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[RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code

Started byAndy Lutomirski <luto@kernel.org>
First post2015-06-16 22:20 +0200
Last post2015-06-16 22:30 +0200
Articles 4 — 1 participant

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  [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code Andy Lutomirski <luto@kernel.org> - 2015-06-16 22:20 +0200
    [RFC/INCOMPLETE 10/13] x86/asm/entry/64: Save all regs on interrupt entry Andy Lutomirski <luto@kernel.org> - 2015-06-16 22:30 +0200
    [RFC/INCOMPLETE 09/13] x86/entry/compat: Migrate compat syscalls to new exit hooks Andy Lutomirski <luto@kernel.org> - 2015-06-16 22:30 +0200
    [RFC/INCOMPLETE 11/13] x86/asm/entry/64: Simplify irq stack pt_regs handling Andy Lutomirski <luto@kernel.org> - 2015-06-16 22:30 +0200

#1166373 — [RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code

FromAndy Lutomirski <luto@kernel.org>
Date2015-06-16 22:20 +0200
Subject[RFC/INCOMPLETE 00/13] x86: Rewrite exit-to-userspace code
Message-ID<pC5uV-4CR-3@gated-at.bofh.it>
This is incomplete, but it's finally good enough that I think it's
time to get other opinions on it.  It is a complete rewrite of the
slow path code that handles exits to user mode.

The exit-to-usermode code is copied in several places and is written
in a nasty combination of asm and C.  It's not at all clear what
it's supposed to do, and the way it's structured makes it very hard
to work with.  For example, it's not even clear why syscall exit
hooks are called only once per syscall right now.  (It seems to be a
side effect of the way that rdi and rdx are handled in the asm loop,
and it seems reliable, but it's still pointlessly complicated.)  The
existing code also makes context tracking overly complicated and
hard to understand.  Finally, it's nearly impossible for anyone to
change what happens on exit to usermode, since the existing code is
so fragile.

I tried to clean it up incrementally, but I decided it was too hard.
Instead, this series just replaces the code.  It seems to work.

Context tracking in particular works very differently now.  The
low-level entry code checks that we're in CONTEXT_USER and switches
to CONTEXT_KERNEL.  The exit code does the reverse.  There is no
need to track what CONTEXT_XYZ state we came from, because we
already know.  Similarly, SCHEDULE_USER is gone, since we can
reschedule if needed by simply calling schedule() from C code.

The main things that are missing are that I haven't done the 32-bit
parts (anyone want to help?) and therefore I haven't deleted the old
C code.  I also think this may break UML for trivial reasons.

Because I haven't converted the 32-bit code yet, all of the now-unnecessary
unnecessary calls to exception_enter are still present in traps.c.

IRQ context tracking is still duplicated.  We should probably clean
it up by changing the core code to supply something like
irq_enter_we_are_already_in_context_kernel.

Thoughts?

Andy Lutomirski (13):
  context_tracking: Add context_tracking_assert_state
  notifiers: Assert that RCU is watching in notify_die
  x86: Move C entry and exit code to arch/x86/entry/common.c
  x86/traps: Assert that we're in CONTEXT_KERNEL in exception entries
  x86/entry: Add enter_from_user_mode and use it in syscalls
  x86/entry: Add new, comprehensible entry and exit hooks
  x86/entry/64: Really create an error-entry-from-usermode code path
  x86/entry/64: Migrate 64-bit syscalls to new exit hooks
  x86/entry/compat: Migrate compat syscalls to new exit hooks
  x86/asm/entry/64: Save all regs on interrupt entry
  x86/asm/entry/64: Simplify irq stack pt_regs handling
  x86/asm/entry/64: Migrate error and interrupt exit work to C
  x86/entry: Remove SCHEDULE_USER and asm/context-tracking.h

 arch/x86/entry/Makefile                 |   1 +
 arch/x86/entry/common.c                 | 372 ++++++++++++++++++++++++++++++++
 arch/x86/entry/entry_64.S               | 176 ++++-----------
 arch/x86/entry/entry_64_compat.S        |   7 +-
 arch/x86/include/asm/context_tracking.h |  10 -
 arch/x86/include/asm/signal.h           |   1 +
 arch/x86/kernel/ptrace.c                | 202 +----------------
 arch/x86/kernel/signal.c                |  28 +--
 arch/x86/kernel/traps.c                 |   9 +
 include/linux/context_tracking.h        |   8 +
 kernel/notifier.c                       |   2 +
 11 files changed, 439 insertions(+), 377 deletions(-)
 create mode 100644 arch/x86/entry/common.c
 delete mode 100644 arch/x86/include/asm/context_tracking.h

