diff mbox series

[bpf-next,v3,6/7] selftests/bpf: detach a struct_ops link from the subsystem managing it.

Message ID 20240510002942.1253354-7-thinker.li@gmail.com (mailing list archive)
State Superseded
Delegated to: BPF
Headers show
Series Notify user space when a struct_ops object is detached/unregistered | expand

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Commit Message

Kui-Feng Lee May 10, 2024, 12:29 a.m. UTC 
Not only a user space program can detach a struct_ops link, the subsystem
managing a link can also detach the link. This patch adds a kfunc to
simulate detaching a link by the subsystem managing it and makes sure user
space programs get notified through epoll.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
---
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   | 42 ++++++++++++
 .../bpf/bpf_testmod/bpf_testmod_kfunc.h       |  1 +
 .../bpf/prog_tests/test_struct_ops_module.c   | 67 +++++++++++++++++++
 .../selftests/bpf/progs/struct_ops_detach.c   |  7 ++
 4 files changed, 117 insertions(+)

Comments

Amery Hung May 21, 2024, 10:56 p.m. UTC | #1 
On Thu, May 09, 2024 at 05:29:41PM -0700, Kui-Feng Lee wrote:
> Not only a user space program can detach a struct_ops link, the subsystem
> managing a link can also detach the link. This patch adds a kfunc to
> simulate detaching a link by the subsystem managing it and makes sure user
> space programs get notified through epoll.
> 
> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> ---
>  .../selftests/bpf/bpf_testmod/bpf_testmod.c   | 42 ++++++++++++
>  .../bpf/bpf_testmod/bpf_testmod_kfunc.h       |  1 +
>  .../bpf/prog_tests/test_struct_ops_module.c   | 67 +++++++++++++++++++
>  .../selftests/bpf/progs/struct_ops_detach.c   |  7 ++
>  4 files changed, 117 insertions(+)
> 
> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> index 1150e758e630..1f347eed6c18 100644
> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> @@ -741,6 +741,38 @@ __bpf_kfunc int bpf_kfunc_call_kernel_getpeername(struct addr_args *args)
>  	return err;
>  }
>  
> +static DEFINE_SPINLOCK(detach_lock);
> +static struct bpf_link *link_to_detach;
> +
> +__bpf_kfunc int bpf_dummy_do_link_detach(void)
> +{
> +	struct bpf_link *link;
> +	int ret = -ENOENT;
> +
> +	/* A subsystem must ensure that a link is valid when detaching the
> +	 * link. In order to achieve that, the subsystem may need to obtain
> +	 * a lock to safeguard a table that holds the pointer to the link
> +	 * being detached. However, the subsystem cannot invoke
> +	 * link->ops->detach() while holding the lock because other tasks
> +	 * may be in the process of unregistering, which could lead to
> +	 * acquiring the same lock and causing a deadlock. This is why
> +	 * bpf_link_inc_not_zero() is used to maintain the link's validity.
> +	 */
> +	spin_lock(&detach_lock);
> +	link = link_to_detach;
> +	/* Make sure the link is still valid by increasing its refcnt */
> +	if (link && IS_ERR(bpf_link_inc_not_zero(link)))
> +		link = NULL;
> +	spin_unlock(&detach_lock);
> +

I know it probably doesn't matter in this example, but where would you set
link_to_detach to NULL if reg and unreg can be called multiple times?

> +	if (link) {
> +		ret = link->ops->detach(link);
> +		bpf_link_put(link);
> +	}
> +
> +	return ret;
> +}

