[PATCH v2 9/9] uprobes/x86: Teach arch_uprobe_post_xol() to restart if possible

From: Oleg Nesterov
Date: Fri Apr 04 2014 - 14:52:56 EST

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", we 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@xxxxxxxxxx>
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 e72903e..b820668 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;

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