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Subject[PATCH v2] x86/orc: Don't bail on stack overflow
Date
If we overflow the stack into a guard page and then try to unwind it
with ORC, it should work well: by construction, there can't be any
meaningful data in the guard page because no writes to the guard page
will have succeeded.

This patch fixes a bug that unwinding from working correctly: if the
starting register state has RSP pointing into a stack guard page, the
ORC unwinder bails out immediately. This patch fixes that: the ORC
unwinder will start the unwind.

I tested this by intentionally overflowing the task stack. The
result is an accurate call trace instead of a trace consisting
purely of '?' entries.

There are a few other bugs that are triggered if the unwinder
encounters a stack overflow after the first step, and Josh has WIP
patches to fix those as well.

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

Changes from v1: better changelog

I'm not going to send Josh's patch yet. He said he'd split it up
nicely, so I'll let him :)

arch/x86/kernel/unwind_orc.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index a3f973b2c97a..7f6e3935666b 100644
--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -553,8 +553,18 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
}

if (get_stack_info((unsigned long *)state->sp, state->task,
- &state->stack_info, &state->stack_mask))
- return;
+ &state->stack_info, &state->stack_mask)) {
+ /*
+ * We weren't on a valid stack. It's possible that
+ * we overflowed a valid stack into a guard page.
+ * See if the next page up is valid so that we can
+ * generate some kind of backtrace if this happens.
+ */
+ void *next_page = (void *)PAGE_ALIGN((unsigned long)regs->sp);
+ if (get_stack_info(next_page, state->task, &state->stack_info,
+ &state->stack_mask))
+ return;
+ }

/*
* The caller can provide the address of the first frame directly
--
2.13.6
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