arm64: debug: call software breakpoint handlers statically

Software breakpoints pass an immediate value in ESR ("comment") that can
be used to call a specialized handler (KGDB, KASAN...).
We do so in two different ways :
 - During early boot, `early_brk64` statically checks against known
   immediates and calls the corresponding handler,
 - During init, handlers are dynamically registered into a list. When
   called, the generic software breakpoint handler will iterate over
   the list to find the appropriate handler.

The dynamic registration does not provide any benefit here as it is not
exported and all its uses are within the arm64 tree. It also depends on an
RCU list, whose safe access currently relies on the non-preemptible state
of `do_debug_exception`.

Replace the list iteration logic in `call_break_hooks` to call
the breakpoint handlers statically if they are enabled, like in
`early_brk64`.
Expose the handlers in their respective headers to be reachable from
`arch/arm64/kernel/debug-monitors.c` at link time.

Unify the naming of the software breakpoint handlers to XXX_brk_handler(),
making it clear they are related and to differentiate from the
hardware breakpoints.

Signed-off-by: Ada Couprie Diaz <ada.coupriediaz@arm.com>
Tested-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Reviewed-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20250707114109.35672-4-ada.coupriediaz@arm.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: Ada Couprie Diaz

arch/arm64/include/asm/kgdb.h

@@ -24,6 +24,9 @@ static inline void arch_kgdb_breakpoint(void)
 extern void kgdb_handle_bus_error(void);
 extern int kgdb_fault_expected;
 
+int kgdb_brk_handler(struct pt_regs *regs, unsigned long esr);
+int kgdb_compiled_brk_handler(struct pt_regs *regs, unsigned long esr);
+
 #endif /* !__ASSEMBLY__ */
 
 /*

arch/arm64/include/asm/kprobes.h

@@ -41,4 +41,12 @@ void __kretprobe_trampoline(void);
 void __kprobes *trampoline_probe_handler(struct pt_regs *regs);
 
 #endif /* CONFIG_KPROBES */
+
+int __kprobes kprobe_brk_handler(struct pt_regs *regs,
+				 unsigned long esr);
+int __kprobes kprobe_ss_brk_handler(struct pt_regs *regs,
+				 unsigned long esr);
+int __kprobes kretprobe_brk_handler(struct pt_regs *regs,
+				 unsigned long esr);
+
 #endif /* _ARM_KPROBES_H */

arch/arm64/include/asm/traps.h

@@ -29,6 +29,12 @@ void arm64_force_sig_fault_pkey(unsigned long far, const char *str, int pkey);
 void arm64_force_sig_mceerr(int code, unsigned long far, short lsb, const char *str);
 void arm64_force_sig_ptrace_errno_trap(int errno, unsigned long far, const char *str);
 
+int bug_brk_handler(struct pt_regs *regs, unsigned long esr);
+int cfi_brk_handler(struct pt_regs *regs, unsigned long esr);
+int reserved_fault_brk_handler(struct pt_regs *regs, unsigned long esr);
+int kasan_brk_handler(struct pt_regs *regs, unsigned long esr);
+int ubsan_brk_handler(struct pt_regs *regs, unsigned long esr);
+
 int early_brk64(unsigned long addr, unsigned long esr, struct pt_regs *regs);
 
 /*

arch/arm64/include/asm/uprobes.h

@@ -28,4 +28,6 @@ struct arch_uprobe {
 	bool simulate;
 };
 
+int uprobe_brk_handler(struct pt_regs *regs, unsigned long esr);
+
 #endif

arch/arm64/kernel/debug-monitors.c

@@ -21,8 +21,11 @@
 #include <asm/cputype.h>
 #include <asm/daifflags.h>
 #include <asm/debug-monitors.h>
+#include <asm/kgdb.h>
+#include <asm/kprobes.h>
 #include <asm/system_misc.h>
 #include <asm/traps.h>
+#include <asm/uprobes.h>
 
 /* Determine debug architecture. */
 u8 debug_monitors_arch(void)
@@ -299,20 +302,47 @@ void unregister_kernel_break_hook(struct break_hook *hook)
 
 static int call_break_hook(struct pt_regs *regs, unsigned long esr)
 {
-	struct break_hook *hook;
-	struct list_head *list;
+	if (user_mode(regs)) {
+		if (IS_ENABLED(CONFIG_UPROBES) &&
+			esr_brk_comment(esr) == UPROBES_BRK_IMM)
+			return uprobe_brk_handler(regs, esr);
+		return DBG_HOOK_ERROR;
+	}
 
-	list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
+	if (esr_brk_comment(esr) == BUG_BRK_IMM)
+		return bug_brk_handler(regs, esr);
 
