[PATCH 0/2] ARM: v7m: handle faults and enable debugging

[PATCH 0/2] ARM: v7m: handle faults and enable debugging

[Ben Wolsieffer]

This series enables real exception handlers on no-MMU systems,
increasing robustness in the face of buggy user- or kernel-space
software. Previously, any fault would trigger the invalid exception
handler, which would hang the system. With this series, faults only
kill the offending process and allow the rest of the system to
continue operating.

The second patch in this series adds support for undefined instruction
hooks, enabling software breakpoints through ptrace. Using this
functionality currently requires a patch to gdb.

This series has been tested on an STM32F746 (Cortex-M7).

I would appreciate feedback in particular on the following questions:
* Is the fault table formatting acceptable? Or should the lines be
  wrapped/shortened?
* Does my chosen mapping between faults and signals make sense?


Ben Wolsieffer (2):
  ARM: v7m: handle faults
  ARM: v7m: support undefined instruction hooks

 arch/arm/include/asm/traps.h |   2 +
 arch/arm/include/asm/v7m.h   |  29 +++++++
 arch/arm/kernel/Makefile     |   2 +-
 arch/arm/kernel/entry-v7m.S  |  68 ++++++++++++++-
 arch/arm/kernel/traps-v7m.c  | 162 +++++++++++++++++++++++++++++++++++
 5 files changed, 258 insertions(+), 5 deletions(-)
 create mode 100644 arch/arm/kernel/traps-v7m.c

-- 
2.37.0


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[PATCH 1/2] ARM: v7m: handle faults

[Ben Wolsieffer]

Currently, any fault on an ARMv7M system will trigger the invalid entry
exception handler, bringing down the entire system.

This patch installs real handlers for the hard fault, memmanage, bus
fault and usage fault exceptions. For kernel space faults, an oops is
triggered, while user space faults kill the offending process with an
appropriate signal. The fault status registers are parsed to construct
an appropriate message and signal.

This is based on a patch from Emcraft Systems' kernel tree [1], but I
have significantly reworked it.

[1] https://github.com/EmcraftSystems/linux-upstream/commit/2882de1d86bd536c855feee582d44722434c2ac9

Signed-off-by: Ben Wolsieffer <Ben.Wolsieffer@hefring.com>
---
 arch/arm/include/asm/v7m.h  |  29 +++++++
 arch/arm/kernel/Makefile    |   2 +-
 arch/arm/kernel/entry-v7m.S |  68 +++++++++++++++-
 arch/arm/kernel/traps-v7m.c | 156 ++++++++++++++++++++++++++++++++++++
 4 files changed, 250 insertions(+), 5 deletions(-)
 create mode 100644 arch/arm/kernel/traps-v7m.c

diff --git a/arch/arm/include/asm/v7m.h b/arch/arm/include/asm/v7m.h
index 4512f7e1918f..b8c636a0578d 100644
--- a/arch/arm/include/asm/v7m.h
+++ b/arch/arm/include/asm/v7m.h
@@ -38,6 +38,35 @@
 #define V7M_SCB_SHCSR_BUSFAULTENA		(1 << 17)
 #define V7M_SCB_SHCSR_MEMFAULTENA		(1 << 16)
 
