[PATCH 3/4] x86/dumpstack: support for unwinding empty irq stacks

[Josh Poimboeuf <jpoimboe@redhat.com>]

When an interrupt happens in entry code while running on a software irq
stack, and the irq stack was empty, regs->sp will contain the stack end
address (e.g., irq_stack_ptr).  If the regs are passed to dump_trace(),
get_stack_info() will report STACK_TYPE_UNKNOWN, causing dump_trace() to
return prematurely without trying to go to the next stack.

Update the bounds checking for software interrupt stacks so that the
ending address is now considered part of the stack.

This means that it's now possible for the 'walk_stack' callbacks --
print_context_stack() and print_context_stack_bp() -- to be called with
an empty stack.  But that's fine; they're already prepared to deal with
that due to their on_stack() checks.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
---
 arch/x86/kernel/dumpstack_32.c | 12 ++++++++++--
 arch/x86/kernel/dumpstack_64.c |  6 +++++-
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/dumpstack_32.c b/arch/x86/kernel/dumpstack_32.c
index c92da5a..50076d4 100644
--- a/arch/x86/kernel/dumpstack_32.c
+++ b/arch/x86/kernel/dumpstack_32.c
@@ -35,7 +35,11 @@ static bool in_hardirq_stack(unsigned long *stack, struct stack_info *info)
 	unsigned long *begin = (unsigned long *)this_cpu_read(hardirq_stack);
 	unsigned long *end   = begin + (THREAD_SIZE / sizeof(long));
 
-	if (stack < begin || stack >= end)
+	/*
+	 * This is a software stack, so 'end' can be a valid stack pointer.
+	 * It just means the stack is empty.
+	 */
+	if (stack < begin || stack > end)
 		return false;
 
 	info->type	= STACK_TYPE_IRQ;
@@ -56,7 +60,11 @@ static bool in_softirq_stack(unsigned long *stack, struct stack_info *info)
 	unsigned long *begin = (unsigned long *)this_cpu_read(softirq_stack);
 	unsigned long *end   = begin + (THREAD_SIZE / sizeof(long));
 
-	if (stack < begin || stack >= end)
+	/*
+	 * This is a software stack, so 'end' can be a valid stack pointer.
+	 * It just means the stack is empty.
+	 */
+	if (stack < begin || stack > end)
 		return false;
 
 	info->type	= STACK_TYPE_SOFTIRQ;
diff --git a/arch/x86/kernel/dumpstack_64.c b/arch/x86/kernel/dumpstack_64.c
index 41813ab..2e708af 100644
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -90,7 +90,11 @@ static bool in_irq_stack(unsigned long *stack, struct stack_info *info)
 	unsigned long *end   = (unsigned long *)this_cpu_read(irq_stack_ptr);
 	unsigned long *begin = end - (IRQ_STACK_SIZE / sizeof(long));
 
-	if (stack < begin || stack >= end)
+	/*
+	 * This is a software stack, so 'end' can be a valid stack pointer.
+	 * It just means the stack is empty.
+	 */
+	if (stack < begin || stack > end)
 		return false;
 
 	info->type	= STACK_TYPE_IRQ;
-- 
2.7.4