On 11/17/22 5:52 PM, Hou Tao wrote:
Hi,
On 11/17/2022 2:48 PM, Martin KaFai Lau wrote:
On 11/15/22 6:48 PM, Hao Luo wrote:
On Tue, Nov 15, 2022 at 5:37 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
On Tue, Nov 15, 2022 at 11:16 AM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
On 11/10/22 10:34 PM, Hou Tao wrote:
From: Hou Tao <houtao1@xxxxxxxxxx>
For many bpf iterator (e.g., cgroup iterator), iterator link acquires
the reference of iteration target in .attach_target(), but iterator link
may be closed before or in the middle of iteration, so iterator will
need to acquire the reference of iteration target as well to prevent
potential use-after-free. To avoid doing the acquisition in
.init_seq_private() for each iterator type, just pin iterator link in
iterator.
iiuc, a link currently will go away when all its fds closed and pinned file
removed. After this change, the link will stay until the last iter is
closed().
Before then, the user space can still "bpftool link show" and even get the
link back by bpf_link_get_fd_by_id(). If this is the case, it would be useful
to explain it in the commit message.
and does this new behavior make sense when comparing with other link types?
I think this is a unique problem in iter link. Because iter link is
the only link type that can generate another object.
Should a similar solution as in the current map iter be used then?
I am thinking, after all link fds are closed and its pinned files are removed,
if it cannot stop the already created iter, it should at least stop new iter
from being created?
Rather than adding the above logic for iterator link, just pinning the start
cgroup in .init_seq_private() will be much simpler.
Yeah, it is better to fix the bug without changing the behavior when all the
link fds are closed and pinned files are removed. In particular this will make
the link iter works differently from other link types.
I can see pinning a link inside an iter is a generic solution for all iter types
but lets continue to explore other ways to refactor this within the kernel if it
is really needed instead of leaking it to the user space. (not saying this
refactoring belongs to the same patch set. lets fix the current bug first.)
prepare_seq_file() has already acquired an extra reference of the currently
attached program, so it is OK to read the iterator after the close of iterator
link fd. So what do you think ?
Right, it is my understanding also that the prog refcnt has been acquired during
the iter creation.
