdef sections in vpanic() with a clean arch_do_panic() hook. Currently s390 and sparc embed their panic handlers directly in vpanic() using preprocessor conditionals, making the common code path harder to maintain. Introduce arch_do_panic() as an architecture extension point called at the end of vpanic(). Architectures can use this hook to implement their specific panic handling without polluting the generic panic code. Remove s390s ifdef block in vpanic() and move the corresponding code block to s390s own arch_do_panic() implementation in architecture specific code. Move sparc panic handling from ifdef blocks to arch_do_panic(). Remove the preprocessor conditionals from vpanic() and place the Stop-A enablement code in architecture-specific files where it belongs. Stop-A enablement markers are now printed after "end Kernel panic" line. To me, there are no better alternatives other than setup.c to put sparc's arch_do_panic() implementation. The other files under arch/sparc/kernel are either divided to *_32.c and *_64.c variants, which mean code duplication, or unrelated. The cleanup reduces vpanic() complexity and establishes a pattern for other architectures needing custom panic behavior. No functional changes, only minor print order changes. This patch (of 3): Introduce a hook for architectures to put their specific panic handlers. s390 and sparc already have ifdef preprocessor checks to execute architecture specific code. Pave the way for vpanic() cleanup. Link: https://lore.kernel.org/20260831-arch_do_panic-v6-1-a1e170a9e7fd@linux.ibm.com Link: https://lore.kernel.org/all/20260730-arch_do_panic-v3-0-d5401e683cdb@linux.ibm.com/ [1] Signed-off-by: Mete Durlu Signed-off-by: Andrew Morton Reviewed-by: Bradley Morgan Suggested-by: Sven Schnelle Reviewed-by: Andrew Morton Cc: Alexander Gordeev Cc: Andreas Larsson Cc: Christian Borntraeger Cc: David S. Miller Cc: Heiko Carstens Cc: Petr Mladek Cc: Vasily Gorbik --- kernel/panic.c | 3 +++ 1 file changed, 3 insertions(+) (limited to 'kernel/panic.c') diff --git a/kernel/panic.c b/kernel/panic.c index 213725b612aa..726a97842232 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -567,6 +567,8 @@ static void panic_other_cpus_shutdown(bool crash_kexec) crash_smp_send_stop(); } +void __weak arch_do_panic(void) {} + /** * vpanic - halt the system * @fmt: The text string to print @@ -756,6 +758,7 @@ void vpanic(const char *fmt, va_list args) #endif pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf); + arch_do_panic(); /* Do not scroll important messages printed above */ suppress_printk = 1; -- cgit v1.2.3 From ed26d6fcf5631330a06ad120d6e8a262efa65dde Mon Sep 17 00:00:00 2001 From: Mete Durlu Date: Mon, 31 Aug 2026 11:57:16 +0200 Subject: s390: implement arch_do_panic Implement s390 specific arch_do_panic() instead of using s390 specific ifdef sections in vpanic() code. disabled_wait() is now called after "end Kernel panic" marker. No functional changes. Link: https://lore.kernel.org/20260831-arch_do_panic-v6-2-a1e170a9e7fd@linux.ibm.com Signed-off-by: Mete Durlu Signed-off-by: Andrew Morton Acked-by: Heiko Carstens Reviewed-by: Bradley Morgan Reviewed-by: Andrew Morton Cc: Alexander Gordeev Cc: Andreas Larsson Cc: Christian Borntraeger Cc: David S. Miller Cc: Petr Mladek Cc: Sven Schnelle Cc: Vasily Gorbik --- arch/s390/kernel/traps.c | 7 +++++++ kernel/panic.c | 3 --- 2 files changed, 7 insertions(+), 3 deletions(-) (limited to 'kernel/panic.c') diff --git a/arch/s390/kernel/traps.c b/arch/s390/kernel/traps.c index b6ba4465f59d..115cb3373247 100644 --- a/arch/s390/kernel/traps.c +++ b/arch/s390/kernel/traps.c @@ -26,6 +26,7 @@ #include #include #include +#include #include #include #include @@ -33,6 +34,7 @@ #include #include #include +#include #include "entry.h" struct pgm_stat { @@ -283,6 +285,11 @@ static void monitor_event_exception(struct pt_regs *regs) } } +void arch_do_panic(void) +{ + disabled_wait(); +} + void kernel_stack_invalid(struct pt_regs *regs) { /* diff --git a/kernel/panic.c b/kernel/panic.c index 726a97842232..ee6e3f9e3900 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -752,9 +752,6 @@ void vpanic(const char *fmt, va_list args) pr_emerg("Press Stop-A (L1-A) from sun keyboard or send break\n" "twice on console to return to the boot prom\n"); } -#endif -#if defined(CONFIG_S390) - disabled_wait(); #endif pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf); -- cgit v1.2.3 From 1d4408578567d1b3c7086be37986164982791f19 Mon Sep 17 00:00:00 2001 From: Mete Durlu Date: Mon, 31 Aug 2026 11:57:17 +0200 Subject: sparc: implement arch_do_panic Implement sparc specific arch_do_panic() instead of using sparc specific ifdef sections in vpanic() code. Reorder arch specific panic handling, sparc's Stop-A messages are now printed after "end Kernel panic" marker. Link: https://lore.kernel.org/20260831-arch_do_panic-v6-3-a1e170a9e7fd@linux.ibm.com Signed-off-by: Mete Durlu Signed-off-by: Andrew Morton Reviewed-by: Bradley Morgan Reviewed-by: Andrew Morton Cc: Alexander Gordeev Cc: Andreas Larsson Cc: Christian Borntraeger Cc: David S. Miller Cc: Heiko Carstens Cc: Petr Mladek Cc: Sven Schnelle Cc: Vasily Gorbik --- arch/sparc/kernel/setup.c | 9 +++++++++ kernel/panic.c | 9 --------- 2 files changed, 9 insertions(+), 9 deletions(-) (limited to 'kernel/panic.c') diff --git a/arch/sparc/kernel/setup.c b/arch/sparc/kernel/setup.c index 4975867d9001..5f43cef80638 100644 --- a/arch/sparc/kernel/setup.c +++ b/arch/sparc/kernel/setup.c @@ -2,6 +2,8 @@ #include #include +#include +#include static const struct ctl_table sparc_sysctl_table[] = { { @@ -36,6 +38,13 @@ static const struct ctl_table sparc_sysctl_table[] = { #endif }; +void arch_do_panic(void) +{ + /* Make sure the user can actually press Stop-A (L1-A) */ + stop_a_enabled = 1; + pr_emerg("Press Stop-A (L1-A) from sun keyboard or send break\n" + "twice on console to return to the boot prom\n"); +} static int __init init_sparc_sysctls(void) { diff --git a/kernel/panic.c b/kernel/panic.c index ee6e3f9e3900..7dda841c16f9 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -744,15 +744,6 @@ void vpanic(const char *fmt, va_list args) reboot_mode = panic_reboot_mode; emergency_restart(); } -#ifdef __sparc__ - { - extern int stop_a_enabled; - /* Make sure the user can actually press Stop-A (L1-A) */ - stop_a_enabled = 1; - pr_emerg("Press Stop-A (L1-A) from sun keyboard or send break\n" - "twice on console to return to the boot prom\n"); - } -#endif pr_emerg("---[ end Kernel panic - not syncing: %s ]---\n", buf); arch_do_panic(); -- cgit v1.2.3 From 033abd1e9bf99f3004ed1932d4880c3c941c3834 Mon Sep 17 00:00:00 2001 From: Feng Tang Date: Wed, 2 Sep 2026 19:48:51 +0800 Subject: panic: remove the unneeded panic_print_get() panic_print_get() was introduced in commit 2683df6539cb ("panic: add note that 'panic_print' parameter is deprecated") to print out warning message of the deprecation of 'panic_print' on read access. Since commit 90f3c123247e ("panic: only warn about deprecated panic_print on write access"), panic_print_get() wrapper is not needed anymore for read access, so remove it and use param_get_ulong() instead. Link: https://lore.kernel.org/20260902114851.77062-1-feng.tang@linux.alibaba.com Signed-off-by: Feng Tang Signed-off-by: Andrew Morton Reviewed-by: Bradley Morgan Reviewed-by: Andrew Morton Reviewed-by: Petr Mladek --- kernel/panic.c | 7 +------ 1 