uprobes: Change handle_trampoline() to flush the frames invalidated by longjmp()

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kernel uses the no longer valid return_instance of func_2().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.

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/20150721134021.GA4773@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index c5f316e06dc0b486389f9d7aef4ade4e67d364a0..93d939c80cd92e73d018f52ea6b1d40a4a18d9aa 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1774,6 +1774,7 @@ static void handle_trampoline(struct pt_regs *regs)
 {
        struct uprobe_task *utask;
        struct return_instance *ri, *next;
+       bool valid;
 
        utask = current->utask;
        if (!utask)
@@ -1783,18 +1784,24 @@ 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, 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;