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Subject[PATCH 5/6] uprobes: teach handle_swbp() to rely on "is_swbp" rather than uprobes_srcu
Currently handle_swbp() assumes that it can't race with unregister,
so it roughly does:

if (find_uprobe(vaddr))

This relies on the not-really-working uprobes_srcu code we are going
to remove.

With this patch we rely on the result of is_swbp_at_addr_fast(bp_vaddr)
if find_uprobe() fails.

If is_swbp == 1, then we hit the normal int3, we should send SIGTRAP.

If is_swbp == 0, we raced with uprobe_unregister(), we simply restart
this insn again.

The "difficult" case is is_swbp == -EFAULT, when we can't read this
memory. In this case I think we should restart too, and this is more
correct compared to the current code which sends SIGTRAP.

Ignoring ENOMEM/etc from get_user_pages(), this can only happen if
another thread unmaps this memory before find_active_uprobe() takes
mmap_sem. It would be better to pretend it was unmapped before this
insn was executed, restart, and get SIGSEGV.
kernel/events/uprobes.c | 18 +++++++++++++++---
1 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 2af458d..ed76ee5 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1538,14 +1538,26 @@ static void handle_swbp(struct pt_regs *regs)
struct uprobe_task *utask;
struct uprobe *uprobe;
unsigned long bp_vaddr;
- int is_swbp;
+ int uninitialized_var(is_swbp);

bp_vaddr = uprobe_get_swbp_addr(regs);
uprobe = find_active_uprobe(bp_vaddr, &is_swbp);

if (!uprobe) {
- /* No matching uprobe; signal SIGTRAP. */
- send_sig(SIGTRAP, current, 0);
+ if (is_swbp > 0) {
+ /* No matching uprobe; signal SIGTRAP. */
+ send_sig(SIGTRAP, current, 0);
+ } else {
+ /*
+ * Either we raced with uprobe_unregister() or we can't
+ * access this memory. The latter is only possible if
+ * another thread plays with our ->mm. In both cases
+ * we can simply restart. If this vma was unmapped we
+ * can pretend this insn was not executed yet and get
+ * the (correct) SIGSEGV after restart.
+ */
+ instruction_pointer_set(regs, bp_vaddr);
+ }


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