-- 
2.4.3

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#1166375 — [RFC/INCOMPLETE 10/13] x86/asm/entry/64: Save all regs on interrupt entry

FromAndy Lutomirski <luto@kernel.org>
Date2015-06-16 22:30 +0200
Subject[RFC/INCOMPLETE 10/13] x86/asm/entry/64: Save all regs on interrupt entry
Message-ID<pC5EB-4Oe-3@gated-at.bofh.it>
In reply to#1166373
To prepare for the big rewrite of the error and interrupt exit
paths, we will need pt_regs completely filled in.  It's already
completely filled in when error_exit runs, so rearrange interrupt
handling to match it.  This will slow down interrupt handling very
slightly (eight instructions), but the simplification it enables
will be more than worth it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_64.S | 29 +++++++++--------------------
 1 file changed, 9 insertions(+), 20 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a5044d7a9d43..43bf5762443c 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -501,21 +501,13 @@ END(irq_entries_start)
 /* 0(%rsp): ~(interrupt number) */
 	.macro interrupt func
 	cld
-	/*
-	 * Since nothing in interrupt handling code touches r12...r15 members
-	 * of "struct pt_regs", and since interrupts can nest, we can save
-	 * four stack slots and simultaneously provide
-	 * an unwind-friendly stack layout by saving "truncated" pt_regs
-	 * exactly up to rbp slot, without these members.
-	 */
-	ALLOC_PT_GPREGS_ON_STACK -RBP
-	SAVE_C_REGS -RBP
-	/* this goes to 0(%rsp) for unwinder, not for saving the value: */
-	SAVE_EXTRA_REGS_RBP -RBP
+	ALLOC_PT_GPREGS_ON_STACK
+	SAVE_C_REGS
+	SAVE_EXTRA_REGS
 
-	leaq	-RBP(%rsp), %rdi		/* arg1 for \func (pointer to pt_regs) */
+	movq	%rsp,%rdi	/* arg1 for \func (pointer to pt_regs) */
 
-	testb	$3, CS-RBP(%rsp)
+	testb	$3, CS(%rsp)
 	jz	1f
 	SWAPGS
 1:
@@ -552,9 +544,7 @@ ret_from_intr:
 	decl	PER_CPU_VAR(irq_count)
 
 	/* Restore saved previous stack */
-	popq	%rsi
-	/* return code expects complete pt_regs - adjust rsp accordingly: */
-	leaq	-RBP(%rsi), %rsp
+	popq	%rsp
 
 	testb	$3, CS(%rsp)
 	jz	retint_kernel
@@ -579,7 +569,7 @@ retint_swapgs:					/* return to user-space */
 	TRACE_IRQS_IRETQ
 
 	SWAPGS
-	jmp	restore_c_regs_and_iret
+	jmp	restore_regs_and_iret
 
 /* Returning to kernel space */
 retint_kernel:
@@ -603,6 +593,8 @@ retint_kernel:
  * At this label, code paths which return to kernel and to user,
  * which come from interrupts/exception and from syscalls, merge.
  */
+restore_regs_and_iret:
+	RESTORE_EXTRA_REGS
 restore_c_regs_and_iret:
 	RESTORE_C_REGS
 	REMOVE_PT_GPREGS_FROM_STACK 8
@@ -673,12 +665,10 @@ retint_signal:
 	jz	retint_swapgs
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_NONE)
-	SAVE_EXTRA_REGS
 	movq	$-1, ORIG_RAX(%rsp)
 	xorl	%esi, %esi			/* oldset */
 	movq	%rsp, %rdi			/* &pt_regs */
 	call	do_notify_resume
-	RESTORE_EXTRA_REGS
 	DISABLE_INTERRUPTS(CLBR_NONE)
 	TRACE_IRQS_OFF
 	GET_THREAD_INFO(%rcx)
@@ -1158,7 +1148,6 @@ END(error_entry)
  */
 ENTRY(error_exit)
 	movl	%ebx, %eax
-	RESTORE_EXTRA_REGS
 	DISABLE_INTERRUPTS(CLBR_NONE)
 	TRACE_IRQS_OFF
 	testl	%eax, %eax
-- 
2.4.3

