[PATCH -tip v3 09/11] x86/kprobes: Push a fake return address at kretprobe_trampoline

[Masami Hiramatsu <mhiramat@xxxxxxxxxx>]

This changes x86/kretprobe stack frame on kretprobe_trampoline
a bit, which now push the kretprobe_trampoline as a fake return
address at the bottom of the stack frame. With this fix, the ORC
unwinder will see the kretprobe_trampoline as a return address.

Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Suggested-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/kprobes/core.c |   31 ++++++++++++++++++++++---------
 1 file changed, 22 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index b31058a152b6..7e7b854e0fdf 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -769,28 +769,31 @@ asm(
 	".global kretprobe_trampoline\n"
 	".type kretprobe_trampoline, @function\n"
 	"kretprobe_trampoline:\n"
-	/* We don't bother saving the ss register */
 #ifdef CONFIG_X86_64
-	"	pushq %rsp\n"
+	/* Push fake return address to tell the unwinder it's a kretprobe */
+	"	pushq $kretprobe_trampoline\n"
 	UNWIND_HINT_FUNC
+	/* Save the sp-8, this will be fixed later */
+	"	pushq %rsp\n"
 	"	pushfq\n"
 	SAVE_REGS_STRING
 	"	movq %rsp, %rdi\n"
 	"	call trampoline_handler\n"
-	/* Replace saved sp with true return address. */
-	"	movq %rax, 19*8(%rsp)\n"
 	RESTORE_REGS_STRING
+	"	addq $8, %rsp\n"
 	"	popfq\n"
 #else
-	"	pushl %esp\n"
+	/* Push fake return address to tell the unwinder it's a kretprobe */
+	"	pushl $kretprobe_trampoline\n"
 	UNWIND_HINT_FUNC
+	/* Save the sp-4, this will be fixed later */
+	"	pushl %esp\n"
 	"	pushfl\n"
 	SAVE_REGS_STRING
 	"	movl %esp, %eax\n"
 	"	call trampoline_handler\n"
-	/* Replace saved sp with true return address. */
-	"	movl %eax, 15*4(%esp)\n"
 	RESTORE_REGS_STRING
+	"	addl $4, %esp\n"
 	"	popfl\n"
 #endif
 	"	ret\n"
@@ -802,8 +805,10 @@ NOKPROBE_SYMBOL(kretprobe_trampoline);
 /*
  * Called from kretprobe_trampoline
  */
-__used __visible void *trampoline_handler(struct pt_regs *regs)
+__used __visible void trampoline_handler(struct pt_regs *regs)
 {
+	unsigned long *frame_pointer;
+
 	/* fixup registers */
 	regs->cs = __KERNEL_CS;
 #ifdef CONFIG_X86_32
@@ -811,8 +816,16 @@ __used __visible void *trampoline_handler(struct pt_regs *regs)
 #endif
 	regs->ip = (unsigned long)&kretprobe_trampoline;
 	regs->orig_ax = ~0UL;
+	regs->sp += sizeof(long);
+	frame_pointer = ((unsigned long *)&regs->sp) + 1;
 
-	return (void *)kretprobe_trampoline_handler(regs, &regs->sp);
+	/* Replace fake return address with real one. */
+	*frame_pointer = kretprobe_trampoline_handler(regs, frame_pointer);
+	/*
+	 * Move flags to sp so that kretprobe_trapmoline can return
+	 * right after popf.
+	 */
+	regs->sp = regs->flags;
 }
 NOKPROBE_SYMBOL(trampoline_handler);