[PATCH] arm64: Remove stack duplicating code from jprobes

[David A. Long]

Because the arm64 calling standard allows stacked function arguments to be
anywhere in the stack frame, do not attempt to duplicate the stack frame for
jprobes handler functions.

Signed-off-by: David A. Long <dave.long@xxxxxxxxxx>
---
 Documentation/kprobes.txt          |  7 +++++++
 arch/arm64/include/asm/kprobes.h   |  2 --
 arch/arm64/kernel/probes/kprobes.c | 31 +++++--------------------------
 3 files changed, 12 insertions(+), 28 deletions(-)

diff --git a/Documentation/kprobes.txt b/Documentation/kprobes.txt
index 1f9b3e2..bd01839 100644
--- a/Documentation/kprobes.txt
+++ b/Documentation/kprobes.txt
@@ -103,6 +103,13 @@ Note that the probed function's args may be passed on the stack
 or in registers.  The jprobe will work in either case, so long as the
 handler's prototype matches that of the probed function.
 
+Note that in some architectures (e.g.: arm64) the stack copy is not
+done, as the actual location of stacked parameters may be outside of
+a reasonable MAX_STACK_SIZE value and because that location cannot be
+determined by the jprobes code. In this case the jprobes user must be
+careful to make certain the calling signature of the function does
+not cause parameters to be passed on the stack.
+
 1.3 Return Probes
 
 1.3.1 How Does a Return Probe Work?
diff --git a/arch/arm64/include/asm/kprobes.h b/arch/arm64/include/asm/kprobes.h
index 61b4915..1737aec 100644
--- a/arch/arm64/include/asm/kprobes.h
+++ b/arch/arm64/include/asm/kprobes.h
@@ -22,7 +22,6 @@
 
 #define __ARCH_WANT_KPROBES_INSN_SLOT
 #define MAX_INSN_SIZE			1
-#define MAX_STACK_SIZE			128
 
 #define flush_insn_slot(p)		do { } while (0)
 #define kretprobe_blacklist_size	0
@@ -47,7 +46,6 @@ struct kprobe_ctlblk {
 	struct prev_kprobe prev_kprobe;
 	struct kprobe_step_ctx ss_ctx;
 	struct pt_regs jprobe_saved_regs;
-	char jprobes_stack[MAX_STACK_SIZE];
 };
 
 void arch_remove_kprobe(struct kprobe *);
diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index bf97685..c6b0f40 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -41,18 +41,6 @@ DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
 static void __kprobes
 post_kprobe_handler(struct kprobe_ctlblk *, struct pt_regs *);
 
-static inline unsigned long min_stack_size(unsigned long addr)
-{
-	unsigned long size;
-
-	if (on_irq_stack(addr, raw_smp_processor_id()))
-		size = IRQ_STACK_PTR(raw_smp_processor_id()) - addr;
-	else
-		size = (unsigned long)current_thread_info() + THREAD_START_SP - addr;
-
-	return min(size, FIELD_SIZEOF(struct kprobe_ctlblk, jprobes_stack));
-}
-
 static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
 {
 	/* prepare insn slot */
@@ -489,20 +477,15 @@ int __kprobes setjmp_pre_handler(struct kprobe *p, struct pt_regs *regs)
 {
 	struct jprobe *jp = container_of(p, struct jprobe, kp);
 	struct kprobe_ctlblk *kcb = get_kprobe_ctlblk();
-	long stack_ptr = kernel_stack_pointer(regs);
 
 	kcb->jprobe_saved_regs = *regs;
 	/*
-	 * As Linus pointed out, gcc assumes that the callee
-	 * owns the argument space and could overwrite it, e.g.
-	 * tailcall optimization. So, to be absolutely safe
-	 * we also save and restore enough stack bytes to cover
-	 * the argument area.
+	 * Since we can't be sure where in the stack frame "stacked"
+	 * pass-by-value arguments are stored we just don't try to
+	 * duplicate any of the stack. Do not use jprobes on functions that
+	 * use more than 64 bytes (after padding each to an 8 byte boundary)
+	 * of arguments, or pass individual arguments larger than 16 bytes.
 	 */
-	kasan_disable_current();
-	memcpy(kcb->jprobes_stack, (void *)stack_ptr,
-	       min_stack_size(stack_ptr));
-	kasan_enable_current();
 
 	instruction_pointer_set(regs, (unsigned long) jp->entry);
 	preempt_disable();
@@ -554,10 +537,6 @@ int __kprobes longjmp_break_handler(struct kprobe *p, struct pt_regs *regs)
 	}
 	unpause_graph_tracing();
 	*regs = kcb->jprobe_saved_regs;
-	kasan_disable_current();
-	memcpy((void *)stack_addr, kcb->jprobes_stack,
-	       min_stack_size(stack_addr));
-	kasan_enable_current();
 	preempt_enable_no_resched();
 	return 1;
 }
-- 
2.5.0