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#1166377 — [RFC/INCOMPLETE 09/13] x86/entry/compat: Migrate compat syscalls to new exit hooks

FromAndy Lutomirski <luto@kernel.org>
Date2015-06-16 22:30 +0200
Subject[RFC/INCOMPLETE 09/13] x86/entry/compat: Migrate compat syscalls to new exit hooks
Message-ID<pC5EB-4Oe-9@gated-at.bofh.it>
In reply to#1166373
This is separate for ease of bisection.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_64_compat.S | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index bb187a6a877c..415afa038edf 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -209,10 +209,10 @@ sysexit_from_sys_call:
 	.endm
 
 	.macro auditsys_exit exit
-	testl	$(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
-	jnz	ia32_ret_from_sys_call
 	TRACE_IRQS_ON
 	ENABLE_INTERRUPTS(CLBR_NONE)
+	testl $(_TIF_ALLWORK_MASK & ~_TIF_SYSCALL_AUDIT), ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
+	jnz ia32_ret_from_sys_call
 	movl	%eax, %esi		/* second arg, syscall return value */
 	cmpl	$-MAX_ERRNO, %eax	/* is it an error ? */
 	jbe	1f
@@ -227,11 +227,10 @@ sysexit_from_sys_call:
 	testl	%edi, ASM_THREAD_INFO(TI_flags, %rsp, SIZEOF_PTREGS)
 	jz	\exit
 	xorl	%eax, %eax		/* Do not leak kernel information */
-	movq	%rax, R11(%rsp)
+	jmp	int_ret_from_sys_call_irqs_off
 	movq	%rax, R10(%rsp)
 	movq	%rax, R9(%rsp)
 	movq	%rax, R8(%rsp)
-	jmp	int_with_check
 	.endm
 
 sysenter_auditsys:
-- 
2.4.3

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#1166380 — [RFC/INCOMPLETE 11/13] x86/asm/entry/64: Simplify irq stack pt_regs handling

FromAndy Lutomirski <luto@kernel.org>
Date2015-06-16 22:30 +0200
Subject[RFC/INCOMPLETE 11/13] x86/asm/entry/64: Simplify irq stack pt_regs handling
Message-ID<pC5EC-4Oe-21@gated-at.bofh.it>
In reply to#1166373
There's no need for both rsi and rdi to point to the original stack.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/entry_64.S | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 43bf5762443c..ab8cbf602d19 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -505,8 +505,6 @@ END(irq_entries_start)
 	SAVE_C_REGS
 	SAVE_EXTRA_REGS
 
-	movq	%rsp,%rdi	/* arg1 for \func (pointer to pt_regs) */
-
 	testb	$3, CS(%rsp)
 	jz	1f
 	SWAPGS
@@ -518,14 +516,14 @@ END(irq_entries_start)
 	 * a little cheaper to use a separate counter in the PDA (short of
 	 * moving irq_enter into assembly, which would be too much work)
 	 */
-	movq	%rsp, %rsi
+	movq	%rsp, %rdi
 	incl	PER_CPU_VAR(irq_count)
 	cmovzq	PER_CPU_VAR(irq_stack_ptr), %rsp
-	pushq	%rsi
+	pushq	%rdi
 	/* We entered an interrupt context - irqs are off: */
 	TRACE_IRQS_OFF
 
-	call	\func
+	call	\func	/* rdi points to pt_regs */
 	.endm
 
 	/*
-- 
2.4.3

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