Re: [patch 8/8] x86/entry: Move irqflags tracing to do_int80_syscall_32()

[Alexandre Chartre]

On 2/25/20 11:08 PM, Thomas Gleixner wrote:
> which cleans up the ASM maze.
>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>   arch/x86/entry/common.c          |    8 +++++++-
>   arch/x86/entry/entry_32.S        |    9 ++-------
>   arch/x86/entry/entry_64_compat.S |   14 +++++---------
>   3 files changed, 14 insertions(+), 17 deletions(-)
>
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -333,6 +333,7 @@ void do_syscall_64_irqs_on(unsigned long
>   {
>   	syscall_entry_fixups();
>   	do_syscall_64_irqs_on(nr, regs);
> +	trace_hardirqs_on();
>   }

trace_hardirqs_on() is already called through syscall_return_slowpath()
(from the previous patch):

do_syscall_64()
  -> do_syscall_64_irqs_on()
    -> syscall_return_slowpath()
      -> trace_hardirqs_on()

>   NOKPROBE_SYMBOL(do_syscall_64);
>   #endif
> @@ -389,6 +390,7 @@ static __always_inline void do_syscall_3
>   {
>   	syscall_entry_fixups();
>   	do_syscall_32_irqs_on(regs);
> +	trace_hardirqs_on();
>   }

Same here:

do_int80_syscall_32()
  -> do_syscall_32_irqs_on()
    -> syscall_return_slowpath()
      -> trace_hardirqs_on()

>   NOKPROBE_SYMBOL(do_int80_syscall_32);
>   
> @@ -468,8 +470,12 @@ static __always_inline long do_fast_sysc
>   /* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
>   __visible notrace long do_fast_syscall_32(struct pt_regs *regs)
>   {
> +	long ret;
> +
>   	syscall_entry_fixups();
> -	return do_fast_syscall_32_irqs_on(regs);
> +	ret = do_fast_syscall_32_irqs_on(regs);
> +	trace_hardirqs_on();
> +	return ret;
>   }
>   NOKPROBE_SYMBOL(do_fast_syscall_32);

Same here:

  do_fast_syscall_32()
    -> do_fast_syscall_32_irqs_on()
      -> do_syscall_32_irqs_on()
        -> syscall_return_slowpath()
          -> trace_hardirqs_on()

Except for one case (if the get_user() call is true in
do_fast_syscall_32_irqs_on()):

  do_fast_syscall_32()
    -> do_fast_syscall_32_irqs_on()
      -> prepare_exit_to_usermode()

So we need to call trace_hardirqs_on() but only in that case:

static __always_inline long do_fast_syscall_32_irqs_on(struct pt_regs *regs)
{
          ...
	if (
#ifdef CONFIG_X86_64
		/*
		 * Micro-optimization: the pointer we're following is explicitly
		 * 32 bits, so it can't be out of range.
		 */
		__get_user(*(u32 *)&regs->bp,
			    (u32 __user __force *)(unsigned long)(u32)regs->sp)
#else
		get_user(*(u32 *)&regs->bp,
			 (u32 __user __force *)(unsigned long)(u32)regs->sp)
#endif
		) {

		/* User code screwed up. */
		local_irq_disable();
		regs->ax = -EFAULT;
		prepare_exit_to_usermode(regs);
                trace_hardirqs_on();                <<<=== HERE
		return 0;	/* Keep it simple: use IRET. */
	}
        ...
}

alex.


> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -811,8 +811,7 @@ SYM_CODE_START(ret_from_fork)
>   	/* When we fork, we trace the syscall return in the child, too. */
>   	movl    %esp, %eax
>   	call    syscall_return_slowpath
> -	STACKLEAK_ERASE
> -	jmp     restore_all_switch_stack
> +	jmp     .Lsyscall_32_done
>   
>   	/* kernel thread */
>   1:	movl	%edi, %eax
> @@ -968,8 +967,7 @@ SYM_FUNC_START(entry_SYSENTER_32)
>   
>   	STACKLEAK_ERASE
>   
> -/* Opportunistic SYSEXIT */
> -	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
> +	/* Opportunistic SYSEXIT */
>   
>   	/*
>   	 * Setup entry stack - we keep the pointer in %eax and do the
> @@ -1072,11 +1070,8 @@ SYM_FUNC_START(entry_INT80_32)
>   	movl	%esp, %eax
>   	call	do_int80_syscall_32
>   .Lsyscall_32_done:
> -
>   	STACKLEAK_ERASE
>   
> -restore_all:
> -	TRACE_IRQS_IRET
>   restore_all_switch_stack:
>   	SWITCH_TO_ENTRY_STACK
>   	CHECK_AND_APPLY_ESPFIX
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -132,8 +132,8 @@ SYM_FUNC_START(entry_SYSENTER_compat)
>   	movq	%rsp, %rdi
>   	call	do_fast_syscall_32
>   	/* XEN PV guests always use IRET path */
> -	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
> -		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
> +	ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
> +		    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
>   	jmp	sysret32_from_system_call
>   
>   .Lsysenter_fix_flags:
> @@ -244,8 +244,8 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_aft
>   	movq	%rsp, %rdi
>   	call	do_fast_syscall_32
>   	/* XEN PV guests always use IRET path */
> -	ALTERNATIVE "testl %eax, %eax; jz .Lsyscall_32_done", \
> -		    "jmp .Lsyscall_32_done", X86_FEATURE_XENPV
> +	ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
> +		    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
>   
>   	/* Opportunistic SYSRET */
>   sysret32_from_system_call:
> @@ -254,7 +254,7 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_aft
>   	 * stack. So let's erase the thread stack right now.
>   	 */
>   	STACKLEAK_ERASE
> -	TRACE_IRQS_ON			/* User mode traces as IRQs on. */
> +
>   	movq	RBX(%rsp), %rbx		/* pt_regs->rbx */
>   	movq	RBP(%rsp), %rbp		/* pt_regs->rbp */
>   	movq	EFLAGS(%rsp), %r11	/* pt_regs->flags (in r11) */
> @@ -393,9 +393,5 @@ SYM_CODE_START(entry_INT80_compat)
>   
>   	movq	%rsp, %rdi
>   	call	do_int80_syscall_32
> -.Lsyscall_32_done:
> -
> -	/* Go back to user mode. */
> -	TRACE_IRQS_ON
>   	jmp	swapgs_restore_regs_and_return_to_usermode
>   SYM_CODE_END(entry_INT80_compat)