mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-24 09:50:04 +00:00
![Nicolas Saenz Julienne](/assets/img/avatar_default.png)
The topic of nesting and reentrancy in the context of early entry code hasn't been addressed so far. So do it. Signed-off-by: Nicolas Saenz Julienne <nsaenzju@redhat.com> Reviewed-by: Frederic Weisbecker <frederic@kernel.org> Reviewed-by: Paul E. McKenney <paulmck@kernel.org> Link: https://lore.kernel.org/r/20220110105044.94423-2-nsaenzju@redhat.com Signed-off-by: Jonathan Corbet <corbet@lwn.net>
279 lines
9.5 KiB
ReStructuredText
279 lines
9.5 KiB
ReStructuredText
Entry/exit handling for exceptions, interrupts, syscalls and KVM
|
|
================================================================
|
|
|
|
All transitions between execution domains require state updates which are
|
|
subject to strict ordering constraints. State updates are required for the
|
|
following:
|
|
|
|
* Lockdep
|
|
* RCU / Context tracking
|
|
* Preemption counter
|
|
* Tracing
|
|
* Time accounting
|
|
|
|
The update order depends on the transition type and is explained below in
|
|
the transition type sections: `Syscalls`_, `KVM`_, `Interrupts and regular
|
|
exceptions`_, `NMI and NMI-like exceptions`_.
|
|
|
|
Non-instrumentable code - noinstr
|
|
---------------------------------
|
|
|
|
Most instrumentation facilities depend on RCU, so intrumentation is prohibited
|
|
for entry code before RCU starts watching and exit code after RCU stops
|
|
watching. In addition, many architectures must save and restore register state,
|
|
which means that (for example) a breakpoint in the breakpoint entry code would
|
|
overwrite the debug registers of the initial breakpoint.
|
|
|
|
Such code must be marked with the 'noinstr' attribute, placing that code into a
|
|
special section inaccessible to instrumentation and debug facilities. Some
|
|
functions are partially instrumentable, which is handled by marking them
|
|
noinstr and using instrumentation_begin() and instrumentation_end() to flag the
|
|
instrumentable ranges of code:
|
|
|
|
.. code-block:: c
|
|
|
|
noinstr void entry(void)
|
|
{
|
|
handle_entry(); // <-- must be 'noinstr' or '__always_inline'
|
|
...
|
|
|
|
instrumentation_begin();
|
|
handle_context(); // <-- instrumentable code
|
|
instrumentation_end();
|
|
|
|
...
|
|
handle_exit(); // <-- must be 'noinstr' or '__always_inline'
|
|
}
|
|
|
|
This allows verification of the 'noinstr' restrictions via objtool on
|
|
supported architectures.
|
|
|
|
Invoking non-instrumentable functions from instrumentable context has no
|
|
restrictions and is useful to protect e.g. state switching which would
|
|
cause malfunction if instrumented.
|
|
|
|
All non-instrumentable entry/exit code sections before and after the RCU
|
|
state transitions must run with interrupts disabled.
|
|
|
|
Syscalls
|
|
--------
|
|
|
|
Syscall-entry code starts in assembly code and calls out into low-level C code
|
|
after establishing low-level architecture-specific state and stack frames. This
|
|
low-level C code must not be instrumented. A typical syscall handling function
|
|
invoked from low-level assembly code looks like this:
|
|
|
|
.. code-block:: c
|
|
|
|
noinstr void syscall(struct pt_regs *regs, int nr)
|
|
{
|
|
arch_syscall_enter(regs);
|
|
nr = syscall_enter_from_user_mode(regs, nr);
|
|
|
|
instrumentation_begin();
|
|
if (!invoke_syscall(regs, nr) && nr != -1)
|
|
result_reg(regs) = __sys_ni_syscall(regs);
|
|
instrumentation_end();
|
|
|
|
syscall_exit_to_user_mode(regs);
|
|
}
|
|
|
|
syscall_enter_from_user_mode() first invokes enter_from_user_mode() which
|
|
establishes state in the following order:
|
|
|
|
* Lockdep
|
|
* RCU / Context tracking
|
|
* Tracing
|
|
|
|
and then invokes the various entry work functions like ptrace, seccomp, audit,
|
|
syscall tracing, etc. After all that is done, the instrumentable invoke_syscall
|
|
function can be invoked. The instrumentable code section then ends, after which
|
|
syscall_exit_to_user_mode() is invoked.
