[PATCH 08/10] uprobes: Change handle_trampoline() to flush the frames invalidated by longjmp()

[Oleg Nesterov <oleg@redhat.com>]

Test-case:

	#include <stdio.h>
	#include <setjmp.h>

	jmp_buf jmp;

	void func_2(void)
	{
		longjmp(jmp, 1);
	}

	void func_1(void)
	{
		if (setjmp(jmp))
			return;
		func_2();
		printf("ERR!! I am running on the caller's stack\n");
	}

	int main(void)
	{
		func_1();
		return 0;
	}

fails if you probe func_1() and func_2() because handle_trampoline()
assumes that the probed function should must return and hit the bp
installed be prepare_uretprobe(). But in this case func_2() does not
return, so when func_1() returns the kernel uses the no longer valid
return_instance of func_2().

Change handle_trampoline() to unwind ->return_instances until we know
that the next chain is alive or NULL, this ensures that the current
chain is the last we need to report and free.

Alternatively, every return_instance could use unique trampoline_vaddr,
in this case we could use it as a key. And this could solve the problem
with sigaltstack() automatically.

But this approach needs more changes, and it puts the "hard" limit on
MAX_URETPROBE_DEPTH. Plus it can not solve another problem partially
fixed by the next patch.

Note: this change has no effect on !x86, the arch-agnostic version of
arch_uretprobe_is_alive() just returns "true".

TODO: as documented by the previous change, arch_uretprobe_is_alive()
can be fooled by sigaltstack/etc.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/events/uprobes.c |   29 ++++++++++++++++++-----------
 1 files changed, 18 insertions(+), 11 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 0f68ea2..0dd7ff7 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1784,6 +1784,7 @@ static void handle_trampoline(struct pt_regs *regs)
 {
 	struct uprobe_task *utask;
 	struct return_instance *ri, *next;
+	bool valid;
 
 	utask = current->utask;
 	if (!utask)
@@ -1793,18 +1794,24 @@ static void handle_trampoline(struct pt_regs *regs)
 	if (!ri)
 		goto sigill;
 
-	next = find_next_ret_chain(ri);
-	/*
-	 * TODO: we should throw out return_instance's invalidated by
-	 * longjmp(), currently we assume that the probed function always
-	 * returns.
-	 */
-	instruction_pointer_set(regs, ri->orig_ret_vaddr);
 	do {
-		handle_uretprobe_chain(ri, regs);
-		ri = free_ret_instance(ri);
-		utask->depth--;
-	} while (ri != next);
+		/*
+		 * We should throw out the frames invalidated by longjmp().
+		 * If this chain is valid, then the next one should be alive
+		 * or NULL; the latter case means that nobody but ri->func
+		 * could hit this trampoline on return. TODO: sigaltstack().
+		 */
+		next = find_next_ret_chain(ri);
+		valid = !next || arch_uretprobe_is_alive(&next->auret, regs);
+
+		instruction_pointer_set(regs, ri->orig_ret_vaddr);
+		do {
+			if (valid)
+				handle_uretprobe_chain(ri, regs);
+			ri = free_ret_instance(ri);
+			utask->depth--;
+		} while (ri != next);
+	} while (!valid);
 
 	utask->return_instances = ri;
 	return;
-- 
1.5.5.1