x86: make lazy %gs optional on x86_32
Impact: pt_regs changed, lazy gs handling made optional, add slight
overhead to SAVE_ALL, simplifies error_code path a bit
On x86_32, %gs hasn't been used by kernel and handled lazily. pt_regs
doesn't have place for it and gs is saved/loaded only when necessary.
In preparation for stack protector support, this patch makes lazy %gs
handling optional by doing the followings.
* Add CONFIG_X86_32_LAZY_GS and place for gs in pt_regs.
* Save and restore %gs along with other registers in entry_32.S unless
LAZY_GS. Note that this unfortunately adds "pushl $0" on SAVE_ALL
even when LAZY_GS. However, it adds no overhead to common exit path
and simplifies entry path with error code.
* Define different user_gs accessors depending on LAZY_GS and add
lazy_save_gs() and lazy_load_gs() which are noop if !LAZY_GS. The
lazy_*_gs() ops are used to save, load and clear %gs lazily.
* Define ELF_CORE_COPY_KERNEL_REGS() which always read %gs directly.
xen and lguest changes need to be verified.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
index c461925..82e6868 100644
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -30,12 +30,13 @@
* 1C(%esp) - %ds
* 20(%esp) - %es
* 24(%esp) - %fs
- * 28(%esp) - orig_eax
- * 2C(%esp) - %eip
- * 30(%esp) - %cs
- * 34(%esp) - %eflags
- * 38(%esp) - %oldesp
- * 3C(%esp) - %oldss
+ * 28(%esp) - %gs saved iff !CONFIG_X86_32_LAZY_GS
+ * 2C(%esp) - orig_eax
+ * 30(%esp) - %eip
+ * 34(%esp) - %cs
+ * 38(%esp) - %eflags
+ * 3C(%esp) - %oldesp
+ * 40(%esp) - %oldss
*
* "current" is in register %ebx during any slow entries.
*/
@@ -101,8 +102,99 @@
#define resume_userspace_sig resume_userspace
#endif
+/*
+ * User gs save/restore
+ *
+ * %gs is used for userland TLS and kernel only uses it for stack
+ * canary which is required to be at %gs:20 by gcc. Read the comment
+ * at the top of stackprotector.h for more info.
+ *
+ * Local labels 98 and 99 are used.
+ */
+#ifdef CONFIG_X86_32_LAZY_GS
+
+ /* unfortunately push/pop can't be no-op */
+.macro PUSH_GS
+ pushl $0
+ CFI_ADJUST_CFA_OFFSET 4
+.endm
+.macro POP_GS pop=0
+ addl $(4 + \pop), %esp
+ CFI_ADJUST_CFA_OFFSET -(4 + \pop)
+.endm
+.macro POP_GS_EX
+.endm
+
+ /* all the rest are no-op */
+.macro PTGS_TO_GS
+.endm
+.macro PTGS_TO_GS_EX
+.endm
+.macro GS_TO_REG reg
+.endm
+.macro REG_TO_PTGS reg
+.endm
+.macro SET_KERNEL_GS reg
+.endm
+
+#else /* CONFIG_X86_32_LAZY_GS */
+
+.macro PUSH_GS
+ pushl %gs
+ CFI_ADJUST_CFA_OFFSET 4
+ /*CFI_REL_OFFSET gs, 0*/
+.endm
+
+.macro POP_GS pop=0
+98: popl %gs
+ CFI_ADJUST_CFA_OFFSET -4
+ /*CFI_RESTORE gs*/
+ .if \pop <> 0
+ add $\pop, %esp
+ CFI_ADJUST_CFA_OFFSET -\pop
+ .endif
+.endm
+.macro POP_GS_EX
