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    SubjectRe: [PATCH] x86/traps: Weaken context tracking entry assertions
    On Thu, Aug 20, 2015 at 10:03:21PM -0700, Andy Lutomirski wrote:
    > We were asserting that we were all the way in CONTEXT_KERNEL when
    > exception handlers were called. While having this be true is, I
    > think, a nice goal (or maybe a variant in which we assert that we're
    > in CONTEXT_KERNEL or some new IRQ context), we're not quite there.
    >
    > In particular, if an IRQ interrupts the SYSCALL prologue and the IRQ
    > handler in turn causes an exception, the exception entry will be
    > called in RCU IRQ mode but with CONTEXT_USER.
    >
    > This is okay (nothing goes wrong), but until we fix up the SYSCALL
    > prologue, we need to avoid warning.

    We can avoid interrupts before the context tracking call but we'll
    never be able to remove all possibility for exceptions. I don't think
    we can assume that without making context tracking more fragile.

    >
    > Signed-off-by: Andy Lutomirski <luto@kernel.org>

    ACK!

    Thanks!

    We can indeed definetly trigger an exception in the kernel entry code
    (syscall, exception, irq) before the user_exit() call and that
    would break the checks. We can fix that later with context tracking
    calls on exception entry code. I still think an exception slow path
    based on static keys is the best way to go there.

    > ---
    > arch/x86/kernel/traps.c | 18 +++++++++---------
    > 1 file changed, 9 insertions(+), 9 deletions(-)
    >
    > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
    > index 86a82eafb96f..45e8d9891fa3 100644
    > --- a/arch/x86/kernel/traps.c
    > +++ b/arch/x86/kernel/traps.c
    > @@ -112,7 +112,7 @@ static inline void preempt_conditional_cli(struct pt_regs *regs)
    > void ist_enter(struct pt_regs *regs)
    > {
    > if (user_mode(regs)) {
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > } else {
    > /*
    > * We might have interrupted pretty much anything. In
    > @@ -282,7 +282,7 @@ static void do_error_trap(struct pt_regs *regs, long error_code, char *str,
    > {
    > siginfo_t info;
    >
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    >
    > if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) !=
    > NOTIFY_STOP) {
    > @@ -364,7 +364,7 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
    > const struct bndcsr *bndcsr;
    > siginfo_t *info;
    >
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > if (notify_die(DIE_TRAP, "bounds", regs, error_code,
    > X86_TRAP_BR, SIGSEGV) == NOTIFY_STOP)
    > return;
    > @@ -442,7 +442,7 @@ do_general_protection(struct pt_regs *regs, long error_code)
    > {
    > struct task_struct *tsk;
    >
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > conditional_sti(regs);
    >
    > if (v8086_mode(regs)) {
    > @@ -496,7 +496,7 @@ dotraplinkage void notrace do_int3(struct pt_regs *regs, long error_code)
    > return;
    >
    > ist_enter(regs);
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > #ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
    > if (kgdb_ll_trap(DIE_INT3, "int3", regs, error_code, X86_TRAP_BP,
    > SIGTRAP) == NOTIFY_STOP)
    > @@ -729,14 +729,14 @@ static void math_error(struct pt_regs *regs, int error_code, int trapnr)
    >
    > dotraplinkage void do_coprocessor_error(struct pt_regs *regs, long error_code)
    > {
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > math_error(regs, error_code, X86_TRAP_MF);
    > }
    >
    > dotraplinkage void
    > do_simd_coprocessor_error(struct pt_regs *regs, long error_code)
    > {
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > math_error(regs, error_code, X86_TRAP_XF);
    > }
    >
    > @@ -749,7 +749,7 @@ do_spurious_interrupt_bug(struct pt_regs *regs, long error_code)
    > dotraplinkage void
    > do_device_not_available(struct pt_regs *regs, long error_code)
    > {
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > BUG_ON(use_eager_fpu());
    >
    > #ifdef CONFIG_MATH_EMULATION
    > @@ -775,7 +775,7 @@ dotraplinkage void do_iret_error(struct pt_regs *regs, long error_code)
    > {
    > siginfo_t info;
    >
    > - CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
    > + rcu_lockdep_assert(rcu_is_watching(), "entry code didn't wake RCU");
    > local_irq_enable();
    >
    > info.si_signo = SIGILL;
    > --
    > 2.4.3
    >


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