uprobes/x86: Make arch_uretprobe_is_alive(RP_CHECK_CALL) more clever

The previous change documents that cleanup_return_instances()
can't always detect the dead frames, the stack can grow. But
there is one special case which imho worth fixing:
arch_uretprobe_is_alive() can return true when the stack didn't
actually grow, but the next "call" insn uses the already
invalidated frame.

Test-case:

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

	jmp_buf jmp;
	int nr = 1024;

	void func_2(void)
	{
		if (--nr == 0)
			return;
		longjmp(jmp, 1);
	}

	void func_1(void)
	{
		setjmp(jmp);
		func_2();
	}

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

If you ret-probe func_1() and func_2() prepare_uretprobe() hits
the MAX_URETPROBE_DEPTH limit and "return" from func_2() is not
reported.

When we know that the new call is not chained, we can do the
more strict check. In this case "sp" points to the new ret-addr,
so every frame which uses the same "sp" must be dead. The only
complication is that arch_uretprobe_is_alive() needs to know was
it chained or not, so we add the new RP_CHECK_CHAIN_CALL enum
and change prepare_uretprobe() to pass RP_CHECK_CALL only if
!chained.

Note: arch_uretprobe_is_alive() could also re-read *sp and check
if this word is still trampoline_vaddr. This could obviously
improve the logic, but I would like to avoid another
copy_from_user() especially in the case when we can't avoid the
false "alive == T" positives.

Tested-by: Pratyush Anand <panand@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Anton Arapov <arapov@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150721134028.GA4786@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/uprobes.c

@@ -989,5 +989,8 @@ arch_uretprobe_hijack_return_addr(unsigned long trampoline_vaddr, struct pt_regs
 bool arch_uretprobe_is_alive(struct return_instance *ret, enum rp_check ctx,
 				struct pt_regs *regs)
 {
-	return regs->sp <= ret->stack;
+	if (ctx == RP_CHECK_CALL) /* sp was just decremented by "call" insn */
+		return regs->sp < ret->stack;
+	else
+		return regs->sp <= ret->stack;
 }

include/linux/uprobes.h

@@ -104,6 +104,7 @@ struct return_instance {
 
 enum rp_check {
 	RP_CHECK_CALL,
+	RP_CHECK_CHAIN_CALL,
 	RP_CHECK_RET,
 };
 

kernel/events/uprobes.c

@@ -1511,10 +1511,11 @@ static unsigned long get_trampoline_vaddr(void)
 	return trampoline_vaddr;
 }
 
-static void cleanup_return_instances(struct uprobe_task *utask, struct pt_regs *regs)
+static void cleanup_return_instances(struct uprobe_task *utask, bool chained,
+					struct pt_regs *regs)
 {
 	struct return_instance *ri = utask->return_instances;
-	enum rp_check ctx = RP_CHECK_CALL;
+	enum rp_check ctx = chained ? RP_CHECK_CHAIN_CALL : RP_CHECK_CALL;
 
 	while (ri && !arch_uretprobe_is_alive(ri, ctx, regs)) {
 		ri = free_ret_instance(ri);
@@ -1528,7 +1529,7 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 	struct return_instance *ri;
 	struct uprobe_task *utask;
 	unsigned long orig_ret_vaddr, trampoline_vaddr;
-	bool chained = false;
+	bool chained;
 
 	if (!get_xol_area())
 		return;
@@ -1554,14 +1555,15 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 		goto fail;
 
 	/* drop the entries invalidated by longjmp() */
-	cleanup_return_instances(utask, regs);
+	chained = (orig_ret_vaddr == trampoline_vaddr);
+	cleanup_return_instances(utask, chained, regs);
 
 	/*
 	 * We don't want to keep trampoline address in stack, rather keep the
 	 * original return address of first caller thru all the consequent
 	 * instances. This also makes breakpoint unwrapping easier.
 	 */
-	if (orig_ret_vaddr == trampoline_vaddr) {
+	if (chained) {
 		if (!utask->return_instances) {
 			/*
 			 * This situation is not possible. Likely we have an
@@ -1570,8 +1572,6 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 			uprobe_warn(current, "handle tail call");
 			goto fail;
 		}
-
-		chained = true;
 		orig_ret_vaddr = utask->return_instances->orig_ret_vaddr;
 	}