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Message-ID: <153156064588.10043.9293018593725922789.stgit@srivatsa-ubuntu>
Date:   Sat, 14 Jul 2018 02:30:46 -0700
From:   "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
To:     gregkh@...uxfoundation.org, stable@...r.kernel.org
Cc:     Denys Vlasenko <dvlasenk@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        "Matt Helsley \(VMware\)" <matt.helsley@...il.com>,
        Alexey Makhalov <amakhalov@...are.com>,
        Bo Gan <ganb@...are.com>, matt.helsley@...il.com,
        rostedt@...dmis.org, amakhalov@...are.com, ganb@...are.com,
        srivatsa@...il.mit.edu, srivatsab@...are.com
Subject: [PATCH 4.4.y 033/101] x86/asm/entry/32: Simplify pushes of zeroed
 pt_regs->REGs

From: Denys Vlasenko <dvlasenk@...hat.com>

commit 778843f934e362ed4ed734520f60a44a78a074b4 upstream

Use of a temporary R8 register here seems to be unnecessary.

"push %r8" is a two-byte insn (it needs REX prefix to specify R8),
"push $0" is two-byte too. It seems just using the latter would be
no worse.

Thus, code had an unnecessary "xorq %r8,%r8" insn.
It probably costs nothing in execution time here since we are probably
limited by store bandwidth at this point, but still.

Run-tested under QEMU: 32-bit calls still work:

 / # ./test_syscall_vdso32
 [RUN]	Executing 6-argument 32-bit syscall via VDSO
 [OK]	Arguments are preserved across syscall
 [NOTE]	R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
 [OK]	R8..R15 did not leak kernel data
 [RUN]	Executing 6-argument 32-bit syscall via INT 80
 [OK]	Arguments are preserved across syscall
 [OK]	R8..R15 did not leak kernel data
 [RUN]	Running tests under ptrace
 [RUN]	Executing 6-argument 32-bit syscall via VDSO
 [OK]	Arguments are preserved across syscall
 [NOTE]	R11 has changed:0000000000200ed7 - assuming clobbered by SYSRET insn
 [OK]	R8..R15 did not leak kernel data
 [RUN]	Executing 6-argument 32-bit syscall via INT 80
 [OK]	Arguments are preserved across syscall
 [OK]	R8..R15 did not leak kernel data

Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
Acked-by: Andy Lutomirski <luto@...nel.org>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Will Drewry <wad@...omium.org>
Cc: linux-kernel@...r.kernel.org
Link: http://lkml.kernel.org/r/1462201010-16846-1-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa@...il.mit.edu>
Reviewed-by: Matt Helsley (VMware) <matt.helsley@...il.com>
Reviewed-by: Alexey Makhalov <amakhalov@...are.com>
Reviewed-by: Bo Gan <ganb@...are.com>
---

 arch/x86/entry/entry_64_compat.S |   45 ++++++++++++++++++--------------------
 1 file changed, 21 insertions(+), 24 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index d03bf0e..e479ff8 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -79,24 +79,23 @@ ENTRY(entry_SYSENTER_compat)
 	ASM_CLAC			/* Clear AC after saving FLAGS */
 
 	pushq	$__USER32_CS		/* pt_regs->cs */
-	xorq    %r8,%r8
-	pushq	%r8			/* pt_regs->ip = 0 (placeholder) */
+	pushq	$0			/* pt_regs->ip = 0 (placeholder) */
 	pushq	%rax			/* pt_regs->orig_ax */
 	pushq	%rdi			/* pt_regs->di */
 	pushq	%rsi			/* pt_regs->si */
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rcx			/* pt_regs->cx */
 	pushq	$-ENOSYS		/* pt_regs->ax */
-	pushq   %r8                     /* pt_regs->r8  = 0 */
-	pushq   %r8                     /* pt_regs->r9  = 0 */
-	pushq   %r8                     /* pt_regs->r10 = 0 */
-	pushq   %r8                     /* pt_regs->r11 = 0 */
+	pushq   $0			/* pt_regs->r8  = 0 */
+	pushq   $0			/* pt_regs->r9  = 0 */
+	pushq   $0			/* pt_regs->r10 = 0 */
+	pushq   $0			/* pt_regs->r11 = 0 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	pushq   %rbp                    /* pt_regs->rbp (will be overwritten) */
-	pushq   %r8                     /* pt_regs->r12 = 0 */
-	pushq   %r8                     /* pt_regs->r13 = 0 */
-	pushq   %r8                     /* pt_regs->r14 = 0 */
-	pushq   %r8                     /* pt_regs->r15 = 0 */
+	pushq   $0			/* pt_regs->r12 = 0 */
+	pushq   $0			/* pt_regs->r13 = 0 */
+	pushq   $0			/* pt_regs->r14 = 0 */
+	pushq   $0			/* pt_regs->r15 = 0 */
 	cld
 
 	/*
@@ -185,17 +184,16 @@ ENTRY(entry_SYSCALL_compat)
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rbp			/* pt_regs->cx (stashed in bp) */
 	pushq	$-ENOSYS		/* pt_regs->ax */
-	xorq    %r8,%r8
-	pushq   %r8                     /* pt_regs->r8  = 0 */
-	pushq   %r8                     /* pt_regs->r9  = 0 */
-	pushq   %r8                     /* pt_regs->r10 = 0 */
-	pushq   %r8                     /* pt_regs->r11 = 0 */
+	pushq   $0			/* pt_regs->r8  = 0 */
+	pushq   $0			/* pt_regs->r9  = 0 */
+	pushq   $0			/* pt_regs->r10 = 0 */
+	pushq   $0			/* pt_regs->r11 = 0 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	pushq   %rbp                    /* pt_regs->rbp (will be overwritten) */
-	pushq   %r8                     /* pt_regs->r12 = 0 */
-	pushq   %r8                     /* pt_regs->r13 = 0 */
-	pushq   %r8                     /* pt_regs->r14 = 0 */
-	pushq   %r8                     /* pt_regs->r15 = 0 */
+	pushq   $0			/* pt_regs->r12 = 0 */
+	pushq   $0			/* pt_regs->r13 = 0 */
+	pushq   $0			/* pt_regs->r14 = 0 */
+	pushq   $0			/* pt_regs->r15 = 0 */
 
 	/*
 	 * User mode is traced as though IRQs are on, and SYSENTER
@@ -292,11 +290,10 @@ ENTRY(entry_INT80_compat)
 	pushq	%rdx			/* pt_regs->dx */
 	pushq	%rcx			/* pt_regs->cx */
 	pushq	$-ENOSYS		/* pt_regs->ax */
-	xorq    %r8,%r8
-	pushq   %r8                     /* pt_regs->r8  = 0 */
-	pushq   %r8                     /* pt_regs->r9  = 0 */
-	pushq   %r8                     /* pt_regs->r10 = 0 */
-	pushq   %r8                     /* pt_regs->r11 = 0 */
+	pushq   $0			/* pt_regs->r8  = 0 */
+	pushq   $0			/* pt_regs->r9  = 0 */
+	pushq   $0			/* pt_regs->r10 = 0 */
+	pushq   $0			/* pt_regs->r11 = 0 */
 	pushq   %rbx                    /* pt_regs->rbx */
 	pushq   %rbp                    /* pt_regs->rbp */
 	pushq   %r12                    /* pt_regs->r12 */

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