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

[Ingo Molnar]

* Richard Weinberger <richard.weinberger@xxxxxxxxx> wrote:

> On Wed, Jun 17, 2015 at 11:48 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> >
> > * Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> >> 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.
> >
> > Modulo the small comments I made about the debug checks interface plus naming
> > details the structure and intention of this series gives me warm fuzzy feelings.
> >
> >> 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.
> >
> > Amen.
> >
> >> 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.
> >
> > Any known bugs beyond UML build breakage?
> >
> >> 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?
> >
> > So assuming you fix the UML build I'm inclined to go for it, even in this
> > incomplete form, to increase testing coverage.
> 
> Andy, can you please share the build breakage you're facing?
> I'll happily help you fixing it.

So they come in the form of:

  ./arch/um/include/shared/kern_util.h:25:12: error: conflicting types for âdo_signalâ

which comes from now x86 also having a do_signal().

The patch below fixes it by harmonizing the UML implementation with the x86 one. 
This improves the UML side a bit, and fixes the build failure.

Thanks,

	Ingo

=========================>
Subject: uml: Fix do_signal() prototype
From: Ingo Molnar <mingo@xxxxxxxxxx>
Date: Wed Jun 17 12:58:37 CEST 2015

Now that x86 exports its do_signal(), the prototypes clash.

Fix the clash and also improve the code a bit: remove the unnecessary
kern_do_signal() indirection. This allows interrupt_end() to share
the 'regs' parameter calculation.

Also remove the unused return code to match x86.

Minimally build and boot tested.

Cc: Richard Weinberger <richard.weinberger@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/um/include/shared/kern_util.h |    3 ++-
 arch/um/kernel/process.c           |    6 ++++--
 arch/um/kernel/signal.c            |    8 +-------
 arch/um/kernel/tlb.c               |    2 +-
 arch/um/kernel/trap.c              |    2 +-
 5 files changed, 9 insertions(+), 12 deletions(-)

Index: tip/arch/um/include/shared/kern_util.h
===================================================================
--- tip.orig/arch/um/include/shared/kern_util.h
+++ tip/arch/um/include/shared/kern_util.h
@@ -22,7 +22,8 @@ extern int kmalloc_ok;
 extern unsigned long alloc_stack(int order, int atomic);
 extern void free_stack(unsigned long stack, int order);
 
-extern int do_signal(void);
+struct pt_regs;
+extern void do_signal(struct pt_regs *regs);
 extern void interrupt_end(void);
 extern void relay_signal(int sig, struct siginfo *si, struct uml_pt_regs *regs);
 
Index: tip/arch/um/kernel/process.c
===================================================================
--- tip.orig/arch/um/kernel/process.c
+++ tip/arch/um/kernel/process.c
@@ -90,12 +90,14 @@ void *__switch_to(struct task_struct *fr
 
 void interrupt_end(void)
 {
+	struct pt_regs *regs = &current->thread.regs;
+
 	if (need_resched())
 		schedule();
 	if (test_thread_flag(TIF_SIGPENDING))
-		do_signal();
+		do_signal(regs);
 	if (test_and_clear_thread_flag(TIF_NOTIFY_RESUME))
-		tracehook_notify_resume(&current->thread.regs);
+		tracehook_notify_resume(regs);
 }
 
 void exit_thread(void)
Index: tip/arch/um/kernel/signal.c
===================================================================
--- tip.orig/arch/um/kernel/signal.c
+++ tip/arch/um/kernel/signal.c
@@ -64,7 +64,7 @@ static void handle_signal(struct ksignal
 	signal_setup_done(err, ksig, singlestep);
 }
 
-static int kern_do_signal(struct pt_regs *regs)
+void do_signal(struct pt_regs *regs)
 {
 	struct ksignal ksig;
 	int handled_sig = 0;
@@ -110,10 +110,4 @@ static int kern_do_signal(struct pt_regs
 	 */
 	if (!handled_sig)
 		restore_saved_sigmask();
-	return handled_sig;
-}
-
-int do_signal(void)
-{
-	return kern_do_signal(&current->thread.regs);
 }
Index: tip/arch/um/kernel/tlb.c
===================================================================
--- tip.orig/arch/um/kernel/tlb.c
+++ tip/arch/um/kernel/tlb.c
@@ -291,7 +291,7 @@ void fix_range_common(struct mm_struct *
 		/* We are under mmap_sem, release it such that current can terminate */
 		up_write(&current->mm->mmap_sem);
 		force_sig(SIGKILL, current);
-		do_signal();
+		do_signal(&current->thread.regs);
 	}
 }
 
Index: tip/arch/um/kernel/trap.c
===================================================================
--- tip.orig/arch/um/kernel/trap.c
+++ tip/arch/um/kernel/trap.c
@@ -173,7 +173,7 @@ static void bad_segv(struct faultinfo fi
 void fatal_sigsegv(void)
 {
 	force_sigsegv(SIGSEGV, current);
-	do_signal();
+	do_signal(&current->thread.regs);
 	/*
 	 * This is to tell gcc that we're not returning - do_signal
 	 * can, in general, return, but in this case, it's not, since
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