[...]
Kuifeng Lee May 22, 2024, 12:31 a.m. UTC | #2 
On 5/21/24 15:56, Amery Hung wrote:
> On Thu, May 09, 2024 at 05:29:41PM -0700, Kui-Feng Lee wrote:
>> Not only a user space program can detach a struct_ops link, the subsystem
>> managing a link can also detach the link. This patch adds a kfunc to
>> simulate detaching a link by the subsystem managing it and makes sure user
>> space programs get notified through epoll.
>>
>> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
>> ---
>>   .../selftests/bpf/bpf_testmod/bpf_testmod.c   | 42 ++++++++++++
>>   .../bpf/bpf_testmod/bpf_testmod_kfunc.h       |  1 +
>>   .../bpf/prog_tests/test_struct_ops_module.c   | 67 +++++++++++++++++++
>>   .../selftests/bpf/progs/struct_ops_detach.c   |  7 ++
>>   4 files changed, 117 insertions(+)
>>
>> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
>> index 1150e758e630..1f347eed6c18 100644
>> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
>> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
>> @@ -741,6 +741,38 @@ __bpf_kfunc int bpf_kfunc_call_kernel_getpeername(struct addr_args *args)
>>   	return err;
>>   }
>>   
>> +static DEFINE_SPINLOCK(detach_lock);
>> +static struct bpf_link *link_to_detach;
>> +
>> +__bpf_kfunc int bpf_dummy_do_link_detach(void)
>> +{
>> +	struct bpf_link *link;
>> +	int ret = -ENOENT;
>> +
>> +	/* A subsystem must ensure that a link is valid when detaching the
>> +	 * link. In order to achieve that, the subsystem may need to obtain
>> +	 * a lock to safeguard a table that holds the pointer to the link
>> +	 * being detached. However, the subsystem cannot invoke
>> +	 * link->ops->detach() while holding the lock because other tasks
>> +	 * may be in the process of unregistering, which could lead to
>> +	 * acquiring the same lock and causing a deadlock. This is why
>> +	 * bpf_link_inc_not_zero() is used to maintain the link's validity.
>> +	 */
>> +	spin_lock(&detach_lock);
>> +	link = link_to_detach;
>> +	/* Make sure the link is still valid by increasing its refcnt */
>> +	if (link && IS_ERR(bpf_link_inc_not_zero(link)))
>> +		link = NULL;
>> +	spin_unlock(&detach_lock);
>> +
> 
> I know it probably doesn't matter in this example, but where would you set
> link_to_detach to NULL if reg and unreg can be called multiple times?

For the same link if there is, reg() can be called only once
except if unreg() has been called for the previous reg() call on the
same link. Unreg() can only be called for once after a reg() call on the
same link.

For struct_ops map with link, unreg() is called by
bpf_struct_ops_map_link_dealloc() and bpf_struct_ops_map_link_detach().
The former one is called for a link only if the refcnt of the link has
dropped to zero. The later one is called for a link only if the refcnt
is not zero, and it holds update_mutex. Once unreg() has been called,
link->map will be cleared as well. So, unreg() should not be called
twice on the same link except it is registered again.

Does that answer your question?

> 
>> +	if (link) {
>> +		ret = link->ops->detach(link);
>> +		bpf_link_put(link);
>> +	}
>> +
>> +	return ret;
>> +}
> 
> [...]
Amery Hung May 22, 2024, 5:33 p.m. UTC | #3 
On Tue, May 21, 2024 at 5:31 PM Kui-Feng Lee <sinquersw@gmail.com> wrote:
>
>
>
> On 5/21/24 15:56, Amery Hung wrote:
> > On Thu, May 09, 2024 at 05:29:41PM -0700, Kui-Feng Lee wrote:
> >> Not only a user space program can detach a struct_ops link, the subsystem
> >> managing a link can also detach the link. This patch adds a kfunc to
> >> simulate detaching a link by the subsystem managing it and makes sure user
> >> space programs get notified through epoll.
> >>
> >> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> >> ---
> >>   .../selftests/bpf/bpf_testmod/bpf_testmod.c   | 42 ++++++++++++
> >>   .../bpf/bpf_testmod/bpf_testmod_kfunc.h       |  1 +
> >>   .../bpf/prog_tests/test_struct_ops_module.c   | 67 +++++++++++++++++++
> >>   .../selftests/bpf/progs/struct_ops_detach.c   |  7 ++
> >>   4 files changed, 117 insertions(+)
> >>
> >> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> >> index 1150e758e630..1f347eed6c18 100644
> >> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> >> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
> >> @@ -741,6 +741,38 @@ __bpf_kfunc int bpf_kfunc_call_kernel_getpeername(struct addr_args *args)
> >>      return err;
> >>   }
> >>
> >> +static DEFINE_SPINLOCK(detach_lock);
> >> +static struct bpf_link *link_to_detach;
> >> +
> >> +__bpf_kfunc int bpf_dummy_do_link_detach(void)
> >> +{
> >> +    struct bpf_link *link;
> >> +    int ret = -ENOENT;
> >> +
> >> +    /* A subsystem must ensure that a link is valid when detaching the
> >> +     * link. In order to achieve that, the subsystem may need to obtain
> >> +     * a lock to safeguard a table that holds the pointer to the link
> >> +     * being detached. However, the subsystem cannot invoke
> >> +     * link->ops->detach() while holding the lock because other tasks
> >> +     * may be in the process of unregistering, which could lead to
> >> +     * acquiring the same lock and causing a deadlock. This is why
> >> +     * bpf_link_inc_not_zero() is used to maintain the link's validity.
> >> +     */
> >> +    spin_lock(&detach_lock);
> >> +    link = link_to_detach;
> >> +    /* Make sure the link is still valid by increasing its refcnt */
> >> +    if (link && IS_ERR(bpf_link_inc_not_zero(link)))
> >> +            link = NULL;
> >> +    spin_unlock(&detach_lock);
> >> +
> >
> > I know it probably doesn't matter in this example, but where would you set
> > link_to_detach to NULL if reg and unreg can be called multiple times?
>
> For the same link if there is, reg() can be called only once
> except if unreg() has been called for the previous reg() call on the
> same link. Unreg() can only be called for once after a reg() call on the
> same link.
>
> For struct_ops map with link, unreg() is called by
> bpf_struct_ops_map_link_dealloc() and bpf_struct_ops_map_link_detach().
> The former one is called for a link only if the refcnt of the link has
> dropped to zero. The later one is called for a link only if the refcnt
> is not zero, and it holds update_mutex. Once unreg() has been called,
> link->map will be cleared as well. So, unreg() should not be called
> twice on the same link except it is registered again.
>
> Does that answer your question?
>