+#define V7M_SCB_CFSR			0x28
+
+#define V7M_SCB_CFSR_DIVBYZERO			(1 << 25)
+#define V7M_SCB_CFSR_UNALIGNED			(1 << 24)
+#define V7M_SCB_CFSR_NOCP			(1 << 19)
+#define V7M_SCB_CFSR_INVPC			(1 << 18)
+#define V7M_SCB_CFSR_INVSTATE			(1 << 17)
+#define V7M_SCB_CFSR_UNDEFINSTR			(1 << 16)
+#define V7M_SCB_CFSR_BFARVALID			(1 << 15)
+#define V7M_SCB_CFSR_LSPERR			(1 << 13)
+#define V7M_SCB_CFSR_STKERR			(1 << 12)
+#define V7M_SCB_CFSR_UNSTKERR			(1 << 11)
+#define V7M_SCB_CFSR_IMPRECISERR		(1 << 10)
+#define V7M_SCB_CFSR_PRECISERR			(1 << 9)
+#define V7M_SCB_CFSR_IBUSERR			(1 << 8)
+#define V7M_SCB_CFSR_MMARVALID			(1 << 7)
+#define V7M_SCB_CFSR_MLSPERR			(1 << 5)
+#define V7M_SCB_CFSR_MSTKERR			(1 << 4)
+#define V7M_SCB_CFSR_MUNSTKERR			(1 << 3)
+#define V7M_SCB_CFSR_DACCVIOL			(1 << 1)
+#define V7M_SCB_CFSR_IACCVIOL			(1 << 0)
+
+#define V7M_SCB_HFSR			0x2c
+#define V7M_SCB_HFSR_FORCED			(1 << 30)
+#define V7M_SCB_HFSR_VECTTBL			(1 << 1)
+
+#define V7M_SCB_MMAR			0x34
+#define V7M_SCB_BFAR			0x38
+
 #define V7M_xPSR_FRAMEPTRALIGN			0x00000200
 #define V7M_xPSR_EXCEPTIONNO			V7M_SCB_ICSR_VECTACTIVE
 
diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
index 553866751e1a..5d3e66ebc77d 100644
--- a/arch/arm/kernel/Makefile
+++ b/arch/arm/kernel/Makefile
@@ -34,7 +34,7 @@ obj-$(CONFIG_ATAGS_PROC)	+= atags_proc.o
 obj-$(CONFIG_DEPRECATED_PARAM_STRUCT) += atags_compat.o
 
 ifeq ($(CONFIG_CPU_V7M),y)
-obj-y		+= entry-v7m.o v7m.o
+obj-y		+= entry-v7m.o v7m.o traps-v7m.o
 else
 obj-y		+= entry-armv.o
 endif
diff --git a/arch/arm/kernel/entry-v7m.S b/arch/arm/kernel/entry-v7m.S
index de8a60363c85..62909731cd1f 100644
--- a/arch/arm/kernel/entry-v7m.S
+++ b/arch/arm/kernel/entry-v7m.S
@@ -84,6 +84,66 @@ __irq_entry:
 	bx	lr
 ENDPROC(__irq_entry)
 
+__hardfault_entry:
+	v7m_exception_entry
+
+	@
+	@ Invoke the Hard Fault handler
+	@ routine called with r0 = struct pt_regs *
+	mov	r0, sp
+	bl	do_hardfault
+
+	@ execute the pending work, including reschedule
+	get_thread_info tsk
+	mov	why, #0
+	b	ret_to_user_from_irq
+ENDPROC(__hardfault_entry)
+
+__memmanage_entry:
+	v7m_exception_entry
+
+	@
+	@ Invoke the Mem Manage handler
+	@ routine called with r0 = struct pt_regs *
+	mov	r0, sp
+	bl	do_memmanage
+
+	@ execute the pending work, including reschedule
+	get_thread_info tsk
+	mov	why, #0
+	b	ret_to_user_from_irq
+ENDPROC(__memmanage_entry)
+
+__busfault_entry:
+	v7m_exception_entry
+
+	@
+	@ Invoke the Bus Fault handler
+	@ routine called with r0 = struct pt_regs *
+	mov	r0, sp
+	bl	do_busfault
+
+	@ execute the pending work, including reschedule
+	get_thread_info tsk
+	mov	why, #0
+	b	ret_to_user_from_irq
+ENDPROC(__busfault_entry)
+
+__usagefault_entry:
+	v7m_exception_entry
+
+	@
+	@ Invoke the Bus Fault handler
+	@ routine called with r0 = struct pt_regs *
+	mov	r0, sp
+	bl	do_usagefault
+
+	@ execute the pending work, including reschedule
+	get_thread_info tsk
+	mov	why, #0
+	b	ret_to_user_from_irq
+ENDPROC(__usagefault_entry)
+
 __pendsv_entry:
 	v7m_exception_entry
 
