linux-kernel - [PATCH 3/4] x86/asm/entry/32: Make register zero-extension more prominent

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Date:	Fri, 27 Mar 2015 11:36:21 +0100
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 3/4] x86/asm/entry/32: Make register zero-extension more prominent

It was mentioned that people keep trying to optimize them out,
introducing bugs. Make them more visible, and add "do not remove"
comment.

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
CC: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Steven Rostedt <rostedt@...dmis.org>
CC: Ingo Molnar <mingo@...nel.org>
CC: Borislav Petkov <bp@...en8.de>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andy Lutomirski <luto@...capital.net>
CC: Oleg Nesterov <oleg@...hat.com>
CC: Frederic Weisbecker <fweisbec@...il.com>
CC: Alexei Starovoitov <ast@...mgrid.com>
CC: Will Drewry <wad@...omium.org>
CC: Kees Cook <keescook@...omium.org>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org
---
 arch/x86/ia32/ia32entry.S | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 7502ff0..dec8c1d 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -122,8 +122,11 @@ ENTRY(ia32_sysenter_target)
 	movq	PER_CPU_VAR(cpu_tss + TSS_sp0), %rsp
 	ENABLE_INTERRUPTS(CLBR_NONE)
 
+	/* Zero-extending 32-bit regs, do not remove */
+	movl	%ebp, %ebp
+	movl	%eax, %eax
+
 	/* Construct iret frame (ss,rsp,rflags,cs,rip) */
- 	movl	%ebp,%ebp		/* zero extension */
 	pushq_cfi $__USER32_DS
 	/*CFI_REL_OFFSET ss,0*/
 	pushq_cfi %rbp
@@ -134,7 +137,6 @@ ENTRY(ia32_sysenter_target)
 	CFI_REGISTER rip,r10
 	pushq_cfi $__USER32_CS
 	/*CFI_REL_OFFSET cs,0*/
-	movl	%eax, %eax
 	/* Store thread_info->sysenter_return in rip stack slot */
 	pushq_cfi %r10
 	CFI_REL_OFFSET rip,0
@@ -329,9 +331,11 @@ ENTRY(ia32_cstar_target)
 	movq	PER_CPU_VAR(kernel_stack),%rsp
 	ENABLE_INTERRUPTS(CLBR_NONE)
 
+	/* Zero-extending 32-bit regs, do not remove */
+	movl	%eax,%eax
+
 	ALLOC_PT_GPREGS_ON_STACK 6*8 /* 6*8: space for orig_ax and iret frame */
 	SAVE_C_REGS_EXCEPT_RCX_R891011
-	movl 	%eax,%eax	/* zero extension */
 	movq	%rax,ORIG_RAX(%rsp)
 	movq	%rcx,RIP(%rsp)
 	CFI_REL_OFFSET rip,RIP
@@ -471,7 +475,9 @@ ENTRY(ia32_syscall)
 	SWAPGS
 	ENABLE_INTERRUPTS(CLBR_NONE)
 
-	movl %eax,%eax
+	/* Zero-extending 32-bit regs, do not remove */
+	movl	%eax,%eax
+
 	pushq_cfi %rax		/* store orig_ax */
 	cld
 	/* note the registers are not zero extended to the sf.
-- 
1.8.1.4

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