file changed, 1 insertion(+), 6 deletions(-) (limited to 'kernel/panic.c') diff --git a/kernel/panic.c b/kernel/panic.c index 7dda841c16f9..50715f14cf04 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -1216,14 +1216,9 @@ static int panic_print_set(const char *val, const struct kernel_param *kp) return param_set_ulong(val, kp); } -static int panic_print_get(char *val, const struct kernel_param *kp) -{ - return param_get_ulong(val, kp); -} - static const struct kernel_param_ops panic_print_ops = { .set = panic_print_set, - .get = panic_print_get, + .get = param_get_ulong, }; __core_param_cb(panic_print, &panic_print_ops, &panic_print, 0644); -- cgit v1.2.3 From c05e2c32cd322ee2899501feda5b746c14207c0e Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Wed, 16 Sep 2026 18:29:52 +0000 Subject: panic: fix redirect CPU race in panic_try_force_cpu() Patch series "panic: fix panic_force_cpu= redirect races and NMI bypass", v7. The panic_force_cpu= parameter redirects a panic to a specific CPU so the crash kernel runs there. The redirect code in panic_try_force_cpu() had two races and an NMI bypass, all found by Sashiko. This series closes them and kills one more hack that the rework turned up. Panic from NMI still goes through the redirect path first, so the crash kernel ends up on the CPU the admin asked for. The redirect buffer is a static 1KB now, no initcall, no fallback message. This patch (of 6): The cmpxchg() in panic_try_force_cpu() makes sure that only one CPU tries to redirect panic() to the requested CPU. It is similar to the cmpxchg() in panic_try_start() which makes sure that only one CPU does the panic(). In both situations, only the winner of cmpxchg() should proceed further. Other CPUs should go offline. There is a bug because the cmpxchg loser returns false and falls through into vpanic(). Two non-target CPUs A and B panic, the requested CPU is C: cpu A cpu B ---------- ---------- panic() panic() vpanic() vpanic() panic_try_force_cpu() panic_try_force_cpu() cmpxchg wins cmpxchg fails redirect = A old_cpu = A IPI -> C return false <- BUG return true panic_try_start() wins panic_smp_self_stop() __crash_kexec() on B (A stops) (target C bypassed) The loser must stop, not fall through. It cannot just return true, though. A CPU that already won the redirect cmpxchg can reenter panic_try_force_cpu() on the same CPU, for example a nested NMI during the message formatting, before the IPI is sent: cpu A (1st) cpu A (nested) ---------- ---------- panic() vpanic() panic_try_force_cpu() cmpxchg wins (redirect = A) vsnprintf(msg) ... <-- NMI, nested panic --> panic() vpanic() panic_try_force_cpu() cmpxchg fails old_cpu == A (this CPU) return true <- would halt panic_smp_self_stop() (IPI never sent, panic abandoned) Check old_cpu against this_cpu so a second call from the same CPU returns false and falls through to panic_try_start() instead. Also fix the panic_in_progress() check. We must not redirect when panic_cpu is already assigned. Return true to stop when the panic is on another CPU, false to proceed when it is this one. Update the panic_try_force_cpu() doc comment for the new return value semantics. Link: https://lore.kernel.org/20260916182957.7788-1-brads@mainlining.org Link: https://lore.kernel.org/20260916182957.7788-2-brads@mainlining.org Fixes: 2e171ab29f91 ("panic: add panic_force_cpu= parameter to redirect panic to a specific CPU") Signed-off-by: Bradley Morgan Signed-off-by: Andrew Morton Reported-by: Sashiko Closes: https://sashiko.dev/#/patchset/20260705164123.18746-1-include@grrlz.net Closes: https://sashiko.dev/#/patchset/20260707172252.4842-1-include@grrlz.net Reviewed-by: Petr Mladek Cc: Guenetr Roeck Cc: Wang