@@ -138,10 +198,10 @@ ENTRY(vector_table)
 	.long	0			@ 0 - Reset stack pointer
 	.long	__invalid_entry		@ 1 - Reset
 	.long	__invalid_entry		@ 2 - NMI
-	.long	__invalid_entry		@ 3 - HardFault
-	.long	__invalid_entry		@ 4 - MemManage
-	.long	__invalid_entry		@ 5 - BusFault
-	.long	__invalid_entry		@ 6 - UsageFault
+	.long	__hardfault_entry	@ 3 - HardFault
+	.long	__memmanage_entry	@ 4 - MemManage
+	.long	__busfault_entry	@ 5 - BusFault
+	.long	__usagefault_entry	@ 6 - UsageFault
 	.long	__invalid_entry		@ 7 - Reserved
 	.long	__invalid_entry		@ 8 - Reserved
 	.long	__invalid_entry		@ 9 - Reserved
diff --git a/arch/arm/kernel/traps-v7m.c b/arch/arm/kernel/traps-v7m.c
new file mode 100644
index 000000000000..5fd9943448e9
--- /dev/null
+++ b/arch/arm/kernel/traps-v7m.c
@@ -0,0 +1,156 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2011 Dmitry Cherukhin, Emcraft Systems
+ * Copyright (C) 2022 Ben Wolsieffer, Hefring Engineering
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ */
+
+#include <linux/io.h>
+#include <linux/sched.h>
+
+#include <asm/linkage.h>
+#include <asm/ptrace.h>
+#include <asm/system_misc.h>
+#include <asm/v7m.h>
+
+enum fault {
+	FAULT_HARDFALT = 3,
+	FAULT_MEMMANAGE = 4,
+	FAULT_BUSFAULT = 5,
+	FAULT_USAGEFAULT = 6
+};
+
+struct exception {
+	const char	*name;
+	int		fault;
+	int		test_bit;
+	int		signo;
+	int		si_code;
+	unsigned int	user_debug;
+};
+
+static const struct exception unknown_exception =
+	{"unknown exception",			0,			0,				SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV};
+
+static const struct exception exceptions[] = {
+	{"escalated to hard fault",		FAULT_HARDFALT,		V7M_SCB_HFSR_FORCED,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"vector read error",			FAULT_HARDFALT,		V7M_SCB_HFSR_VECTTBL,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"divide by 0",				FAULT_USAGEFAULT,	V7M_SCB_CFSR_DIVBYZERO,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"illegal unaligned access",		FAULT_USAGEFAULT,	V7M_SCB_CFSR_UNALIGNED,		SIGBUS,		BUS_ADRALN,	UDBG_BUS},
+	{"no coprocessor",			FAULT_USAGEFAULT,	V7M_SCB_CFSR_NOCP,		SIGILL,		ILL_COPROC,	UDBG_UNDEFINED},
+	{"return to invalid PC",		FAULT_USAGEFAULT,	V7M_SCB_CFSR_INVPC,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"invalid ISA state",			FAULT_USAGEFAULT,	V7M_SCB_CFSR_INVSTATE,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"undefined instruction",		FAULT_USAGEFAULT,	V7M_SCB_CFSR_UNDEFINSTR,	SIGILL,		ILL_ILLOPC,	UDBG_UNDEFINED},
+	{"floating point state error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_LSPERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
+	{"exception stack push error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_STKERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
+	{"exception stack pop error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_UNSTKERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
+	{"imprecise data bus error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_IMPRECISERR,	SIGBUS,		BUS_ADRERR,	UDBG_BUS},
+	{"precise data bus error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_PRECISERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
+	{"pre-fetch error",			FAULT_BUSFAULT,		V7M_SCB_CFSR_IBUSERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
