powerpc: Remove ksp_limit on ppc64

We've been keeping that field in thread_struct for a while, it contains
the "limit" of the current stack pointer and is meant to be used for
detecting stack overflows.

It has a few problems however:

 - First, it was never actually *used* on 64-bit. Set and updated but
not actually exploited

 - When switching stack to/from irq and softirq stacks, it's update
is racy unless we hard disable interrupts, which is costly. This
is fine on 32-bit as we don't soft-disable there but not on 64-bit.

Thus rather than fixing 2 in order to implement 1 in some hypothetical
future, let's remove the code completely from 64-bit. In order to avoid
a clutter of ifdef's, we remove the updates from C code completely
during interrupt stack switching, and instead maintain it from the
asm helper that is used to do the stack switching in the first place.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

4 files changed, 20 insertions, 14 deletions

diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index d8958be5f31a..502c7a4e73f7 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -80,10 +80,11 @@ int main(void)
         DEFINE(TASKTHREADPPR, offsetof(struct task_struct, thread.ppr));
 #else
         DEFINE(THREAD_INFO, offsetof(struct task_struct, stack));
+        DEFINE(THREAD_INFO_GAP, _ALIGN_UP(sizeof(struct thread_info), 16));
+        DEFINE(KSP_LIMIT, offsetof(struct thread_struct, ksp_limit));
 #endif /* CONFIG_PPC64 */
 
         DEFINE(KSP, offsetof(struct thread_struct, ksp));
-        DEFINE(KSP_LIMIT, offsetof(struct thread_struct, ksp_limit));
         DEFINE(PT_REGS, offsetof(struct thread_struct, regs));
 #ifdef CONFIG_BOOKE
         DEFINE(THREAD_NORMSAVES, offsetof(struct thread_struct, normsave[0]));
diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c
index 2234a1276a77..57d286a78f86 100644
--- a/arch/powerpc/kernel/irq.c
+++ b/arch/powerpc/kernel/irq.c
@@ -496,7 +496,6 @@ void do_IRQ(struct pt_regs *regs)
 {
         struct pt_regs *old_regs = set_irq_regs(regs);
         struct thread_info *curtp, *irqtp;
-        unsigned long saved_sp_limit;
 
         /* Switch to the irq stack to handle this */
         curtp = current_thread_info();
@@ -509,12 +508,6 @@ void do_IRQ(struct pt_regs *regs)
                 return;
         }
 
-        /* Adjust the stack limit */
-        saved_sp_limit = current->thread.ksp_limit;
-        current->thread.ksp_limit = (unsigned long)irqtp +
-                _ALIGN_UP(sizeof(struct thread_info), 16);
-
-
         /* Prepare the thread_info in the irq stack */
         irqtp->task = curtp->task;
         irqtp->flags = 0;
@@ -526,7 +519,6 @@ void do_IRQ(struct pt_regs *regs)
         call_do_irq(regs, irqtp);
 
         /* Restore stack limit */
-        current->thread.ksp_limit = saved_sp_limit;
         irqtp->task = NULL;
 
         /* Copy back updates to the thread_info */
@@ -604,16 +596,12 @@ void irq_ctx_init(void)
 static inline void do_softirq_onstack(void)
 {
         struct thread_info *curtp, *irqtp;
-        unsigned long saved_sp_limit = current->thread.ksp_limit;
 
         curtp = current_thread_info();
         irqtp = softirq_ctx[smp_processor_id()];
         irqtp->task = curtp->task;
         irqtp->flags = 0;
-        current->thread.ksp_limit = (unsigned long)irqtp +
-                                    _ALIGN_UP(sizeof(struct thread_info), 16);
         call_do_softirq(irqtp);
-        current->thread.ksp_limit = saved_sp_limit;
         irqtp->task = NULL;
 
         /* Set any flag that may have been set on the
diff --git a/arch/powerpc/kernel/misc_32.S b/arch/powerpc/kernel/misc_32.S
index 7da3882a3622..2b0ad9845363 100644
--- a/arch/powerpc/kernel/misc_32.S
+++ b/arch/powerpc/kernel/misc_32.S
@@ -36,25 +36,41 @@
 
         .text
 
+/*
+ * We store the saved ksp_limit in the unused part
+ * of the STACK_FRAME_OVERHEAD
+ */
 _GLOBAL(call_do_softirq)
         mflr    r0
         stw     r0,4(r1)
+        lwz     r10,THREAD+KSP_LIMIT(r2)
+        addi    r11,r3,THREAD_INFO_GAP
         stwu    r1,THREAD_SIZE-STACK_FRAME_OVERHEAD(r3)
         mr      r1,r3
+        stw     r10,8(r1)
+        stw     r11,THREAD+KSP_LIMIT(r2)
         bl      __do_softirq
+        lwz     r10,8(r1)
         lwz     r1,0(r1)
         lwz     r0,4(r1)
+        stw     r10,THREAD+KSP_LIMIT(r2)
         mtlr    r0
         blr
 
 _GLOBAL(call_do_irq)
         mflr    r0
         stw     r0,4(r1)
+        lwz     r10,THREAD+KSP_LIMIT(r2)
+        addi    r11,r3,THREAD_INFO_GAP
         stwu    r1,THREAD_SIZE-STACK_FRAME_OVERHEAD(r4)
         mr      r1,r4
+        stw     r10,8(r1)
+        stw     r11,THREAD+KSP_LIMIT(r2)
         bl      __do_irq
+        lwz     r10,8(r1)
         lwz     r1,0(r1)
         lwz     r0,4(r1)
+        stw     r10,THREAD+KSP_LIMIT(r2)
         mtlr    r0
         blr
 
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index 6f428da53e20..96d2fdf3aa9e 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1000,9 +1000,10 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
         kregs = (struct pt_regs *) sp;
         sp -= STACK_FRAME_OVERHEAD;
         p->thread.ksp = sp;
+#ifdef CONFIG_PPC32
         p->thread.ksp_limit = (unsigned long)task_stack_page(p) +
                                 _ALIGN_UP(sizeof(struct thread_info), 16);
-
+#endif
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
         p->thread.ptrace_bps[0] = NULL;
 #endif