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SubjectRe: [RFC v2 PATCH 6/7] x86/entry: get rid of ALLOC_PT_GPREGS_ON_STACK and SAVE_AND_CLEAR_REGS
On Wed, Feb 07, 2018 at 12:44:41PM -0800, Linus Torvalds wrote:
> On Wed, Feb 7, 2018 at 12:15 PM, Dominik Brodowski
> <linux@dominikbrodowski.net> wrote:
> >
> > Note: The testb $3, CS(%rsp) instruction in idtentry() does not need
> > modification. Previously %rsp was manually decreased by 15*8; with
> > this patch, %rsp is decreased by 15 pushq instructions. Moreover,
> > error_entry did and does the exact same test (with offset=8) after
> > the registers have been moved/pushed and cleared.
>
> So this has the problem that now those save/clear instructions will
> all be done in that idtentry macro.
>
> So now that code will be duplicated for all the users of that macro.
>
> The old code did the saving in the common error_entry and
> paranoid_entry routines, in order to be able to share all the code,
> and making the duplicated stub functions generated by the idtentry
> macro smaller.
>
> Now, admittedly the new push sequence is much smaller than the old
> movq sequence, so the duplication doesn't hurt as much, but it's still
> likely quite noticeable.
>
> So this removes lines of asm code, but it adds a lot of instructions
> to the end result thanks to the macro, I think.

Indeed, that is the case (see below). However, if we want to switch to
PUSH instructions and do this in a routine which is call'ed and which
ret'urns, %rsp needs to be moved around even more often than the old
ALLOC_PT_GPREGS_ON_STACK macro did (which you wanted to get rid of,
IIUYC). Or do I miss something?

text data bss dec hex filename
19500 0 0 19500 4c2c arch/x86/entry/entry_64.o-orig
19510 0 0 19510 4c36 arch/x86/entry/entry_64.o-3_of_7
21105 0 0 21105 5271 arch/x86/entry/entry_64.o-5_of_7
24307 0 0 24307 5ef3 arch/x86/entry/entry_64.o-7_of_7

In any case, here's a v2.1 for this patch 6/7, which silences an objtool
warning in error_entry.

Thanks,
Dominik

----------------------------------------------------
From: Dominik Brodowski <linux@dominikbrodowski.net>
Date: Wed, 7 Feb 2018 20:56:13 +0100
Subject: [PATCH] x86/entry: get rid of ALLOC_PT_GPREGS_ON_STACK and SAVE_AND_CLEAR_REGS

Previously, error_entry() and paranoid_entry() saved the GP registers
onto stack space previously allocated by its callers. Combine these two
steps in the callers, and use the generic PUSH_AND_CLEAR_REGS macro
for that.

Note: The testb $3, CS(%rsp) instruction in idtentry() does not need
modification. Previously %rsp was manually decreased by 15*8; with
this patch, %rsp is decreased by 15 pushq instructions. Moreover,
error_entry did and does the exact same test (with offset=8) after
the registers have been moved/pushed and cleared.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index d6a97e2945ee..59675010c9a0 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -97,46 +97,6 @@ For 32-bit we have the following conventions - kernel is built with

