x86/asm/entry: Get rid of KERNEL_STACK_OFFSET

PER_CPU_VAR(kernel_stack) was set up in a way where it points
five stack slots below the top of stack.

Presumably, it was done to avoid one "sub $5*8,%rsp"
in syscall/sysenter code paths, where iret frame needs to be
created by hand.

Ironically, none of them benefits from this optimization,
since all of them need to allocate additional data on stack
(struct pt_regs), so they still have to perform subtraction.

This patch eliminates KERNEL_STACK_OFFSET.

PER_CPU_VAR(kernel_stack) now points directly to top of stack.
pt_regs allocations are adjusted to allocate iret frame as well.
Hopefully we can merge it later with 32-bit specific
PER_CPU_VAR(cpu_current_top_of_stack) variable...

Net result in generated code is that constants in several insns
are changed.

This change is necessary for changing struct pt_regs creation
in SYSCALL64 code path from MOV to PUSH instructions.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Acked-by: Borislav Petkov <bp@suse.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Link: http://lkml.kernel.org/r/1426785469-15125-2-git-send-email-dvlasenk@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

8 files changed, 11 insertions, 16 deletions

diff --git a/arch/x86/ia32/ia32entry.S b/arch/x86/ia32/ia32entry.S
index 50190e15c1b6..acbff3fb96a1 100644
--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -311,7 +311,7 @@ ENDPROC(ia32_sysenter_target)
 ENTRY(ia32_cstar_target)
         CFI_STARTPROC32 simple
         CFI_SIGNAL_FRAME
-        CFI_DEF_CFA     rsp,KERNEL_STACK_OFFSET
+        CFI_DEF_CFA     rsp,0
         CFI_REGISTER    rip,rcx
         /*CFI_REGISTER  rflags,r11*/
         SWAPGS_UNSAFE_STACK
@@ -323,7 +323,7 @@ ENTRY(ia32_cstar_target)
          * disabled irqs and here we enable it straight after entry:
          */
         ENABLE_INTERRUPTS(CLBR_NONE)
-        ALLOC_PT_GPREGS_ON_STACK 8      /* +8: space for orig_ax */
+        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)
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index ae9c2f13b476..ad0ee3423da5 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -172,7 +172,6 @@ struct thread_info {
 #define _TIF_WORK_CTXSW_NEXT (_TIF_WORK_CTXSW)
 
 #define STACK_WARN              (THREAD_SIZE/8)
-#define KERNEL_STACK_OFFSET     (5*(BITS_PER_LONG/8))
 
 /*
  * macros/functions for gaining access to the thread information structure
@@ -201,10 +200,10 @@ static inline unsigned long current_stack_pointer(void)
 
 #else /* !__ASSEMBLY__ */
 
-/* how to get the thread information struct from ASM */
+/* Load thread_info address into "reg" */
 #define GET_THREAD_INFO(reg) \
         _ASM_MOV PER_CPU_VAR(kernel_stack),reg ; \
-        _ASM_SUB $(THREAD_SIZE-KERNEL_STACK_OFFSET),reg ;
+        _ASM_SUB $(THREAD_SIZE),reg ;
 
 /*
  * ASM operand which evaluates to thread_info address
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 66d62aef7214..002216ab9145 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1116,7 +1116,7 @@ static __init int setup_disablecpuid(char *arg)
 __setup("clearcpuid=", setup_disablecpuid);
 
 DEFINE_PER_CPU(unsigned long, kernel_stack) =
-        (unsigned long)&init_thread_union - KERNEL_STACK_OFFSET + THREAD_SIZE;
+        (unsigned long)&init_thread_union + THREAD_SIZE;
 EXPORT_PER_CPU_SYMBOL(kernel_stack);
 
 #ifdef CONFIG_X86_64
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index 8076df982a18..eae69bba3a2c 100644
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -225,7 +225,7 @@ ENDPROC(native_usergs_sysret64)
 ENTRY(system_call)
         CFI_STARTPROC   simple
         CFI_SIGNAL_FRAME
-        CFI_DEF_CFA     rsp,KERNEL_STACK_OFFSET
+        CFI_DEF_CFA     rsp,0
         CFI_REGISTER    rip,rcx
         /*CFI_REGISTER  rflags,r11*/
         SWAPGS_UNSAFE_STACK
@@ -242,9 +242,8 @@ GLOBAL(system_call_after_swapgs)
          * so we can enable interrupts only after we're done with using rsp_scratch:
          */
         movq    %rsp,PER_CPU_VAR(rsp_scratch)
-        /* kernel_stack is set so that 5 slots (iret frame) are preallocated */
         movq    PER_CPU_VAR(kernel_stack),%rsp
-        ALLOC_PT_GPREGS_ON_STACK 8              /* +8: space for orig_ax */
+        ALLOC_PT_GPREGS_ON_STACK 6*8 /* 6*8: space for orig_ax and iret frame */
         movq    %rcx,RIP(%rsp)
         movq    PER_CPU_VAR(rsp_scratch),%rcx
         movq    %r11,EFLAGS(%rsp)
diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index c5e987022ca0..8ed2106b06da 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -308,7 +308,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
         load_sp0(tss, next);
         this_cpu_write(kernel_stack,
                        (unsigned long)task_stack_page(next_p) +
-                       THREAD_SIZE - KERNEL_STACK_OFFSET);
+                       THREAD_SIZE);
         this_cpu_write(cpu_current_top_of_stack,
                        (unsigned long)task_stack_page(next_p) +
                        THREAD_SIZE);
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index da8b74598d90..4baaa972f52a 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -410,8 +410,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
         load_sp0(tss, next);
 
         this_cpu_write(kernel_stack,
-                  (unsigned long)task_stack_page(next_p) +
-                  THREAD_SIZE - KERNEL_STACK_OFFSET);
+                (unsigned long)task_stack_page(next_p) + THREAD_SIZE);
 
         /*
          * Now maybe reload the debug registers and handle I/O bitmaps
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 759388c538cf..7b20ffd2fffc 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -813,8 +813,7 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
         initial_gs = per_cpu_offset(cpu);
 #endif
         per_cpu(kernel_stack, cpu) =
-                (unsigned long)task_stack_page(idle) -
-                KERNEL_STACK_OFFSET + THREAD_SIZE;
+                (unsigned long)task_stack_page(idle) + THREAD_SIZE;
         early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
         initial_code = (unsigned long)start_secondary;
         stack_start  = idle->thread.sp;
diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index 08e8489c47f1..765b7684f858 100644
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -452,8 +452,7 @@ static int xen_cpu_up(unsigned int cpu, struct task_struct *idle)
         clear_tsk_thread_flag(idle, TIF_FORK);
 #endif
         per_cpu(kernel_stack, cpu) =
-                (unsigned long)task_stack_page(idle) -
-                KERNEL_STACK_OFFSET + THREAD_SIZE;
+                (unsigned long)task_stack_page(idle) + THREAD_SIZE;
 
         xen_setup_runstate_info(cpu);
         xen_setup_timer(cpu);