Subject: Re: [PATCH v2 bpf-next 2/2] selftests: bpf: Add a new test for bare tracepoints

From: Yonghong Song <[email-masked]>
Date: Mon, 18 Jan 2021 09:48:53 -0800

On 1/18/21 4:18 AM, Qais Yousef wrote:
> On 01/16/21 18:11, Yonghong Song wrote:
>>
>>
>> On 1/16/21 10:21 AM, Qais Yousef wrote:
>>> Reuse module_attach infrastructure to add a new bare tracepoint to check
>>> we can attach to it as a raw tracepoint.
>>>
>>> Signed-off-by: Qais Yousef <[email-masked]>
>>> ---
>>>    .../bpf/bpf_testmod/bpf_testmod-events.h      |  6 +++++
>>>    .../selftests/bpf/bpf_testmod/bpf_testmod.c   | 21 ++++++++++++++-
>>>    .../selftests/bpf/bpf_testmod/bpf_testmod.h   |  6 +++++
>>>    .../selftests/bpf/prog_tests/module_attach.c  | 27 +++++++++++++++++++
>>>    .../selftests/bpf/progs/test_module_attach.c  | 10 +++++++
>>>    5 files changed, 69 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod-events.h b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod-events.h
>>> index b83ea448bc79..89c6d58e5dd6 100644
>>> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod-events.h
>>> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod-events.h
>>> @@ -28,6 +28,12 @@ TRACE_EVENT(bpf_testmod_test_read,
>>>    		  __entry->pid, __entry->comm, __entry->off, __entry->len)
>>>    );
>>> +/* A bare tracepoint with no event associated with it */
>>> +DECLARE_TRACE(bpf_testmod_test_write_bare,
>>> +	TP_PROTO(struct task_struct *task, struct bpf_testmod_test_write_ctx *ctx),
>>> +	TP_ARGS(task, ctx)
>>> +);
>>> +
>>>    #endif /* _BPF_TESTMOD_EVENTS_H */
>>>    #undef TRACE_INCLUDE_PATH
>>> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
>>> index 2df19d73ca49..e900adad2276 100644
>>> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
>>> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c
>>> @@ -28,9 +28,28 @@ bpf_testmod_test_read(struct file *file, struct kobject *kobj,
>>>    EXPORT_SYMBOL(bpf_testmod_test_read);
>>>    ALLOW_ERROR_INJECTION(bpf_testmod_test_read, ERRNO);
>>> +noinline ssize_t
>>> +bpf_testmod_test_write(struct file *file, struct kobject *kobj,
>>> +		      struct bin_attribute *bin_attr,
>>> +		      char *buf, loff_t off, size_t len)
>>> +{
>>> +	struct bpf_testmod_test_write_ctx ctx = {
>>> +		.buf = buf,
>>> +		.off = off,
>>> +		.len = len,
>>> +	};
>>> +
>>> +	trace_bpf_testmod_test_write_bare(current, &ctx);
>>> +
>>> +	return -EIO; /* always fail */
>>> +}
>>> +EXPORT_SYMBOL(bpf_testmod_test_write);
>>> +ALLOW_ERROR_INJECTION(bpf_testmod_test_write, ERRNO);
>>> +
>>>    static struct bin_attribute bin_attr_bpf_testmod_file __ro_after_init = {
>>
>> Do we need to remove __ro_after_init?
> 
> I don't think so. The structure should still remain RO AFAIU.

okay.

> 
>>
>>> -	.attr = { .name = "bpf_testmod", .mode = 0444, },
>>> +	.attr = { .name = "bpf_testmod", .mode = 0666, },
>>>    	.read = bpf_testmod_test_read,
>>> +	.write = bpf_testmod_test_write,
>>>    };
>>>    static int bpf_testmod_init(void)
>>> diff --git a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.h b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.h
>>> index b81adfedb4f6..b3892dc40111 100644
>>> --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.h
>>> +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.h
>>> @@ -11,4 +11,10 @@ struct bpf_testmod_test_read_ctx {
>>>    	size_t len;
>>>    };
>>> +struct bpf_testmod_test_write_ctx {
>>> +	char *buf;
>>> +	loff_t off;
>>> +	size_t len;
>>> +};
>>> +
>>>    #endif /* _BPF_TESTMOD_H */
>>> diff --git a/tools/testing/selftests/bpf/prog_tests/module_attach.c b/tools/testing/selftests/bpf/prog_tests/module_attach.c
>>> index 50796b651f72..e4605c0b5af1 100644
>>> --- a/tools/testing/selftests/bpf/prog_tests/module_attach.c
>>> +++ b/tools/testing/selftests/bpf/prog_tests/module_attach.c
>>> @@ -21,9 +21,34 @@ static int trigger_module_test_read(int read_sz)
>>>    	return 0;
>>>    }
>>> +static int trigger_module_test_write(int write_sz)
>>> +{
>>> +	int fd, err;
>>
>> Init err = 0?
> 
> I don't see what difference this makes.
> 
>>
>>> +	char *buf = malloc(write_sz);
>>> +
>>> +	if (!buf)
>>> +		return -ENOMEM;
>>
>> Looks like we already non-negative value, so return ENOMEM?
> 
> We already set err=-errno. So shouldn't we return negative too?

Oh, yes, return -ENOMEM sounds right here.

> 
>>
>>> +
>>> +	memset(buf, 'a', write_sz);
>>> +	buf[write_sz-1] = '\0';
>>> +
>>> +	fd = open("/sys/kernel/bpf_testmod", O_WRONLY);
>>> +	err = -errno;
>>> +	if (CHECK(fd < 0, "testmod_file_open", "failed: %d\n", err))
>>> +		goto out;
>>
>> Change the above to
>> 	fd = open("/sys/kernel/bpf_testmod", O_WRONLY);
>> 	if (CHECK(fd < 0, "testmod_file_open", "failed: %d\n", errno)) {

Here it should be ... "failed: %d\n", -errno.

>> 		err = -errno;
>> 		goto out;
>> 	}
> 
> I kept the code consistent with the definition of trigger_module_test_read().

The original patch code:

+static int trigger_module_test_write(int write_sz)
+{
+	int fd, err;
+	char *buf = malloc(write_sz);
+
+	if (!buf)
+		return -ENOMEM;
+
+	memset(buf, 'a', write_sz);
+	buf[write_sz-1] = '\0';
+
+	fd = open("/sys/kernel/bpf_testmod", O_WRONLY);
+	err = -errno;
+	if (CHECK(fd < 0, "testmod_file_open", "failed: %d\n", err))
+		goto out;
+
+	write(fd, buf, write_sz);
+	close(fd);
+out:
+	free(buf);
+
+	return 0;
+}

Even for "fd < 0" case, it "goto out" and "return 0". We should return
error code here instead of 0.

Second, "err = -errno" is set before checking fd < 0. If fd >= 0, err 
might inherit an postive errno from previous failure.
In trigger_module_test_write(), it is okay since the err is only used
when fd < 0:
         err = -errno;
         if (CHECK(fd < 0, "testmod_file_open", "failed: %d\n", err))
                 return err;

My above rewrite intends to use "err" during final "return" statement,
so I put assignment of "err = -errno" inside the CHECK branch.
But there are different ways to implement this properly.

> 
> I'll leave it up to the maintainer to pick up the style changes if they prefer
> it this way.
> 
> Thanks for the ack and for the review.

No problem.

> Cheers
> --
> Qais Yousef