[PATCH 9/7] uprobes/x86: Teach arch_uprobe_post_xol() to restart if possible

[Oleg Nesterov <oleg@redhat.com>]

SIGILL after the failed arch_uprobe_post_xol() should only be used as
a last resort, we should try to restart the probed insn if possible.

Currently only adjust_ret_addr() can fail, and this can only happen if
another thread unmapped our stack after we executed "call" out-of-line.
Most probably the application if buggy, but even in this case it can
have a handler for SIGSEGV/etc. And in theory it can be even correct
and do something non-trivial with its memory.

Of course we can't restart unconditionally, so arch_uprobe_post_xol()
does this only if ->post_xol() returns -ERESTART even if currently this
is the only possible error.

Note: this is not "perfect" because handler_chain() we actually do not
want the extra handler_chain() after restart, but I think this is the
best solution we can realistically do without too much uglifications.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/kernel/uprobes.c |   15 +++++++++++----
 1 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 3bdfb6e..78c4bb3 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -443,16 +443,17 @@ static int default_post_xol_op(struct arch_uprobe *auprobe, struct pt_regs *regs
 {
 	struct uprobe_task *utask = current->utask;
 	long correction = (long)(utask->vaddr - utask->xol_vaddr);
-	int ret = 0;
 
 	handle_riprel_post_xol(auprobe, regs, &correction);
 	if (auprobe->fixups & UPROBE_FIX_IP)
 		regs->ip += correction;
 
-	if (auprobe->fixups & UPROBE_FIX_CALL)
-		ret = adjust_ret_addr(regs->sp, correction);
+	if (auprobe->fixups & UPROBE_FIX_CALL) {
+		if (adjust_ret_addr(regs->sp, correction))
+			return -ERESTART;
+	}
 
-	return ret;
+	return 0;
 }
 
 static struct uprobe_xol_ops default_xol_ops = {
@@ -599,6 +600,12 @@ int arch_uprobe_post_xol(struct arch_uprobe *auprobe, struct pt_regs *regs)
 		int err = auprobe->ops->post_xol(auprobe, regs);
 		if (err) {
 			arch_uprobe_abort_xol(auprobe, regs);
+			/*
+			 * Restart the probed insn. ->post_xol() must ensure
+			 * this is really possible if it returns -ERESTART.
+			 */
+			if (err == -ERESTART)
+				return 0;
 			return err;
 		}
 	}
-- 
1.5.5.1