Jinchao Cc: Wim Van Sebroeck Cc: --- kernel/panic.c | 19 ++++++++++++------- 1 file changed, 12 insertions(+), 7 deletions(-) (limited to 'kernel/panic.c') diff --git a/kernel/panic.c b/kernel/panic.c index 50715f14cf04..08072bfae422 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -371,8 +371,9 @@ int __weak panic_smp_redirect_cpu(int target_cpu, void *msg) * for the crash kernel to function correctly. This function redirects * panic handling to the CPU specified via the panic_force_cpu= boot parameter. * - * Returns false if panic should proceed on current CPU. - * Returns true if panic was redirected. + * Returns true when this CPU must stop: the panic was redirected or is + * already running on another CPU. + * Returns false when panic() should proceed on this CPU. */ __printf(1, 0) static bool panic_try_force_cpu(const char *fmt, va_list args) @@ -396,16 +397,20 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) return false; } - /* Another panic already in progress */ + /* + * Don't redirect when a panic is already in progress. Stop this + * CPU when it's another one, proceed when it's this one. + */ if (panic_in_progress()) - return false; + return panic_on_other_cpu(); /* - * Only one CPU can do the redirect. Use atomic cmpxchg to ensure - * we don't race with another CPU also trying to redirect. + * Only one CPU can do the redirection. Others should go offline. + * Continue with panic() when we already tried the redirection + * from this CPU before, for example via nmi_panic(). */ if (!atomic_try_cmpxchg(&panic_redirect_cpu, &old_cpu, this_cpu)) - return false; + return old_cpu != this_cpu; /* * Use dynamically allocated buffer if available, otherwise -- cgit v1.2.3 From 50ce3620bee380cd2e136803ff66cc8c997d6d7c Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Wed, 16 Sep 2026 18:29:53 +0000 Subject: panic: flatten nmi_panic control flow panic() is __noreturn, so the else after panic_try_start() is dead. Drop it so the force_cpu path can be added cleanly on top. Link: https://lore.kernel.org/20260916182957.7788-3-brads@mainlining.org Signed-off-by: Bradley Morgan Signed-off-by: Andrew Morton Reviewed-by: Petr Mladek Cc: Guenetr Roeck Cc: Sashiko Cc: Wang Jinchao Cc: Wim Van Sebroeck Cc: --- kernel/panic.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'kernel/panic.c') diff --git a/kernel/panic.c b/kernel/panic.c index 08072bfae422..5646d4fb82b7 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -517,7 +517,8 @@ void nmi_panic(struct pt_regs *regs, const char *msg) { if (panic_try_start()) panic("%s", msg); - else if (panic_on_other_cpu()) + + if (panic_on_other_cpu()) nmi_panic_self_stop(regs); } EXPORT_SYMBOL(nmi_panic); -- cgit v1.2.3 From e97834b034a3d4de1f3c305a6dfcef7bb14989ad Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Wed, 16 Sep 2026 18:29:54 +0000 Subject: panic: fix va_list reuse in panic_try_force_cpu() vsnprintf() consumes the caller's va_list. When the redirect fails, vpanic() reuses it for the panic message, which is undefined behavior. Use va_copy(). Link: https://lore.kernel.org/20260916182957.7788-4-brads@mainlining.org Fixes: 2e171ab29f91 ("panic: add panic_force_cpu= parameter to redirect panic to a specific CPU") Signed-off-by: Bradley Morgan Signed-off-by: Andrew Morton Reviewed-by: Petr Mladek Cc: Guenetr Roeck Cc: Sashiko Cc: Wang Jinchao Cc: Wim Van Sebroeck Cc: --- kernel/panic.