|
|
|
|
syscall_exit_to_user_mode() handles all work which needs to be done before
|
|
returning to user space like tracing, audit, signals, task work etc. After
|
|
that it invokes exit_to_user_mode() which again handles the state
|
|
transition in the reverse order:
|
|
|
|
* Tracing
|
|
* RCU / Context tracking
|
|
* Lockdep
|
|
|
|
syscall_enter_from_user_mode() and syscall_exit_to_user_mode() are also
|
|
available as fine grained subfunctions in cases where the architecture code
|
|
has to do extra work between the various steps. In such cases it has to
|
|
ensure that enter_from_user_mode() is called first on entry and
|
|
exit_to_user_mode() is called last on exit.
|
|
|
|
Do not nest syscalls. Nested systcalls will cause RCU and/or context tracking
|
|
to print a warning.
|
|
|
|
KVM
|
|
---
|
|
|
|
Entering or exiting guest mode is very similar to syscalls. From the host
|
|
kernel point of view the CPU goes off into user space when entering the
|
|
guest and returns to the kernel on exit.
|
|
|
|
kvm_guest_enter_irqoff() is a KVM-specific variant of exit_to_user_mode()
|
|
and kvm_guest_exit_irqoff() is the KVM variant of enter_from_user_mode().
|
|
The state operations have the same ordering.
|
|
|
|
Task work handling is done separately for guest at the boundary of the
|
|
vcpu_run() loop via xfer_to_guest_mode_handle_work() which is a subset of
|
|
the work handled on return to user space.
|
|
|
|
Do not nest KVM entry/exit transitions because doing so is nonsensical.
|
|
|
|
Interrupts and regular exceptions
|
|
---------------------------------
|
|
|
|
Interrupts entry and exit handling is slightly more complex than syscalls
|
|
and KVM transitions.
|
|
|
|
If an interrupt is raised while the CPU executes in user space, the entry
|
|
and exit handling is exactly the same as for syscalls.
|
|
|
|
If the interrupt is raised while the CPU executes in kernel space the entry and
|
|
exit handling is slightly different. RCU state is only updated when the
|
|
interrupt is raised in the context of the CPU's idle task. Otherwise, RCU will
|
|
already be watching. Lockdep and tracing have to be updated unconditionally.
|
|
|
|
irqentry_enter() and irqentry_exit() provide the implementation for this.
|
|
|
|
The architecture-specific part looks similar to syscall handling:
|
|
|
|
.. code-block:: c
|
|
|
|
noinstr void interrupt(struct pt_regs *regs, int nr)
|
|
{
|
|
arch_interrupt_enter(regs);
|
|
state = irqentry_enter(regs);
|
|
|
|
instrumentation_begin();
|
|
|
|
irq_enter_rcu();
|
|
invoke_irq_handler(regs, nr);
|
|
irq_exit_rcu();
|
|
|
|
instrumentation_end();
|
|
|
|
irqentry_exit(regs, state);
|
|
}
|
|
|
|
Note that the invocation of the actual interrupt handler is within a
|
|
irq_enter_rcu() and irq_exit_rcu() pair.
|
|
|
|
irq_enter_rcu() updates the preemption count which makes in_hardirq()
|
|
return true, handles NOHZ tick state and interrupt time accounting. This
|
|
means that up to the point where irq_enter_rcu() is invoked in_hardirq()
|
|
returns false.
|
|
|
|
irq_exit_rcu() handles interrupt time accounting, undoes the preemption
|
|
count update and eventually handles soft interrupts and NOHZ tick state.
|
|
|
|
In theory, the preemption count could be updated in irqentry_enter(). In
|
|
practice, deferring this update to irq_enter_rcu() allows the preemption-count
|
|
code to be traced, while also maintaining symmetry with irq_exit_rcu() and
|
|
irqentry_exit(), which are described in the next paragraph. The only downside
|
|
is that the early entry code up to irq_enter_rcu() must be aware that the
|
|
preemption count has not yet been updated with the HARDIRQ_OFFSET state.
