powerpc/32: Implement validation of emergency stack

VMAP stack added an emergency stack on powerpc/32 for when there is
a stack overflow, but failed to add stack validation for that
emergency stack. That validation is required for show stack.

Implement it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/2439d50b019f758db4a6d7b238b06441ab109799.1724156805.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/thread_info.h

@@ -226,6 +226,10 @@ static inline int arch_within_stack_frames(const void * const stack,
 	return BAD_STACK;
 }
 
+#ifdef CONFIG_PPC32
+extern void *emergency_ctx[];
+#endif
+
 #endif	/* !__ASSEMBLY__ */
 
 #endif /* __KERNEL__ */

arch/powerpc/kernel/process.c

@@ -2175,10 +2175,10 @@ static inline int valid_irq_stack(unsigned long sp, struct task_struct *p,
 	return 0;
 }
 
+#ifdef CONFIG_PPC64
 static inline int valid_emergency_stack(unsigned long sp, struct task_struct *p,
 					unsigned long nbytes)
 {
-#ifdef CONFIG_PPC64
 	unsigned long stack_page;
 	unsigned long cpu = task_cpu(p);
 
@@ -2206,10 +2206,26 @@ static inline int valid_emergency_stack(unsigned long sp, struct task_struct *p,
 	if (sp >= stack_page && sp <= stack_page + THREAD_SIZE - nbytes)
 		return 1;
 # endif
-#endif
 
 	return 0;
 }
+#else
+static inline int valid_emergency_stack(unsigned long sp, struct task_struct *p,
+					unsigned long nbytes)
+{
+	unsigned long stack_page;
+	unsigned long cpu = task_cpu(p);
+
+	if (!IS_ENABLED(CONFIG_VMAP_STACK))
+		return 0;
+
+	stack_page = (unsigned long)emergency_ctx[cpu] - THREAD_SIZE;
+	if (sp >= stack_page && sp <= stack_page + THREAD_SIZE - nbytes)
+		return 1;
+
+	return 0;
+}
+#endif
 
 /*
  * validate the stack frame of a particular minimum size, used for when we are