[PATCH v2 1/5] uprobes: document the usage of mm->mmap_lock

[Oleg Nesterov <oleg@redhat.com>]

The comment above uprobe_write_opcode() is wrong, unapply_uprobe() calls
it under mmap_read_lock() and this is correct.

And it is completely unclear why register_for_each_vma() takes mmap_lock
for writing, add a comment to explain that mmap_write_lock() is needed to
avoid the following race:

	- A task T hits the bp installed by uprobe and calls
	  find_active_uprobe()

	- uprobe_unregister() removes this uprobe/bp

	- T calls find_uprobe() which returns NULL

	- another uprobe_register() installs the bp at the same address

	- T calls is_trap_at_addr() which returns true

	- T returns to handle_swbp() and gets SIGTRAP.

Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/events/uprobes.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 73cc47708679..73dd12b09a7b 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -453,7 +453,7 @@ static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm,
  * @vaddr: the virtual address to store the opcode.
  * @opcode: opcode to be written at @vaddr.
  *
- * Called with mm->mmap_lock held for write.
+ * Called with mm->mmap_lock held for read or write.
  * Return 0 (success) or a negative errno.
  */
 int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
@@ -1046,7 +1046,13 @@ register_for_each_vma(struct uprobe *uprobe, struct uprobe_consumer *new)
 
 		if (err && is_register)
 			goto free;
-
+		/*
+		 * We take mmap_lock for writing to avoid the race with
+		 * find_active_uprobe() which takes mmap_lock for reading.
+		 * Thus this install_breakpoint() can not make
+		 * is_trap_at_addr() true right after find_uprobe()
+		 * returns NULL in find_active_uprobe().
+		 */
 		mmap_write_lock(mm);
 		vma = find_vma(mm, info->vaddr);
 		if (!vma || !valid_vma(vma, is_register) ||
-- 
2.25.1.362.g51ebf55