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Subject[PATCH V7 2/6] x86/entry: Switch the stack after error_entry() returns
Date
From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

error_entry() calls sync_regs(), and fixup_bad_iret() before sync_regs()
if it is a fault from bad IRET, to copy the pt_regs to the kernel stack
and switches the kernel stack directly after sync_regs().

But error_entry() itself is also a function call, so the code has to
stash the address error_entry() is going to return to, in %r12 and
makes the work complicated.

Move the code of switching stack after error_entry() and get rid of the
need to handle the return address.

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
---
arch/x86/entry/entry_64.S | 16 ++++++----------
1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index ecbfca3cc18c..ca3e99e08a44 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -326,6 +326,8 @@ SYM_CODE_END(ret_from_fork)
.macro idtentry_body cfunc has_error_code:req

call error_entry
+ movq %rax, %rsp /* switch to the task stack if from userspace */
+ ENCODE_FRAME_POINTER
UNWIND_HINT_REGS

movq %rsp, %rdi /* pt_regs pointer into 1st argument*/
@@ -1002,14 +1004,10 @@ SYM_CODE_START_LOCAL(error_entry)
/* We have user CR3. Change to kernel CR3. */
SWITCH_TO_KERNEL_CR3 scratch_reg=%rax

+ leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */
.Lerror_entry_from_usermode_after_swapgs:
/* Put us onto the real thread stack. */
- popq %r12 /* save return addr in %12 */
- movq %rsp, %rdi /* arg0 = pt_regs pointer */
call sync_regs
- movq %rax, %rsp /* switch stack */
- ENCODE_FRAME_POINTER
- pushq %r12
RET

/*
@@ -1041,6 +1039,7 @@ SYM_CODE_START_LOCAL(error_entry)
*/
.Lerror_entry_done_lfence:
FENCE_SWAPGS_KERNEL_ENTRY
+ leaq 8(%rsp), %rax /* return pt_regs pointer */
RET

.Lbstep_iret:
@@ -1061,12 +1060,9 @@ SYM_CODE_START_LOCAL(error_entry)
* Pretend that the exception came from user mode: set up pt_regs
* as if we faulted immediately after IRET.
*/
- popq %r12 /* save return addr in %12 */
- movq %rsp, %rdi /* arg0 = pt_regs pointer */
+ leaq 8(%rsp), %rdi /* arg0 = pt_regs pointer */
call fixup_bad_iret
- mov %rax, %rsp
- ENCODE_FRAME_POINTER
- pushq %r12
+ mov %rax, %rdi
jmp .Lerror_entry_from_usermode_after_swapgs
SYM_CODE_END(error_entry)

--
2.19.1.6.gb485710b
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