Re: [PATCH bpf] selftests/bpf: Fix freplace_link segfault in tailcalls prog test

[Leon Hwang <leon.hwang@linux.dev>]

Hi Tengda,

On 2025/1/21 20:56, Tengda Wu wrote:
> There are two bpf_link__destroy(freplace_link) calls in
> test_tailcall_bpf2bpf_freplace(). After the first bpf_link__destroy()
> is called, if the following bpf_map_{update,delete}_elem() throws an
> exception, it will jump to the "out" label and call bpf_link__destroy()
> again, causing double free and eventually leading to a segfault.
> 

Thank you for pointing this out.

> Fix this issue by moving bpf_link__destroy() out of the "out" label
> and only calling it when freplace_link exists and has not been freed.
> 

I think it would be better to reset freplace_link to NULL immediately
after the first bpf_link__destroy(freplace_link) call. This would help
avoid potential double-free scenarios.

I’ve tested the following diff, which implements this change:

diff --git a/tools/testing/selftests/bpf/prog_tests/tailcalls.c
b/tools/testing/selftests/bpf/prog_tests/tailcalls.c
index 544144620ca61..a12fa0521ccc0 100644
--- a/tools/testing/selftests/bpf/prog_tests/tailcalls.c
+++ b/tools/testing/selftests/bpf/prog_tests/tailcalls.c
@@ -1602,6 +1602,7 @@ static void test_tailcall_bpf2bpf_freplace(void)
        err = bpf_link__destroy(freplace_link);
        if (!ASSERT_OK(err, "destroy link"))
                goto out;
+       freplace_link = NULL;

        /* OK to update prog_array map then delete element from the map. */

Thanks,
Leon

> Fixes: 021611d33e78 ("selftests/bpf: Add test to verify tailcall and freplace restrictions")
> Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
> ---
>  tools/testing/selftests/bpf/prog_tests/tailcalls.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/tailcalls.c b/tools/testing/selftests/bpf/prog_tests/tailcalls.c
> index 544144620ca6..028439dd2c5f 100644
> --- a/tools/testing/selftests/bpf/prog_tests/tailcalls.c
> +++ b/tools/testing/selftests/bpf/prog_tests/tailcalls.c
> @@ -1624,7 +1624,7 @@ static void test_tailcall_bpf2bpf_freplace(void)
>  	freplace_link = bpf_program__attach_freplace(freplace_skel->progs.entry_freplace,
>  						     prog_fd, "subprog_tc");
>  	if (!ASSERT_ERR_PTR(freplace_link, "attach_freplace failure"))
> -		goto out;
> +		goto out_free_link;
>  
>  	err = bpf_map_delete_elem(map_fd, &key);
>  	if (!ASSERT_OK(err, "delete_elem from jmp_table"))
> @@ -1638,11 +1638,11 @@ static void test_tailcall_bpf2bpf_freplace(void)
>  		goto out;
>  
>  	err = bpf_map_update_elem(map_fd, &key, &prog_fd, BPF_ANY);
> -	if (!ASSERT_ERR(err, "update jmp_table failure"))
> -		goto out;
> +	ASSERT_ERR(err, "update jmp_table failure");
>  
> -out:
> +out_free_link:
>  	bpf_link__destroy(freplace_link);
> +out:
>  	tailcall_freplace__destroy(freplace_skel);
>  	tc_bpf2bpf__destroy(tc_skel);
>  }