#define SIZEOF_PTREGS 21*8

- .macro ALLOC_PT_GPREGS_ON_STACK
- addq $-(15*8), %rsp
- .endm
-
- .macro SAVE_AND_CLEAR_REGS offset=0
- /*
- * Save registers and sanitize registers of values that a
- * speculation attack might otherwise want to exploit. The
- * lower registers are likely clobbered well before they
- * could be put to use in a speculative execution gadget.
- * Interleave XOR with PUSH for better uop scheduling:
- */
- movq %rdi, 14*8+\offset(%rsp)
- movq %rsi, 13*8+\offset(%rsp)
- movq %rdx, 12*8+\offset(%rsp)
- movq %rcx, 11*8+\offset(%rsp)
- movq %rax, 10*8+\offset(%rsp)
- movq %r8, 9*8+\offset(%rsp)
- xorq %r8, %r8 /* nospec r8 */
- movq %r9, 8*8+\offset(%rsp)
- xorq %r9, %r9 /* nospec r9 */
- movq %r10, 7*8+\offset(%rsp)
- xorq %r10, %r10 /* nospec r10 */
- movq %r11, 6*8+\offset(%rsp)
- xorq %r11, %r11 /* nospec r11 */
- movq %rbx, 5*8+\offset(%rsp)
- xorl %ebx, %ebx /* nospec rbx */
- movq %rbp, 4*8+\offset(%rsp)
- xorl %ebp, %ebp /* nospec rbp */
- movq %r12, 3*8+\offset(%rsp)
- xorq %r12, %r12 /* nospec r12 */
- movq %r13, 2*8+\offset(%rsp)
- xorq %r13, %r13 /* nospec r13 */
- movq %r14, 1*8+\offset(%rsp)
- xorq %r14, %r14 /* nospec r14 */
- movq %r15, 0*8+\offset(%rsp)
- xorq %r15, %r15 /* nospec r15 */
- UNWIND_HINT_REGS offset=\offset
- .endm
-
.macro PUSH_AND_CLEAR_REGS rdx=%rdx rax=%rax
/*
* Push registers and sanitize registers of values that a
@@ -211,7 +171,7 @@ For 32-bit we have the following conventions - kernel is built with
* is just setting the LSB, which makes it an invalid stack address and is also
* a signal to the unwinder that it's a pt_regs pointer in disguise.
*
- * NOTE: This macro must be used *after* SAVE_AND_CLEAR_REGS because it corrupts
+ * NOTE: This macro must be used *after* PUSH_AND_CLEAR_REGS because it corrupts
* the original rbp.
*/
.macro ENCODE_FRAME_POINTER ptregs_offset=0
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 9c4fe360db42..c6e5d44eb322 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -871,7 +871,9 @@ ENTRY(\sym)
pushq $-1 /* ORIG_RAX: no syscall to restart */
.endif

- ALLOC_PT_GPREGS_ON_STACK
+ /* Save all registers in pt_regs */
+ PUSH_AND_CLEAR_REGS
+ ENCODE_FRAME_POINTER

.if \paranoid < 2
testb $3, CS(%rsp) /* If coming from userspace, switch stacks */
@@ -1121,15 +1123,13 @@ idtentry machine_check do_mce has_error_code=0 paranoid=1
#endif

/*
- * Save all registers in pt_regs, and switch gs if needed.
+ * Switch gs if needed.
* Use slow, but surefire "are we in kernel?" check.
* Return: ebx=0: need swapgs on exit, ebx=1: otherwise
*/
ENTRY(paranoid_entry)
UNWIND_HINT_FUNC
cld
- SAVE_AND_CLEAR_REGS 8
- ENCODE_FRAME_POINTER 8
movl $1, %ebx
movl $MSR_GS_BASE, %ecx
rdmsr
@@ -1173,14 +1173,13 @@ ENTRY(paranoid_exit)
END(paranoid_exit)

/*
- * Save all registers in pt_regs, and switch gs if needed.
+ * Switch gs if needed.
* Return: EBX=0: came from user mode; EBX=1: otherwise
*/
ENTRY(error_entry)
UNWIND_HINT_FUNC
+ UNWIND_HINT_REGS offset=8
cld
- SAVE_AND_CLEAR_REGS 8
- ENCODE_FRAME_POINTER 8
testb $3, CS+8(%rsp)
jz .Lerror_kernelspace

@@ -1571,7 +1570,8 @@ end_repeat_nmi:
* frame to point back to repeat_nmi.
*/
pushq $-1 /* ORIG_RAX: no syscall to restart */
- ALLOC_PT_GPREGS_ON_STACK
+ PUSH_AND_CLEAR_REGS
+ ENCODE_FRAME_POINTER

/*
* Use paranoid_entry to handle SWAPGS, but no need to use paranoid_exit
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