Thanks for the detailed explanation. That makes sense to me.

> >
> >> +    if (link) {
> >> +            ret = link->ops->detach(link);
> >> +            bpf_link_put(link);
> >> +    }
> >> +
> >> +    return ret;
> >> +}
> >
> > [...]
diff mbox series

Patch

diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
index 1150e758e630..1f347eed6c18 100644
--- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
@@ -741,6 +741,38 @@  __bpf_kfunc int bpf_kfunc_call_kernel_getpeername(struct addr_args *args)
 	return err;
 }
 
+static DEFINE_SPINLOCK(detach_lock);
+static struct bpf_link *link_to_detach;
+
+__bpf_kfunc int bpf_dummy_do_link_detach(void)
+{
+	struct bpf_link *link;
+	int ret = -ENOENT;
+
+	/* A subsystem must ensure that a link is valid when detaching the
+	 * link. In order to achieve that, the subsystem may need to obtain
+	 * a lock to safeguard a table that holds the pointer to the link
+	 * being detached. However, the subsystem cannot invoke
+	 * link->ops->detach() while holding the lock because other tasks
+	 * may be in the process of unregistering, which could lead to
+	 * acquiring the same lock and causing a deadlock. This is why
+	 * bpf_link_inc_not_zero() is used to maintain the link's validity.
+	 */
+	spin_lock(&detach_lock);
+	link = link_to_detach;
+	/* Make sure the link is still valid by increasing its refcnt */
+	if (link && IS_ERR(bpf_link_inc_not_zero(link)))
+		link = NULL;
+	spin_unlock(&detach_lock);
+
+	if (link) {
+		ret = link->ops->detach(link);
+		bpf_link_put(link);
+	}
+
+	return ret;
+}
+
 BTF_KFUNCS_START(bpf_testmod_check_kfunc_ids)
 BTF_ID_FLAGS(func, bpf_testmod_test_mod_kfunc)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test1)
@@ -777,6 +809,7 @@  BTF_ID_FLAGS(func, bpf_kfunc_call_kernel_sendmsg, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_kfunc_call_sock_sendmsg, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_kfunc_call_kernel_getsockname, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_kfunc_call_kernel_getpeername, KF_SLEEPABLE)
+BTF_ID_FLAGS(func, bpf_dummy_do_link_detach)
 BTF_KFUNCS_END(bpf_testmod_check_kfunc_ids)
 
 static int bpf_testmod_ops_init(struct btf *btf)
@@ -829,11 +862,20 @@  static int bpf_dummy_reg(void *kdata, struct bpf_link *link)
 	if (ops->test_2)
 		ops->test_2(4, ops->data);
 
+	spin_lock(&detach_lock);
+	if (!link_to_detach)
+		link_to_detach = link;
+	spin_unlock(&detach_lock);
+
 	return 0;
 }
 
 static void bpf_dummy_unreg(void *kdata, struct bpf_link *link)
 {
+	spin_lock(&detach_lock);
+	if (link == link_to_detach)
+		link_to_detach = NULL;
+	spin_unlock(&detach_lock);
 }
 
 static int bpf_testmod_test_1(void)
diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_kfunc.h b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_kfunc.h
index b0d586a6751f..19131baf4a9e 100644
--- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_kfunc.h
+++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod_kfunc.h
@@ -121,6 +121,7 @@  void bpf_kfunc_call_test_fail1(struct prog_test_fail1 *p);
 void bpf_kfunc_call_test_fail2(struct prog_test_fail2 *p);
 void bpf_kfunc_call_test_fail3(struct prog_test_fail3 *p);
 void bpf_kfunc_call_test_mem_len_fail1(void *mem, int len);
+int bpf_dummy_do_link_detach(void) __ksym;
 
 void bpf_kfunc_common_test(void) __ksym;
 
diff --git a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
index bbcf12696a6b..f4000bf04752 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_struct_ops_module.c
@@ -2,6 +2,7 @@ 
 /* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
 #include <test_progs.h>
 #include <time.h>
+#include <network_helpers.h>
 
