[PATCH 18/24] alpha: select SYSCTL_EXCEPTION_TRACE

[Matt Turner <mattst88@gmail.com>]

Report unhandled user faults the way other architectures do, and hook up
the debug.exception-trace sysctl that controls the reporting.

The message gives the faulting address, the PC and the user stack
pointer, and is rate limited.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Matt Turner <mattst88@gmail.com>
---
 arch/alpha/Kconfig    |  1 +
 arch/alpha/mm/fault.c | 29 +++++++++++++++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git ./arch/alpha/Kconfig ./arch/alpha/Kconfig
index c4b4a9b8d770..4c60c442207c 100644
--- ./arch/alpha/Kconfig
+++ ./arch/alpha/Kconfig
@@ -59,6 +59,7 @@ config ALPHA
 	select MMU_GATHER_NO_RANGE
 	select MMU_GATHER_RCU_TABLE_FREE
 	select SPARSEMEM_EXTREME if SPARSEMEM
+	select SYSCTL_EXCEPTION_TRACE
 	select ZONE_DMA
 	select TRACE_IRQFLAGS_SUPPORT
 	select ARCH_WANT_FRAME_POINTERS
diff --git ./arch/alpha/mm/fault.c ./arch/alpha/mm/fault.c
index a9816bbc9f34..dfe427d93072 100644
--- ./arch/alpha/mm/fault.c
+++ ./arch/alpha/mm/fault.c
@@ -27,8 +27,32 @@
 #include <linux/uaccess.h>
 #include <linux/perf_event.h>
 
+#include <asm/pal.h>
+
 extern void die_if_kernel(char *,struct pt_regs *,long, unsigned long *);
 
+int show_unhandled_signals = 1;
+
+static void
+show_signal_msg(struct pt_regs *regs, unsigned long address, int signo,
+		const char *desc)
+{
+	const char *level;
+
+	if (!show_unhandled_signals || !unhandled_signal(current, signo))
+		return;
+	if (!printk_ratelimit())
+		return;
+
+	/* Losing init is fatal, so shout about it.  */
+	level = task_pid_nr(current) > 1 ? KERN_INFO : KERN_EMERG;
+
+	printk("%s%s[%d]: %s at %016lx pc %016lx sp %016lx\n",
+	       level, current->comm, task_pid_nr(current), desc,
+	       address, regs->pc, rdusp());
+	print_vma_addr(KERN_CONT "  in ", regs->pc);
+	printk(KERN_CONT "\n");
+}
 
 /*
  * Force a new ASN for a task.
@@ -217,12 +241,17 @@ do_page_fault(unsigned long address, unsigned long mmcsr,
 	mmap_read_unlock(mm);
 	/* Send a sigbus, regardless of whether we were in kernel
 	   or user mode.  */
+	if (user_mode(regs))
+		show_signal_msg(regs, address, SIGBUS, "bus error");
 	force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *) address);
 	if (!user_mode(regs))
 		goto no_context;
 	return;
 
  do_sigsegv:
+	show_signal_msg(regs, address, SIGSEGV,
+			si_code == SEGV_MAPERR ? "unmapped access"
+					       : "access violation");
 	force_sig_fault(SIGSEGV, si_code, (void __user *) address);
 	return;
 
-- 
2.54.0