[PATCH v2 11/11] uprobes/x86: Make arch_uretprobe_is_alive(RP_CHECK_CALL) more clever

[Oleg Nesterov <oleg@redhat.com>]

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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/kernel/uprobes.c |    5 ++++-
 include/linux/uprobes.h   |    1 +
 kernel/events/uprobes.c   |   14 +++++++-------
 3 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 67eb168..a5c59f2 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -997,5 +997,8 @@ 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;
 }
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index c0a5402..0bdc72f 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -104,6 +104,7 @@ struct return_instance {
 
 enum rp_check {
 	RP_CHECK_CALL,
+	RP_CHECK_CHAIN_CALL,
 	RP_CHECK_RET,
 };
 
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index df5661a..0f370ef 100644
--- a/kernel/events/uprobes.c
+++ b/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;
 	}
 
-- 
1.5.5.1