From ea263bc472eb89dd8696df3f910e299f3f22526b Mon Sep 17 00:00:00 2001
From: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
Date: Fri, 17 Jul 2020 02:39:52 +0000
Subject: [PATCH] compel: x86 parasite_service entry point simplification

We don't need to push 0 on the stack. This seems to be a remnant of the
initial commit of 2011 that had a `pushq $0`.

The 16 bytes %rsp alignment was added with commit 2a0cea29 in 2012.
This is no longer necessary as we already guarantee that %rsp is 16
bytes aligned. A BUG_ON() is added to enforce this guarantee.

Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
---
 compel/arch/x86/plugins/std/parasite-head.S | 12 ++----------
 compel/src/lib/infect.c                     | 11 +++++++++++
 2 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/compel/arch/x86/plugins/std/parasite-head.S b/compel/arch/x86/plugins/std/parasite-head.S
index 2225568b9..4fb38d1f1 100644
--- a/compel/arch/x86/plugins/std/parasite-head.S
+++ b/compel/arch/x86/plugins/std/parasite-head.S
@@ -6,14 +6,6 @@
 # error 64-bit parasite should compile with CONFIG_X86_64
 #endif
 
-.macro PARASITE_ENTRY num
-	subq	$16, %rsp
-	andq	$~15, %rsp
-	pushq	$\num
-	movq	%rsp, %rbp
-	call	parasite_service
-.endm
-
 #ifdef CONFIG_COMPAT
 .code32
 ENTRY(__export_parasite_head_start_compat)
@@ -21,7 +13,7 @@ ENTRY(__export_parasite_head_start_compat)
 	jmp	$__USER_CS,$1f
 1:
 .code64
-	PARASITE_ENTRY 0
+	call	parasite_service
 	pushq	$__USER32_CS
 	xor	%r11, %r11
 	movl	$2f, %r11d
@@ -43,6 +35,6 @@ END(__export_parasite_head_start_compat)
 #endif
 
 ENTRY(__export_parasite_head_start)
-	PARASITE_ENTRY 0
+	call	parasite_service
 	int	$0x03
 END(__export_parasite_head_start)
diff --git a/compel/src/lib/infect.c b/compel/src/lib/infect.c
index 78fe1f1a6..da02d1744 100644
--- a/compel/src/lib/infect.c
+++ b/compel/src/lib/infect.c
@@ -948,6 +948,17 @@ int compel_infect(struct parasite_ctl *ctl, unsigned long nr_threads, unsigned l
 	p += PARASITE_STACK_SIZE;
 	ctl->rstack = ctl->remote_map + p;
 
+	/*
+	 * x86-64 ABI requires a 16 bytes aligned stack.
+	 * It is already the case as RESTORE_STACK_SIGFRAME is a multiple of
+	 * 64, and PARASITE_STACK_SIZE is 0x4000.
+	 */
+	if ((unsigned long)ctl->rstack & (16-1)) {
+		pr_err("BUG: stack is not 16 bytes aligned: %p\n", ctl->rstack);
+		ctl->rstack = (void *)round_down((unsigned long)ctl->rstack, 16);
+	}
+
+
 	if (nr_threads > 1) {
 		p += PARASITE_STACK_SIZE;
 		ctl->r_thread_stack = ctl->remote_map + p;