-	/*
-	 * Since brk exception disables interrupt, this function is
-	 * entirely not preemptible, and we can use rcu list safely here.
-	 */
-	list_for_each_entry_rcu(hook, list, node) {
-		if ((esr_brk_comment(esr) & ~hook->mask) == hook->imm)
-			return hook->fn(regs, esr);
+	if (IS_ENABLED(CONFIG_CFI_CLANG) && esr_is_cfi_brk(esr))
+		return cfi_brk_handler(regs, esr);
+
+	if (esr_brk_comment(esr) == FAULT_BRK_IMM)
+		return reserved_fault_brk_handler(regs, esr);
+
+	if (IS_ENABLED(CONFIG_KASAN_SW_TAGS) &&
+		(esr_brk_comment(esr) & ~KASAN_BRK_MASK) == KASAN_BRK_IMM)
+		return kasan_brk_handler(regs, esr);
+
+	if (IS_ENABLED(CONFIG_UBSAN_TRAP) && esr_is_ubsan_brk(esr))
+		return ubsan_brk_handler(regs, esr);
+
+	if (IS_ENABLED(CONFIG_KGDB)) {
+		if (esr_brk_comment(esr) == KGDB_DYN_DBG_BRK_IMM)
+			return kgdb_brk_handler(regs, esr);
+		if (esr_brk_comment(esr) == KGDB_COMPILED_DBG_BRK_IMM)
+			return kgdb_compiled_brk_handler(regs, esr);
 	}
 
+	if (IS_ENABLED(CONFIG_KPROBES)) {
+		if (esr_brk_comment(esr) == KPROBES_BRK_IMM)
+			return kprobe_brk_handler(regs, esr);
+		if (esr_brk_comment(esr) == KPROBES_BRK_SS_IMM)
+			return kprobe_ss_brk_handler(regs, esr);
+	}
+
+	if (IS_ENABLED(CONFIG_KRETPROBES) &&
+		esr_brk_comment(esr) == KRETPROBES_BRK_IMM)
+		return kretprobe_brk_handler(regs, esr);
+
 	return DBG_HOOK_ERROR;
 }
 NOKPROBE_SYMBOL(call_break_hook);

arch/arm64/kernel/kgdb.c

@@ -234,21 +234,21 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
 	return err;
 }
 
-static int kgdb_brk_fn(struct pt_regs *regs, unsigned long esr)
+int kgdb_brk_handler(struct pt_regs *regs, unsigned long esr)
 {
 	kgdb_handle_exception(1, SIGTRAP, 0, regs);
 	return DBG_HOOK_HANDLED;
 }
-NOKPROBE_SYMBOL(kgdb_brk_fn)
+NOKPROBE_SYMBOL(kgdb_brk_handler)
 
-static int kgdb_compiled_brk_fn(struct pt_regs *regs, unsigned long esr)
+int kgdb_compiled_brk_handler(struct pt_regs *regs, unsigned long esr)
 {
 	compiled_break = 1;
 	kgdb_handle_exception(1, SIGTRAP, 0, regs);
 
 	return DBG_HOOK_HANDLED;
 }
-NOKPROBE_SYMBOL(kgdb_compiled_brk_fn);
+NOKPROBE_SYMBOL(kgdb_compiled_brk_handler);
 
 static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned long esr)
 {
@@ -260,16 +260,6 @@ static int kgdb_step_brk_fn(struct pt_regs *regs, unsigned long esr)
 }
 NOKPROBE_SYMBOL(kgdb_step_brk_fn);
 
-static struct break_hook kgdb_brkpt_hook = {
-	.fn		= kgdb_brk_fn,
-	.imm		= KGDB_DYN_DBG_BRK_IMM,
-};
-
-static struct break_hook kgdb_compiled_brkpt_hook = {
-	.fn		= kgdb_compiled_brk_fn,
-	.imm		= KGDB_COMPILED_DBG_BRK_IMM,
-};
-
 static struct step_hook kgdb_step_hook = {
 	.fn		= kgdb_step_brk_fn
 };
@@ -316,8 +306,6 @@ int kgdb_arch_init(void)
 	if (ret != 0)
 		return ret;
 
-	register_kernel_break_hook(&kgdb_brkpt_hook);
-	register_kernel_break_hook(&kgdb_compiled_brkpt_hook);
 	register_kernel_step_hook(&kgdb_step_hook);
 	return 0;
 }
@@ -329,8 +317,6 @@ int kgdb_arch_init(void)
  */
 void kgdb_arch_exit(void)
 {
-	unregister_kernel_break_hook(&kgdb_brkpt_hook);
-	unregister_kernel_break_hook(&kgdb_compiled_brkpt_hook);
 	unregister_kernel_step_hook(&kgdb_step_hook);
 	unregister_die_notifier(&kgdb_notifier);
 }

arch/arm64/kernel/probes/kprobes.c

@@ -292,8 +292,8 @@ int __kprobes kprobe_fault_handler(struct pt_regs *regs, unsigned int fsr)
 	return 0;
 }
 