+	{"floating point state error",		FAULT_MEMMANAGE,	V7M_SCB_CFSR_MLSPERR,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"exception stack push error",		FAULT_MEMMANAGE,	V7M_SCB_CFSR_MSTKERR,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"exception stack pop error",		FAULT_MEMMANAGE,	V7M_SCB_CFSR_MUNSTKERR,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
+	{"data access violation",		FAULT_MEMMANAGE,	V7M_SCB_CFSR_DACCVIOL,		SIGSEGV,	SEGV_ACCERR,	UDBG_SEGV},
+	{"instruction access violation",	FAULT_MEMMANAGE,	V7M_SCB_CFSR_IACCVIOL,		SIGSEGV,	SEGV_ACCERR,	UDBG_SEGV},
+	{NULL}
+};
+
+/*
+ * Common routine for high-level exception handlers.
+ * @param regs		state of registers when the exception occurs
+ * @param fault		IPSR, the fault number
+ */
+static void traps_v7m_common(struct pt_regs *regs, int fault)
+{
+	unsigned long status;
+	unsigned long hstatus;
+	unsigned long cstatus;
+	unsigned long addr;
+	size_t i;
+	const struct exception *exc = &unknown_exception;
+
+
+	hstatus = readl(BASEADDR_V7M_SCB + V7M_SCB_HFSR);
+	cstatus = readl(BASEADDR_V7M_SCB + V7M_SCB_CFSR);
+
+	if (cstatus & V7M_SCB_CFSR_MMARVALID && (fault == FAULT_MEMMANAGE ||
+			(fault == FAULT_HARDFALT && hstatus & V7M_SCB_HFSR_FORCED))) {
+		addr = readl(BASEADDR_V7M_SCB + V7M_SCB_MMAR);
+	} else if (cstatus & V7M_SCB_CFSR_BFARVALID && (fault == FAULT_BUSFAULT ||
+			(fault == FAULT_HARDFALT && hstatus & V7M_SCB_HFSR_FORCED))) {
+		addr = readl(BASEADDR_V7M_SCB + V7M_SCB_BFAR);
+	} else {
+		addr = instruction_pointer(regs);
+	}
+
+	writel(hstatus, BASEADDR_V7M_SCB + V7M_SCB_HFSR);
+	writel(cstatus, BASEADDR_V7M_SCB + V7M_SCB_CFSR);
+
+	for (i = 0; exceptions[i].name != NULL; ++i) {
+		if (fault != exceptions[i].fault)
+			continue;
+
+		status = fault == FAULT_HARDFALT ? hstatus : cstatus;
+		if (!(status & exceptions[i].test_bit))
+			continue;
+
+		exc = &exceptions[i];
+		break;
+	}
+
+#ifdef CONFIG_DEBUG_USER
+	if (user_mode(regs) && (user_debug & exc->user_debug)) {
+		pr_info("%s (%d): %s: addr=0x%px\n",
+			current->comm, task_pid_nr(current), exc->name, (void *)addr);
+		__show_regs(regs);
+	}
+#endif
+
+	arm_notify_die(exc->name, regs, exc->signo, exc->si_code,
+		       (void __user *)addr, 0, fault);
+}
+
+/*
+ * High-level exception handler for exception 3 (Hard Fault).
+ * @param regs		state of registers when the exception occurred
+ */
+asmlinkage void do_hardfault(struct pt_regs *regs)
+{
+	traps_v7m_common(regs, FAULT_HARDFALT);
+}
+
+/*
+ * High-level exception handler for exception 4 (Mem Manage).
+ * @param regs		state of registers when the exception occurred
+ */
+asmlinkage void do_memmanage(struct pt_regs *regs)
+{
+	traps_v7m_common(regs, FAULT_MEMMANAGE);
+}
+
+/*
+ * High-level exception handler for exception 5 (Bus Fault).
+ * @param regs		state of registers when the exception occurred
+ */
+asmlinkage void do_busfault(struct pt_regs *regs)
+{
+	traps_v7m_common(regs, FAULT_BUSFAULT);
+}
+
+/*
+ * High-level exception handler for exception 6 (Usage Fault).
+ * @param regs		state of registers when the exception occurred
+ */
+asmlinkage void do_usagefault(struct pt_regs *regs)
+{
+	traps_v7m_common(regs, FAULT_USAGEFAULT);
+}
-- 
2.37.0


