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

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

[Tengda Wu]

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.

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.

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);
 }
-- 
2.34.1

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

[Leon Hwang]

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);
>  }

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

[Tengda Wu]

Hi Leon,

On 2025/1/21 22:27, Leon Hwang wrote:
> 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.

What a great suggestion, I can't believe I didn't think of it! Haha.
I will resend a v2 version later. Thanks, Leon.

> 
> 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);
>>  }
>