+.pushsection .fixup, "ax"
+99: movl $0, (%esp)
+ jmp 98b
+.section __ex_table, "a"
+ .align 4
+ .long 98b, 99b
+.popsection
+.endm
+
+.macro PTGS_TO_GS
+98: mov PT_GS(%esp), %gs
+.endm
+.macro PTGS_TO_GS_EX
+.pushsection .fixup, "ax"
+99: movl $0, PT_GS(%esp)
+ jmp 98b
+.section __ex_table, "a"
+ .align 4
+ .long 98b, 99b
+.popsection
+.endm
+
+.macro GS_TO_REG reg
+ movl %gs, \reg
+ /*CFI_REGISTER gs, \reg*/
+.endm
+.macro REG_TO_PTGS reg
+ movl \reg, PT_GS(%esp)
+ /*CFI_REL_OFFSET gs, PT_GS*/
+.endm
+.macro SET_KERNEL_GS reg
+ xorl \reg, \reg
+ movl \reg, %gs
+.endm
+
+#endif /* CONFIG_X86_32_LAZY_GS */
+
.macro SAVE_ALL
cld
+ PUSH_GS
pushl %fs
CFI_ADJUST_CFA_OFFSET 4
/*CFI_REL_OFFSET fs, 0;*/
@@ -138,6 +230,7 @@
movl %edx, %es
movl $(__KERNEL_PERCPU), %edx
movl %edx, %fs
+ SET_KERNEL_GS %edx
.endm
.macro RESTORE_INT_REGS
@@ -164,7 +257,7 @@
CFI_RESTORE eax
.endm
-.macro RESTORE_REGS
+.macro RESTORE_REGS pop=0
RESTORE_INT_REGS
1: popl %ds
CFI_ADJUST_CFA_OFFSET -4
@@ -175,6 +268,7 @@
3: popl %fs
CFI_ADJUST_CFA_OFFSET -4
/*CFI_RESTORE fs;*/
+ POP_GS \pop
.pushsection .fixup, "ax"
4: movl $0, (%esp)
jmp 1b
@@ -188,6 +282,7 @@
.long 2b, 5b
.long 3b, 6b
.popsection
+ POP_GS_EX
.endm
.macro RING0_INT_FRAME
@@ -368,6 +463,7 @@
xorl %ebp,%ebp
TRACE_IRQS_ON
1: mov PT_FS(%esp), %fs
+ PTGS_TO_GS
ENABLE_INTERRUPTS_SYSEXIT
#ifdef CONFIG_AUDITSYSCALL
@@ -416,6 +512,7 @@
.align 4
.long 1b,2b
.popsection
+ PTGS_TO_GS_EX
ENDPROC(ia32_sysenter_target)
# system call handler stub
@@ -458,8 +555,7 @@
restore_nocheck:
TRACE_IRQS_IRET
restore_nocheck_notrace:
- RESTORE_REGS
- addl $4, %esp # skip orig_eax/error_code
+ RESTORE_REGS 4 # skip orig_eax/error_code
CFI_ADJUST_CFA_OFFSET -4
irq_return:
INTERRUPT_RETURN
@@ -1078,7 +1174,10 @@
CFI_ADJUST_CFA_OFFSET 4
ALIGN
error_code:
- /* the function address is in %fs's slot on the stack */
+ /* the function address is in %gs's slot on the stack */
+ pushl %fs
+ CFI_ADJUST_CFA_OFFSET 4
+ /*CFI_REL_OFFSET fs, 0*/
pushl %es
CFI_ADJUST_CFA_OFFSET 4
/*CFI_REL_OFFSET es, 0*/
@@ -1107,20 +1206,15 @@
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET ebx, 0
cld
- pushl %fs
- CFI_ADJUST_CFA_OFFSET 4
- /*CFI_REL_OFFSET fs, 0*/
movl $(__KERNEL_PERCPU), %ecx
movl %ecx, %fs
UNWIND_ESPFIX_STACK
- popl %ecx
- CFI_ADJUST_CFA_OFFSET -4
- /*CFI_REGISTER es, ecx*/
- movl PT_FS(%esp), %edi # get the function address
+ GS_TO_REG %ecx
+ movl PT_GS(%esp), %edi # get the function address
movl PT_ORIG_EAX(%esp), %edx # get the error code
movl $-1, PT_ORIG_EAX(%esp) # no syscall to restart
- mov %ecx, PT_FS(%esp)
- /*CFI_REL_OFFSET fs, ES*/
+ REG_TO_PTGS %ecx
+ SET_KERNEL_GS %ecx
movl $(__USER_DS), %ecx
movl %ecx, %ds
movl %ecx, %es