 #include <sys/epoll.h>
 
@@ -297,6 +298,70 @@  static void test_detach_link(void)
 	struct_ops_detach__destroy(skel);
 }
 
+/* Detach a link from the subsystem that the link was registered to */
+static void test_subsystem_detach(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, topts,
+		    .data_in = &pkt_v4,
+		    .data_size_in = sizeof(pkt_v4));
+	struct epoll_event ev, events[2];
+	struct struct_ops_detach *skel;
+	struct bpf_link *link = NULL;
+	int fd, epollfd = -1, nfds;
+	int prog_fd;
+	int err;
+
+	skel = struct_ops_detach__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "struct_ops_detach_open_and_load"))
+		return;
+
+	link = bpf_map__attach_struct_ops(skel->maps.testmod_do_detach);
+	if (!ASSERT_OK_PTR(link, "attach_struct_ops"))
+		goto cleanup;
+
+	fd = bpf_link__fd(link);
+	if (!ASSERT_GE(fd, 0, "link_fd"))
+		goto cleanup;
+
+	prog_fd = bpf_program__fd(skel->progs.start_detach);
+	if (!ASSERT_GE(prog_fd, 0, "start_detach_fd"))
+		goto cleanup;
+
+	/* Do detachment from the registered subsystem */
+	err = bpf_prog_test_run_opts(prog_fd, &topts);
+	if (!ASSERT_OK(err, "start_detach_run"))
+		goto cleanup;
+
+	if (!ASSERT_EQ(topts.retval, 0, "start_detach_run_retval"))
+		goto cleanup;
+
+	epollfd = epoll_create1(0);
+	if (!ASSERT_GE(epollfd, 0, "epoll_create1"))
+		goto cleanup;
+
+	ev.events = EPOLLHUP;
+	ev.data.fd = fd;
+	err = epoll_ctl(epollfd, EPOLL_CTL_ADD, fd, &ev);
+	if (!ASSERT_OK(err, "epoll_ctl"))
+		goto cleanup;
+
+	/* Wait for EPOLLHUP */
+	nfds = epoll_wait(epollfd, events, 2, 5000);
+	if (!ASSERT_EQ(nfds, 1, "epoll_wait"))
+		goto cleanup;
+
+	if (!ASSERT_EQ(events[0].data.fd, fd, "epoll_wait_fd"))
+		goto cleanup;
+	if (!ASSERT_TRUE(events[0].events & EPOLLHUP, "events[0].events"))
+		goto cleanup;
+
+cleanup:
+	if (epollfd >= 0)
+		close(epollfd);
+	bpf_link__destroy(link);
+	struct_ops_detach__destroy(skel);
+}
+
 void serial_test_struct_ops_module(void)
 {
 	if (test__start_subtest("struct_ops_load"))
@@ -311,5 +376,7 @@  void serial_test_struct_ops_module(void)
 		test_struct_ops_forgotten_cb();
 	if (test__start_subtest("test_detach_link"))
 		test_detach_link();
+	if (test__start_subtest("test_subsystem_detach"))
+		test_subsystem_detach();
 }
 
diff --git a/tools/testing/selftests/bpf/progs/struct_ops_detach.c b/tools/testing/selftests/bpf/progs/struct_ops_detach.c
index 45eacc2ca657..5c742b0df04d 100644
--- a/tools/testing/selftests/bpf/progs/struct_ops_detach.c
+++ b/tools/testing/selftests/bpf/progs/struct_ops_detach.c
@@ -2,8 +2,15 @@ 
 /* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
 #include <vmlinux.h>
 #include "../bpf_testmod/bpf_testmod.h"
+#include "../bpf_testmod/bpf_testmod_kfunc.h"
 
 char _license[] SEC("license") = "GPL";
 
 SEC(".struct_ops.link")
 struct bpf_testmod_ops testmod_do_detach;
+
+SEC("tc")
+int start_detach(void *skb)
+{
+	return bpf_dummy_do_link_detach();
+}