Re: [PATCH bpf v1 2/2] selftests/bpf: Add test for SOCK_OPS_GET_SK with same src/dst register

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Sat Apr 4, 2026 at 10:09 AM EDT, Jiayuan Chen wrote:
> Add a selftest that verifies SOCK_OPS_GET_SK() correctly returns NULL
> when dst_reg == src_reg and is_fullsock == 0.
>
> The BPF program reads ctx->is_fullsock, then loads ctx->sk using the
> same register for source and destination. When is_fullsock == 0, sk
> must be NULL. The test triggers this path via a TCP handshake (which
> causes TCP_NEW_SYN_RECV state) and checks:
>  - null_seen == 1: the is_fullsock==0 path was hit with correct NULL sk
>  - bug_detected == 0: sk was never non-NULL when is_fullsock==0

Can you add a test for dst_reg != src_reg, too?

>
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  .../bpf/prog_tests/sock_ops_get_sk.c          | 62 +++++++++++++++++++
>  .../selftests/bpf/progs/sock_ops_get_sk.c     | 44 +++++++++++++
>  2 files changed, 106 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c
>  create mode 100644 tools/testing/selftests/bpf/progs/sock_ops_get_sk.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c b/tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c
> new file mode 100644
> index 0000000000000..9eaf97786c1d9
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/sock_ops_get_sk.c
> @@ -0,0 +1,62 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <test_progs.h>
> +#include "cgroup_helpers.h"
> +#include "network_helpers.h"
> +#include "sock_ops_get_sk.skel.h"
> +
> +/*
> + * Test that reading ctx->sk with dst_reg == src_reg in a sock_ops program
> + * correctly returns NULL when is_fullsock == 0 (TCP_NEW_SYN_RECV state).
> + *
> + * The bug was in SOCK_OPS_GET_SK() macro which failed to zero the destination
> + * register when it was the same as the source register and the socket was not
> + * a full socket. This left the ctx pointer in the register, which then passed
> + * the NULL check and could be used as a socket pointer, causing OOB access.

The reader of this file will not have context on the original bug, and
the dst_reg == src_reg condition is more obvious in the BPF file. Can
you move it there?

> + */
> +void test_sock_ops_get_sk(void)
> +{
> +	struct sock_ops_get_sk *skel;
> +	int cgroup_fd, server_fd, client_fd;
> +	int prog_fd, err;
> +
> +	cgroup_fd = test__join_cgroup("/sock_ops_get_sk");
> +	if (!ASSERT_GE(cgroup_fd, 0, "join_cgroup"))
> +		return;
> +
> +	skel = sock_ops_get_sk__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "skel_open_load"))
> +		goto close_cgroup;
> +
> +	prog_fd = bpf_program__fd(skel->progs.sock_ops_get_sk_same_reg);
> +	err = bpf_prog_attach(prog_fd, cgroup_fd, BPF_CGROUP_SOCK_OPS, 0);
> +	if (!ASSERT_OK(err, "prog_attach"))
> +		goto destroy_skel;
> +
> +	server_fd = start_server(AF_INET, SOCK_STREAM, NULL, 0, 0);
> +	if (!ASSERT_GE(server_fd, 0, "start_server"))
> +		goto detach;
> +
> +	/* Trigger TCP handshake which causes TCP_NEW_SYN_RECV state where
> +	 * is_fullsock == 0. With the bug, ctx->sk loaded with same src/dst
> +	 * register would return a stale ctx pointer instead of NULL.
> +	 */
> +	client_fd = connect_to_fd(server_fd, 0);
> +	if (!ASSERT_GE(client_fd, 0, "connect_to_fd"))
> +		goto close_server;
> +
> +	close(client_fd);
> +
> +	/* Verify that the is_fullsock == 0 path was hit and sk was NULL */
> +	ASSERT_EQ(skel->bss->null_seen, 1, "null_seen");
> +	ASSERT_EQ(skel->bss->bug_detected, 0, "bug_not_detected");
> +
> +close_server:
> +	close(server_fd);
> +detach:
> +	bpf_prog_detach(cgroup_fd, BPF_CGROUP_SOCK_OPS);
> +destroy_skel:
> +	sock_ops_get_sk__destroy(skel);
> +close_cgroup:
> +	close(cgroup_fd);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/sock_ops_get_sk.c b/tools/testing/selftests/bpf/progs/sock_ops_get_sk.c
> new file mode 100644
> index 0000000000000..4a75614b21eb5
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/sock_ops_get_sk.c
> @@ -0,0 +1,44 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include "vmlinux.h"
> +#include <bpf/bpf_helpers.h>
> +#include "bpf_misc.h"
> +
> +/*
> + * Test that SOCK_OPS_GET_SK() correctly returns NULL when dst_reg == src_reg
> + * and is_fullsock == 0 (e.g., during TCP_NEW_SYN_RECV). Before the fix, the

Again, writing "the fix" here is vague.

> + * macro failed to zero the destination register, leaving a stale ctx pointer
> + * that bypassed the NULL check.
> + */
> +
> +int bug_detected;
> +int null_seen;
> +
> +SEC("sockops")
> +__naked void sock_ops_get_sk_same_reg(void)
> +{
> +	asm volatile (
> +		"r7 = *(u32 *)(r1 + %[is_fullsock_off]);"
> +		"r1 = *(u64 *)(r1 + %[sk_off]);"
> +		"if r7 != 0 goto 2f;"
> +		"if r1 == 0 goto 1f;"
> +		"r1 = %[bug_detected] ll;"
> +		"r2 = 1;"
> +		"*(u32 *)(r1 + 0) = r2;"
> +		"goto 2f;"
> +	"1:"
> +		"r1 = %[null_seen] ll;"
> +		"r2 = 1;"
> +		"*(u32 *)(r1 + 0) = r2;"
> +	"2:"
> +		"r0 = 1;"
> +		"exit;"
> +		:
> +		: __imm_const(is_fullsock_off, offsetof(struct bpf_sock_ops, is_fullsock)),
> +		  __imm_const(sk_off, offsetof(struct bpf_sock_ops, sk)),
> +		  __imm_addr(bug_detected),
> +		  __imm_addr(null_seen)
> +		: __clobber_all);
> +}
> +
> +char _license[] SEC("license") = "GPL";