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context_tracking: Don't implement exception_enter/exit() on CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK

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The typical steps with context tracking are:

1) Task runs in userspace
2) Task enters the kernel (syscall/exception/IRQ)
3) Task switches from context tracking state CONTEXT_USER to
   CONTEXT_KERNEL (user_exit())
4) Task does stuff in kernel
5) Task switches from context tracking state CONTEXT_KERNEL to
   CONTEXT_USER (user_enter())
6) Task exits the kernel

If an exception fires between 5) and 6), the pt_regs and the context
tracking disagree on the context of the faulted/trapped instruction.
CONTEXT_KERNEL must be set before the exception handler, that's
unconditional for those handlers that want to be able to call into
schedule(), but CONTEXT_USER must be restored when the exception exits
whereas pt_regs tells that we are resuming to kernel space.

This can't be fixed with storing the context tracking state in a per-cpu
or per-task variable since another exception may fire onto the current
one and overwrite the saved state. Also the task can schedule. So it
has to be stored in a per task stack.

This is how exception_enter()/exception_exit() paper over the problem:

5) Task switches from context tracking state CONTEXT_KERNEL to
   CONTEXT_USER (user_enter())
5.1) Exception fires
5.2) prev_state = exception_enter() // save CONTEXT_USER to prev_state
                                    // and set CONTEXT_KERNEL
5.3) Exception handler
5.4) exception_enter(prev_state) // restore CONTEXT_USER
5.5) Exception resumes
6) Task exits the kernel

The condition to live without exception_enter()/exception_exit() is to
forbid exceptions and IRQs between 2) and 3) and between 5) and 6), or if
any is allowed to trigger, it won't call into context tracking, eg: NMIs,
and it won't schedule. These requirements are met by architectures
supporting CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK and those can
therefore afford not to implement this hack.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201117151637.259084-3-frederic@kernel.org
This commit is contained in:
Frederic Weisbecker 2020-11-17 16:16:34 +01:00 committed by Peter Zijlstra
parent 83c2da2e60
commit 179a9cf792
1 changed files with 4 additions and 2 deletions
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6
include/linux/context_tracking.h
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@ -51,7 +51,8 @@ static inline enum ctx_state exception_enter(void)
{
enum ctx_state prev_ctx;
if (!context_tracking_enabled())
if (IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) ||
!context_tracking_enabled())
return 0;
prev_ctx = this_cpu_read(context_tracking.state);
@ -63,7 +64,8 @@ static inline enum ctx_state exception_enter(void)
static inline void exception_exit(enum ctx_state prev_ctx)
{
if (context_tracking_enabled()) {
if (!IS_ENABLED(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK) &&
context_tracking_enabled()) {
if (prev_ctx != CONTEXT_KERNEL)
context_tracking_enter(prev_ctx);
}