-static int __kprobes
-kprobe_breakpoint_handler(struct pt_regs *regs, unsigned long esr)
+int __kprobes
+kprobe_brk_handler(struct pt_regs *regs, unsigned long esr)
 {
 	struct kprobe *p, *cur_kprobe;
 	struct kprobe_ctlblk *kcb;
@@ -336,13 +336,8 @@ kprobe_breakpoint_handler(struct pt_regs *regs, unsigned long esr)
 	return DBG_HOOK_HANDLED;
 }
 
-static struct break_hook kprobes_break_hook = {
-	.imm = KPROBES_BRK_IMM,
-	.fn = kprobe_breakpoint_handler,
-};
-
-static int __kprobes
-kprobe_breakpoint_ss_handler(struct pt_regs *regs, unsigned long esr)
+int __kprobes
+kprobe_ss_brk_handler(struct pt_regs *regs, unsigned long esr)
 {
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
 	unsigned long addr = instruction_pointer(regs);
@@ -360,13 +355,8 @@ kprobe_breakpoint_ss_handler(struct pt_regs *regs, unsigned long esr)
 	return DBG_HOOK_ERROR;
 }
 
-static struct break_hook kprobes_break_ss_hook = {
-	.imm = KPROBES_BRK_SS_IMM,
-	.fn = kprobe_breakpoint_ss_handler,
-};
-
-static int __kprobes
-kretprobe_breakpoint_handler(struct pt_regs *regs, unsigned long esr)
+int __kprobes
+kretprobe_brk_handler(struct pt_regs *regs, unsigned long esr)
 {
 	if (regs->pc != (unsigned long)__kretprobe_trampoline)
 		return DBG_HOOK_ERROR;
@@ -375,11 +365,6 @@ kretprobe_breakpoint_handler(struct pt_regs *regs, unsigned long esr)
 	return DBG_HOOK_HANDLED;
 }
 
-static struct break_hook kretprobes_break_hook = {
-	.imm = KRETPROBES_BRK_IMM,
-	.fn = kretprobe_breakpoint_handler,
-};
-
 /*
  * Provide a blacklist of symbols identifying ranges which cannot be kprobed.
  * This blacklist is exposed to userspace via debugfs (kprobes/blacklist).
@@ -422,9 +407,5 @@ int __kprobes arch_trampoline_kprobe(struct kprobe *p)
 
 int __init arch_init_kprobes(void)
 {
-	register_kernel_break_hook(&kprobes_break_hook);
-	register_kernel_break_hook(&kprobes_break_ss_hook);
-	register_kernel_break_hook(&kretprobes_break_hook);
-
 	return 0;
 }

arch/arm64/kernel/probes/kprobes_trampoline.S

@@ -12,7 +12,7 @@
 SYM_CODE_START(__kretprobe_trampoline)
 	/*
 	 * Trigger a breakpoint exception. The PC will be adjusted by
-	 * kretprobe_breakpoint_handler(), and no subsequent instructions will
+	 * kretprobe_brk_handler(), and no subsequent instructions will
 	 * be executed from the trampoline.
 	 */
 	brk #KRETPROBES_BRK_IMM

arch/arm64/kernel/probes/uprobes.c

@@ -173,7 +173,7 @@ int arch_uprobe_exception_notify(struct notifier_block *self,
 	return NOTIFY_DONE;
 }
 
-static int uprobe_breakpoint_handler(struct pt_regs *regs,
+int uprobe_brk_handler(struct pt_regs *regs,
 				     unsigned long esr)
 {
 	if (uprobe_pre_sstep_notifier(regs))
@@ -194,20 +194,13 @@ static int uprobe_single_step_handler(struct pt_regs *regs,
 	return DBG_HOOK_ERROR;
 }
 
-/* uprobe breakpoint handler hook */
-static struct break_hook uprobes_break_hook = {
-	.imm = UPROBES_BRK_IMM,
-	.fn = uprobe_breakpoint_handler,
-};
-
 /* uprobe single step handler hook */
 static struct step_hook uprobes_step_hook = {
 	.fn = uprobe_single_step_handler,
 };
 
 static int __init arch_init_uprobes(void)
 {
-	register_user_break_hook(&uprobes_break_hook);
 	register_user_step_hook(&uprobes_step_hook);
 
 	return 0;