lkml.org 
[lkml]   [2022]   [Mar]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
From
Subject[PATCH V3 3/7] x86/entry: Switch the stack after error_entry() returns
Date
From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

error_entry() calls sync_regs() to settle/copy the pt_regs and switches
the stack directly after sync_regs(). But error_entry() itself is also
a function call, the switching has to handle the return address of it
together, which causes the work complicated and tangly.

Switching to the stack after error_entry() makes the code simpler and
intuitive.

The behavior/logic is unchanged:
1) (opt) feed fixup_bad_iret() with the pt_regs pushed by ASM code
2) (opt) fixup_bad_iret() moves the partial pt_regs up
3) feed sync_regs() with the pt_regs pushed by ASM code or returned
by fixup_bad_iret()
4) sync_regs() copies the whole pt_regs to kernel stack if needed
5) after error_entry() and switching %rsp, it is in kernel stack with
the pt_regs

Changes only in calling:
Old code switches to copied pt_regs immediately twice in
error_entry() while new code switches to the copied pt_regs only
once after error_entry() returns.
It is correct since sync_regs() doesn't need to be called close
to the pt_regs it handles.

Old code stashes the return-address of error_entry() in a scratch
register and new code doesn't stash it.
It relies on the fact that fixup_bad_iret() and sync_regs() don't
corrupt the return-address of error_entry() on the stack. But even
the old code also relies on the fact that fixup_bad_iret() and
sync_regs() don't corrupt the return-address of themselves.
They are the same reliances and are assured.

After this change, error_entry() will not do fancy things with the stack
except when in the prolog which will be fixed in the next patch ("move
PUSH_AND_CLEAR_REGS out of error_entry"). This patch and the next patch
can't be swapped because the next patch relies on this patch's stopping
fiddling with the return-address of error_entry(), otherwise the objtool
would complain.

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 37505331b7f1..7768cdd0c7ed 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 stack settled by sync_regs() */
+ ENCODE_FRAME_POINTER
UNWIND_HINT_REGS

movq %rsp, %rdi /* pt_regs pointer into 1st argument*/
@@ -1014,14 +1016,10 @@ 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

/*
@@ -1053,6 +1051,7 @@ 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:
@@ -1073,12 +1072,9 @@ 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
\
 
 \ /
  Last update: 2022-03-15 08:39    [W:0.820 / U:0.088 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site