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    SubjectRe: [PATCH V2 3/6] x86_32/sysenter: switch to the task stack without emptying the entry stack
    On Mon, Jan 25, 2021 at 11:35 AM Lai Jiangshan <jiangshanlai@gmail.com> wrote:
    >
    > From: Lai Jiangshan <laijs@linux.alibaba.com>
    >
    > Like the way x86_64 uses the "old" stack, we can save the entry stack
    > pointer to a register and switch to the task stack. So that we have
    > space on the "old" stack to save more things or scratch registers.
    >
    > Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
    > ---
    > arch/x86/entry/entry_32.S | 11 +++++------
    > 1 file changed, 5 insertions(+), 6 deletions(-)
    >
    > diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
    > index 3b4d1a63d1f0..4513702ba45d 100644
    > --- a/arch/x86/entry/entry_32.S
    > +++ b/arch/x86/entry/entry_32.S
    > @@ -905,19 +905,18 @@ SYM_FUNC_START(entry_SYSENTER_32)
    > pushl %eax
    > BUG_IF_WRONG_CR3 no_user_check=1
    > SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
    > - popl %eax
    > - popfl
    >
    > - /* Stack empty again, switch to task stack */
    > - movl TSS_entry2task_stack(%esp), %esp
    > + /* Switch to task stack */
    > + movl %esp, %eax
    > + movl (2*4+TSS_entry2task_stack)(%esp), %esp
    >
    > .Lsysenter_past_esp:
    > pushl $__USER_DS /* pt_regs->ss */
    > pushl $0 /* pt_regs->sp (placeholder) */
    > - pushfl /* pt_regs->flags (except IF = 0) */
    > + pushl 4(%eax) /* pt_regs->flags (except IF = 0) */

    __KERNEL_DS isn't loaded at this point, so this needs an explicit %ss:
    override. You probably didn't catch this because the default
    __USER_DS was still loaded.

    > pushl $__USER_CS /* pt_regs->cs */
    > pushl $0 /* pt_regs->ip = 0 (placeholder) */
    > - pushl %eax /* pt_regs->orig_ax */
    > + pushl (%eax) /* pt_regs->orig_ax */

    Add an %ss: override here too.

    > SAVE_ALL pt_regs_ax=$-ENOSYS /* save rest, stack already switched */
    >
    > /*
    > --
    > 2.19.1.6.gb485710b
    >

    --
    Brian Gerst

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