|
|
|
|
Note that irq_exit_rcu() must remove HARDIRQ_OFFSET from the preemption count
|
|
before it handles soft interrupts, whose handlers must run in BH context rather
|
|
than irq-disabled context. In addition, irqentry_exit() might schedule, which
|
|
also requires that HARDIRQ_OFFSET has been removed from the preemption count.
|
|
|
|
Even though interrupt handlers are expected to run with local interrupts
|
|
disabled, interrupt nesting is common from an entry/exit perspective. For
|
|
example, softirq handling happens within an irqentry_{enter,exit}() block with
|
|
local interrupts enabled. Also, although uncommon, nothing prevents an
|
|
interrupt handler from re-enabling interrupts.
|
|
|
|
Interrupt entry/exit code doesn't strictly need to handle reentrancy, since it
|
|
runs with local interrupts disabled. But NMIs can happen anytime, and a lot of
|
|
the entry code is shared between the two.
|
|
|
|
NMI and NMI-like exceptions
|
|
---------------------------
|
|
|
|
NMIs and NMI-like exceptions (machine checks, double faults, debug
|
|
interrupts, etc.) can hit any context and must be extra careful with
|
|
the state.
|
|
|
|
State changes for debug exceptions and machine-check exceptions depend on
|
|
whether these exceptions happened in user-space (breakpoints or watchpoints) or
|
|
in kernel mode (code patching). From user-space, they are treated like
|
|
interrupts, while from kernel mode they are treated like NMIs.
|
|
|
|
NMIs and other NMI-like exceptions handle state transitions without
|
|
distinguishing between user-mode and kernel-mode origin.
|
|
|
|
The state update on entry is handled in irqentry_nmi_enter() which updates
|
|
state in the following order:
|
|
|
|
* Preemption counter
|
|
* Lockdep
|
|
* RCU / Context tracking
|
|
* Tracing
|
|
|
|
The exit counterpart irqentry_nmi_exit() does the reverse operation in the
|
|
reverse order.
|
|
|
|
Note that the update of the preemption counter has to be the first
|
|
operation on enter and the last operation on exit. The reason is that both
|
|
lockdep and RCU rely on in_nmi() returning true in this case. The
|
|
preemption count modification in the NMI entry/exit case must not be
|
|
traced.
|
|
|
|
Architecture-specific code looks like this:
|
|
|
|
.. code-block:: c
|
|
|
|
noinstr void nmi(struct pt_regs *regs)
|
|
{
|
|
arch_nmi_enter(regs);
|
|
state = irqentry_nmi_enter(regs);
|
|
|
|
instrumentation_begin();
|
|
nmi_handler(regs);
|
|
instrumentation_end();
|
|
|
|
irqentry_nmi_exit(regs);
|
|
}
|
|
|
|
and for e.g. a debug exception it can look like this:
|
|
|
|
.. code-block:: c
|
|
|
|
noinstr void debug(struct pt_regs *regs)
|
|
{
|
|
arch_nmi_enter(regs);
|
|
|
|
debug_regs = save_debug_regs();
|
|
|
|
if (user_mode(regs)) {
|
|
state = irqentry_enter(regs);
|
|
|
|
instrumentation_begin();
|
|
user_mode_debug_handler(regs, debug_regs);
|
|
instrumentation_end();
|
|
|
|
irqentry_exit(regs, state);
|
|
} else {
|
|
state = irqentry_nmi_enter(regs);
|
|
|
|
instrumentation_begin();
|
|
kernel_mode_debug_handler(regs, debug_regs);
|
|
instrumentation_end();
|
|
|
|
irqentry_nmi_exit(regs, state);
|
|
}
|
|
}
|
|
|
|
There is no combined irqentry_nmi_if_kernel() function available as the
|
|
above cannot be handled in an exception-agnostic way.
|
|
|
|
NMIs can happen in any context. For example, an NMI-like exception triggered
|
|
while handling an NMI. So NMI entry code has to be reentrant and state updates
|
|
need to handle nesting.
|