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) (limited to 'kernel/panic.c') diff --git a/kernel/panic.c b/kernel/panic.c index 5646d4fb82b7..7388eb81a1c4 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -417,7 +417,12 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) * fall back to static message for early boot panics or allocation failure. */ if (panic_force_buf) { - vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, args); + va_list ap; + + /* Do not consume args, the caller reuses it if we fail */ + va_copy(ap, args); + vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, ap); + va_end(ap); msg = panic_force_buf; } else { msg = "Redirected panic (buffer unavailable)"; -- cgit v1.2.3 From 9cbe4c3344734464263ac4561fe4f639d332fc6e Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Wed, 16 Sep 2026 18:29:55 +0000 Subject: panic: restore variable arguments to nmi_panic() nmi_panic() used to accept variable arguments until commit ebc41f20d77f ("panic: change nmi_panic from macro to function") flattened it to a final message string. vpanic() did not exist back then, so the function had to format through panic("%s", msg). Bring the variable arguments back and format with vpanic() directly. The next patch makes nmi_panic() try the panic_force_cpu= redirect before claiming panic_cpu. panic_try_force_cpu() needs to print the message but it is used also by panic() which accepts variable argument list. The string could be formatted only when a CPU gets assigned to process the redirection. Otherwise, there might be a race when writing to the `panic_force_buf`. No current caller passes a string with format specifiers. The closest one is hpwdt_pretimeout(), which builds panic_msg with hex_byte_pack() and has only two variants, both plain strings. But the new __printf() annotation on nmi_panic() would warn with -Wformat-security there because the buffer is passed directly as the format argument, so switch it to nmi_panic(regs, "%s", panic_msg). [pmladek@suse.com: changelog fix] Link: https://lore.kernel.org/arJxFybmtD7OBIcL@pathway.suse.cz Link: https://lore.kernel.org/20260916182957.7788-5-brads@mainlining.org Signed-off-by: Bradley Morgan Signed-off-by: Andrew Morton Suggested-by: Petr Mladek Reviewed-by: Petr Mladek Cc: Guenetr Roeck Cc: Sashiko Cc: Wang Jinchao Cc: Wim Van Sebroeck Cc: --- drivers/watchdog/hpwdt.c | 2 +- include/linux/panic.h | 3 ++- kernel/panic.c | 10 ++++++++-- 3 files changed, 11 insertions(+), 4 deletions(-) (limited to 'kernel/panic.c') diff --git a/drivers/watchdog/hpwdt.c b/drivers/watchdog/hpwdt.c index 8af1fad2de0b..78227d200afe 100644 --- a/drivers/watchdog/hpwdt.c +++ b/drivers/watchdog/hpwdt.c @@ -199,7 +199,7 @@ static int hpwdt_pretimeout(unsigned int ulReason, struct pt_regs *regs) } hex_byte_pack(panic_msg, nmistat); - nmi_panic(regs, panic_msg); + nmi_panic(regs, "%s", panic_msg); return NMI_HANDLED; } diff --git a/include/linux/panic.h b/include/linux/panic.h index 98dd7dfd27de..17e61b61c45f 100644 --- a/include/linux/panic.h +++ b/include/linux/panic.h @@ -13,7 +13,8 @@ __printf(1, 2) void panic(const char *fmt, ...) __noreturn __cold; __printf(1, 0) void vpanic(const char *fmt, va_list args) __noreturn __cold; -void nmi_panic(struct pt_regs *regs, const char *msg); +__printf(2, 3) +void nmi_panic(struct pt_regs *regs, const char *fmt, ...); void check_panic_on_warn(const char *origin); extern void oops_enter(void); extern void oops_exit(void); diff --git a/kernel/panic.c b/kernel/panic.c index 7388eb81a1c4..e240ca06faab 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -518,13 +518,19 @@ EXPORT_SYMBOL(panic_on_other_cpu); * nmi_panic_self_stop() which can provide architecture dependent code such * as saving register state for crash dump. */ -void nmi_panic(struct pt_regs *regs, const char *msg) +void nmi_panic(struct pt_regs *regs, const char *fmt, ...) { + va_list args; + + va_start(args, fmt); + if (panic_try_start()) - panic("%s", msg); + vpanic(fmt, args); if (panic_on_other_cpu()) nmi_panic_self_stop(regs); + + va_end(args); } EXPORT_SYMBOL(nmi_panic); -- cgit v1.2.3 From 9e3a9fd1f684bd59b70ced64b6709c11e13bd4c7 Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Wed, 16 Sep 2026 18:29:56 +0000 Subject: panic: allow force_cpu redirect from an NMI nmi_panic() claims panic_cpu via panic_try_start() before calling panic(). When the panic later reaches panic_try_force_cpu(), the panic_in_progress() check sees panic_cpu set and refuses to redirect. The crash kernel runs on the CPU that took the NMI instead of the CPU requested with panic_force_cpu=: nmi_panic() panic_try_start() wins, panic_cpu = X panic("%s", msg) vpanic() panic_try_force_cpu() panic_in_progress() true, panic_cpu is X return false redirect bypassed panic_try_start() already won __crash_kexec() on X, not the requested CPU Try the redirect before claiming panic_cpu instead, as suggested by Petr Mladek. nmi_panic() now calls panic_try_force_cpu() first and claims panic_cpu only when no redirect happened. The requested CPU claims panic_cpu itself when it runs panic(), so panic_cpu does not need to be handed off. panic_try_force_cpu() copies the arguments before formatting (patch 3), so nmi_panic() can pass them to vpanic() again when no redirect happens. The redirect IPI is sent with smp_call_function_single_async(), which is not guaranteed to work from NMI context. Treat it as best effort. It is worth the risk because the redirection is only used when the crash kernel would not work on the panicking CPU anyway. Keep returning when the panic is already running on this CPU. A nested NMI, for example with unknown_nmi_panic while this CPU is inside panic(), must return and let the interrupted panic() continue instead of parking the CPU in nmi_panic_self_stop(). Mark the redirecting CPU offline before stopping it, like vpanic() does, so that panic_other_cpus_shutdown() on the target CPU does not wait for it. Link: https://lore.kernel.org/20260916182957.7788-6-brads@mainlining.org Fixes: 2e171ab29f91 ("panic: add panic_force_cpu= parameter to redirect panic to a specific CPU") Signed-off-by: Bradley Morgan Signed-off-by: Andrew Morton Reported-by: Sashiko Closes: https://sashiko.dev/#/patchset/20260708164312.19044-1-include@grrlz.net Reviewed-by: Petr Mladek Cc: Guenetr Roeck Cc: Wang Jinchao Cc: Wim Van Sebroeck Cc: --- kernel/panic.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) (limited to 'kernel/panic.c') diff --git a/kernel/panic.c b/kernel/panic.c index e240ca06faab..29c981926f5f 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -513,10 +513,11 @@ bool panic_on_other_cpu(void) EXPORT_SYMBOL(panic_on_other_cpu); /* - * A variant of panic() called from NMI context. We return if we've already - * panicked on this CPU. If another CPU already panicked, loop in - * nmi_panic_self_stop() which can provide architecture dependent code such - * as saving register state for crash dump. + * A variant of panic() called from NMI context. The panic is first + * redirected to the CPU requested via panic_force_cpu=, when configured. + * We return if we've already panicked on this CPU. If another CPU already + * panicked, loop in nmi_panic_self_stop() which can provide architecture + * dependent code for saving register state for crash dump. */ void nmi_panic(struct pt_regs *regs, const char *fmt, ...) { @@ -524,6 +525,16 @@ void nmi_panic(struct pt_regs *regs, const char *fmt, ...) va_start(args, fmt); + /* Try to redirect to the requested CPU before claiming panic_cpu. */ + if (panic_try_force_cpu(fmt, args)) { + /* + * Mark ourselves offline so panic_other_cpus_shutdown() won't + * wait for us on architectures that check num_online_cpus(). + */ + set_cpu_online(raw_smp_processor_id(), false); + nmi_panic_self_stop(regs); + } + if (panic_try_start()) vpanic(fmt, args); -- cgit v1.2.3 From 56de113da14706d6a1b28648e9ff5c261f536201 Mon Sep 17 00:00:00 2001 From: Bradley Morgan Date: Wed, 16 Sep 2026 18:29:57 +0000 Subject: panic: kill the "buffer unavailable" redirect fallback The redirect buffer is a disgusting terrible hack. panic_force_buf is kmalloc'ed in a late_initcall, and until then the redirect delivers this as the panic message: Redirected panic (buffer unavailable) The whole point of the redirect is to hand the panic message to the target CPU, so the crash kernel boots knowing it panicked and not why. And that window is the entire boot, from the early_param to the late_initcall, which is exactly when you most want the message. Make it a static 1KB buffer and kill the initcall. The cost is 1KB of .bss in SMP crash dump builds, and it is only ever touched when panic_force_cpu= is set anyway. The local msg variable is gone too, the buffer goes directly to panic_smp_redirect_cpu(). Link: https://lore.kernel.org/20260916182957.7788-7-brads@mainlining.org Signed-off-by: Bradley Morgan Signed-off-by: Andrew Morton Suggested-by: Petr Mladek Reviewed-by: Petr Mladek Cc: Guenetr Roeck Cc: Sashiko Cc: Wang Jinchao Cc: Wim Van Sebroeck Cc: --- kernel/panic.c | 34 +++++++--------------------------- 1 file changed, 7 insertions(+), 27 deletions(-) (limited to 'kernel/panic.c') diff --git a/kernel/panic.c b/kernel/panic.c index 29c981926f5f..170744163fc2 100644 --- a/kernel/panic.c +++ b/kernel/panic.c @@ -307,7 +307,7 @@ atomic_t panic_cpu = ATOMIC_INIT(PANIC_CPU_INVALID); atomic_t panic_redirect_cpu = ATOMIC_INIT(PANIC_CPU_INVALID); #if defined(CONFIG_SMP) && defined(CONFIG_CRASH_DUMP) -static char *panic_force_buf; +static char panic_force_buf[PANIC_MSG_BUFSZ]; static int __init panic_force_cpu_setup(char *str) { @@ -326,17 +326,6 @@ static int __init panic_force_cpu_setup(char *str) } early_param("panic_force_cpu", panic_force_cpu_setup); -static int __init panic_force_cpu_late_init(void) -{ - if (panic_force_cpu < 0) - return 0; - - panic_force_buf = kmalloc(PANIC_MSG_BUFSZ, GFP_KERNEL); - - return 0; -} -late_initcall(panic_force_cpu_late_init); - static void do_panic_on_target_cpu(void *info) { panic("%s", (char *)info); @@ -380,7 +369,7 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) { int this_cpu = raw_smp_processor_id(); int old_cpu = PANIC_CPU_INVALID; - const char *msg; + va_list ap; /* Feature not enabled via boot parameter */ if (panic_force_cpu < 0) @@ -413,20 +402,11 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) return old_cpu != this_cpu; /* - * Use dynamically allocated buffer if available, otherwise - * fall back to static message for early boot panics or allocation failure. + * Do not consume args, the caller reuses them if we fail. */ - if (panic_force_buf) { - va_list ap; - - /* Do not consume args, the caller reuses it if we fail */ - va_copy(ap, args); - vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, ap); - va_end(ap); - msg = panic_force_buf; - } else { - msg = "Redirected panic (buffer unavailable)"; - } + va_copy(ap, args); + vsnprintf(panic_force_buf, PANIC_MSG_BUFSZ, fmt, ap); + va_end(ap); console_verbose(); bust_spinlocks(1); @@ -441,7 +421,7 @@ static bool panic_try_force_cpu(const char *fmt, va_list args) dump_stack(); } - if (panic_smp_redirect_cpu(panic_force_cpu, (void *)msg) != 0) { + if (panic_smp_redirect_cpu(panic_force_cpu, panic_force_buf) != 0) { atomic_set(&panic_redirect_cpu, PANIC_CPU_INVALID); pr_warn("panic: failed to redirect to CPU %d, continuing on CPU %d\n", panic_force_cpu, this_cpu); -- cgit v1.2.3