[PATCH 4/7] uprobes: teach find_active_uprobe() to provide the "is_swbp" info

[Oleg Nesterov <oleg@redhat.com>]

A separate patch to simplify the review, and for the documentation.

The patch adds another "int *is_swbp" argument to find_active_uprobe(),
so far its only caller doesn't use this info.

With this patch find_active_uprobe() additionally does:

	- if find_vma() + ->vm_start check fails, *is_swbp = -EFAULT

	- otherwise, if valid_vma() + find_uprobe() fails, it holds
	  the result of is_swbp_at_addr(), can be negative too. The
	  latter is only possible if we raced with another thread
	  which did munmap/etc after we hit this bp.

IOW. If find_active_uprobe(&is_swbp) returns NULL, the caller can
look at is_swbp to figure out whether the current insn is bp or not,
or detect the race with another thread if it is negative.

Note: I think that performance-wise this change is fine. This adds
is_swbp_at_addr(), but only if we raced with uprobe_unregister() or
if we hit the "normal" int3 but this mm has uprobes as well. And even
in this case the slow read_opcode() path is very unlikely, this insn
recently triggered do_int3(), __copy_from_user_inatomic() shouldn't
fail in the likely case.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/events/uprobes.c |   11 ++++++++---
 1 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index eaf4d55..ee3df70 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1489,7 +1489,7 @@ static bool can_skip_sstep(struct uprobe *uprobe, struct pt_regs *regs)
 	return false;
 }
 
-static struct uprobe *find_active_uprobe(unsigned long bp_vaddr)
+static struct uprobe *find_active_uprobe(unsigned long bp_vaddr, int *is_swbp)
 {
 	struct mm_struct *mm = current->mm;
 	struct uprobe *uprobe = NULL;
@@ -1497,7 +1497,6 @@ static struct uprobe *find_active_uprobe(unsigned long bp_vaddr)
 
 	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, bp_vaddr);
-
 	if (vma && vma->vm_start <= bp_vaddr) {
 		if (valid_vma(vma, false)) {
 			struct inode *inode;
@@ -1508,6 +1507,11 @@ static struct uprobe *find_active_uprobe(unsigned long bp_vaddr)
 			offset += (vma->vm_pgoff << PAGE_SHIFT);
 			uprobe = find_uprobe(inode, offset);
 		}
+
+		if (!uprobe)
+			*is_swbp = is_swbp_at_addr(mm, bp_vaddr);
+	} else {
+		*is_swbp = -EFAULT;
 	}
 
 	srcu_read_unlock_raw(&uprobes_srcu, current->uprobe_srcu_id);
@@ -1526,9 +1530,10 @@ static void handle_swbp(struct pt_regs *regs)
 	struct uprobe_task *utask;
 	struct uprobe *uprobe;
 	unsigned long bp_vaddr;
+	int is_swbp;
 
 	bp_vaddr = uprobe_get_swbp_addr(regs);
-	uprobe = find_active_uprobe(bp_vaddr);
+	uprobe = find_active_uprobe(bp_vaddr, &is_swbp);
 
 	if (!uprobe) {
 		/* No matching uprobe; signal SIGTRAP. */
-- 
1.5.5.1