[PATCH 2/2] RISC-V: Add some comments about the shadow and overflow stacks

Palmer Dabbelt
2022-11-30T02:35:57Z

It took me a while to page all this back in when trying to review the
recent spin_shadow_stack, so I figured I'd just write up some comments.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
---
arch/riscv/kernel/traps.c | 20 +++++++++++++-------
1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index acdfcacd7e57..336d4aadadb1 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -200,18 +200,18 @@ void __init trap_init(void)
}

#ifdef CONFIG_VMAP_STACK
+/*
+ * Extra stack space that allows us to provide panic messages when the kernel
+ * has overflowed its stack.
+ */
static DEFINE_PER_CPU(unsigned long [OVERFLOW_STACK_SIZE/sizeof(long)],
overflow_stack)__aligned(16);
/*
- * shadow stack, handled_ kernel_ stack_ overflow(in kernel/entry.S) is used
- * to get per-cpu overflow stack(get_overflow_stack).
+ * A temporary stack for use by handle_kernel_stack_overflow. This is used so
+ * we can call into C code to get the per-hart overflow stack. Usage of this
+ * stack must be protected by spin_shadow_stack.
*/
long shadow_stack[SHADOW_OVERFLOW_STACK_SIZE/sizeof(long)] __aligned(16);
-asmlinkage unsigned long get_overflow_stack(void)
-{
- return (unsigned long)this_cpu_ptr(overflow_stack) +
- OVERFLOW_STACK_SIZE;
-}

/*
* A pseudo spinlock to protect the shadow stack from being used by multiple
@@ -222,6 +222,12 @@ asmlinkage unsigned long get_overflow_stack(void)
*/
unsigned long spin_shadow_stack;

+asmlinkage unsigned long get_overflow_stack(void)
+{
+ return (unsigned long)this_cpu_ptr(overflow_stack) +
+ OVERFLOW_STACK_SIZE;
+}
+
asmlinkage void handle_bad_stack(struct pt_regs *regs)
{
unsigned long tsk_stk = (unsigned long)current->stack;
--
2.38.1