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[PATCH 2/2] ARM: v7m: support undefined instruction hooks

[Ben Wolsieffer]

Call the common ARM undefined instruction handler, which handles running
hooks to enable ptrace breakpoints and other features.

Signed-off-by: Ben Wolsieffer <Ben.Wolsieffer@hefring.com>
---
 arch/arm/include/asm/traps.h | 2 ++
 arch/arm/kernel/traps-v7m.c  | 8 +++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/arm/include/asm/traps.h b/arch/arm/include/asm/traps.h
index 987fefb0a4db..e4253f4a86e1 100644
--- a/arch/arm/include/asm/traps.h
+++ b/arch/arm/include/asm/traps.h
@@ -19,6 +19,8 @@ struct undef_hook {
 void register_undef_hook(struct undef_hook *hook);
 void unregister_undef_hook(struct undef_hook *hook);
 
+void do_undefinstr(struct pt_regs *regs);
+
 static inline int __in_irqentry_text(unsigned long ptr)
 {
 	extern char __irqentry_text_start[];
diff --git a/arch/arm/kernel/traps-v7m.c b/arch/arm/kernel/traps-v7m.c
index 5fd9943448e9..b324499e1010 100644
--- a/arch/arm/kernel/traps-v7m.c
+++ b/arch/arm/kernel/traps-v7m.c
@@ -20,6 +20,7 @@
 #include <asm/linkage.h>
 #include <asm/ptrace.h>
 #include <asm/system_misc.h>
+#include <asm/traps.h>
 #include <asm/v7m.h>
 
 enum fault {
@@ -49,7 +50,6 @@ static const struct exception exceptions[] = {
 	{"no coprocessor",			FAULT_USAGEFAULT,	V7M_SCB_CFSR_NOCP,		SIGILL,		ILL_COPROC,	UDBG_UNDEFINED},
 	{"return to invalid PC",		FAULT_USAGEFAULT,	V7M_SCB_CFSR_INVPC,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
 	{"invalid ISA state",			FAULT_USAGEFAULT,	V7M_SCB_CFSR_INVSTATE,		SIGSEGV,	SEGV_MAPERR,	UDBG_SEGV},
-	{"undefined instruction",		FAULT_USAGEFAULT,	V7M_SCB_CFSR_UNDEFINSTR,	SIGILL,		ILL_ILLOPC,	UDBG_UNDEFINED},
 	{"floating point state error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_LSPERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
 	{"exception stack push error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_STKERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
 	{"exception stack pop error",		FAULT_BUSFAULT,		V7M_SCB_CFSR_UNSTKERR,		SIGBUS,		BUS_ADRERR,	UDBG_BUS},
@@ -95,6 +95,12 @@ static void traps_v7m_common(struct pt_regs *regs, int fault)
 	writel(hstatus, BASEADDR_V7M_SCB + V7M_SCB_HFSR);
 	writel(cstatus, BASEADDR_V7M_SCB + V7M_SCB_CFSR);
 
+	if (fault == FAULT_USAGEFAULT && cstatus & V7M_SCB_CFSR_UNDEFINSTR) {
+		/* Handle undefined instruction hooks */
+		do_undefinstr(regs);
+		return;
+	}
+
 	for (i = 0; exceptions[i].name != NULL; ++i) {
 		if (fault != exceptions[i].fault)
 			continue;
-- 
2.37.0


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