[PATCH bpf-next] bpf: remove useless variable

[PATCH bpf-next] bpf: remove useless variable

[Andrea Claudi]

bytes is initialized to end - start at the beginning of this function,
and is never changed. Remove it making the code a bit more readable.

Suggested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
---
 net/core/filter.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index aa274679965d..f31a0de14216 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2170,8 +2170,8 @@ static const struct bpf_func_proto bpf_msg_cork_bytes_proto = {
 BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
 	   u32, end, u64, flags)
 {
-	u32 len = 0, offset = 0, copy = 0, poffset = 0, bytes = end - start;
 	u32 first_sge, last_sge, i, shift, bytes_sg_total;
+	u32 len = 0, offset = 0, copy = 0, poffset = 0;
 	struct scatterlist *sge;
 	u8 *raw, *to, *from;
 	struct page *page;
@@ -2196,7 +2196,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
 	/* The start may point into the sg element so we need to also
 	 * account for the headroom.
 	 */
-	bytes_sg_total = start - offset + bytes;
+	bytes_sg_total = end - offset;
 	if (!msg->sg.copy[i] && bytes_sg_total <= len)
 		goto out;
 
@@ -2279,7 +2279,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
 		      msg->sg.end - shift;
 out:
 	msg->data = sg_virt(&msg->sg.data[first_sge]) + start - offset;
-	msg->data_end = msg->data + bytes;
+	msg->data_end = msg->data + end - start;
 	return 0;
 }
 
-- 
2.19.2

Re: [PATCH bpf-next] bpf: remove useless variable

[Stefano Brivio]

On Thu, 13 Dec 2018 22:44:57 +0100
Andrea Claudi <aclaudi@redhat.com> wrote:

> bytes is initialized to end - start at the beginning of this function,
> and is never changed. Remove it making the code a bit more readable.
> 
> Suggested-by: Stefano Brivio <sbrivio@redhat.com>
> Signed-off-by: Andrea Claudi <aclaudi@redhat.com>

Reviewed-by: Stefano Brivio <sbrivio@redhat.com>

-- 
Stefano

Re: [PATCH bpf-next] bpf: remove useless variable

[Martin Lau]

On Thu, Dec 13, 2018 at 10:44:57PM +0100, Andrea Claudi wrote:
> bytes is initialized to end - start at the beginning of this function,
> and is never changed. Remove it making the code a bit more readable.
> 
> Suggested-by: Stefano Brivio <sbrivio@redhat.com>
> Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
The change looks correct.
I found the original code more intuitive to read though.
Daniel/John, thoughts?

> ---
>  net/core/filter.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index aa274679965d..f31a0de14216 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -2170,8 +2170,8 @@ static const struct bpf_func_proto bpf_msg_cork_bytes_proto = {
>  BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
>  	   u32, end, u64, flags)
>  {
> -	u32 len = 0, offset = 0, copy = 0, poffset = 0, bytes = end - start;
>  	u32 first_sge, last_sge, i, shift, bytes_sg_total;
> +	u32 len = 0, offset = 0, copy = 0, poffset = 0;
>  	struct scatterlist *sge;
>  	u8 *raw, *to, *from;
>  	struct page *page;
> @@ -2196,7 +2196,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
>  	/* The start may point into the sg element so we need to also
>  	 * account for the headroom.
>  	 */
> -	bytes_sg_total = start - offset + bytes;
> +	bytes_sg_total = end - offset;
>  	if (!msg->sg.copy[i] && bytes_sg_total <= len)
>  		goto out;
>  
> @@ -2279,7 +2279,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
>  		      msg->sg.end - shift;
>  out:
>  	msg->data = sg_virt(&msg->sg.data[first_sge]) + start - offset;
> -	msg->data_end = msg->data + bytes;
> +	msg->data_end = msg->data + end - start;
>  	return 0;
>  }
>  
> -- 
> 2.19.2
> 

Re: [PATCH bpf-next] bpf: remove useless variable

[Stefano Brivio]

On Fri, 14 Dec 2018 17:03:02 +0000
Martin Lau <kafai@fb.com> wrote:

> On Thu, Dec 13, 2018 at 10:44:57PM +0100, Andrea Claudi wrote:
> > bytes is initialized to end - start at the beginning of this function,
> > and is never changed. Remove it making the code a bit more readable.
> > 
> > Suggested-by: Stefano Brivio <sbrivio@redhat.com>
> > Signed-off-by: Andrea Claudi <aclaudi@redhat.com>  
> The change looks correct.
> I found the original code more intuitive to read though.
> Daniel/John, thoughts?

In detail: the idea behind my suggestion was that:

	bytes_sg_total = start - offset + bytes;

sounds like: start from 'start', move back by 'offset' (where am I
now?), add 'bytes' (what is 'bytes'? Oh, it's end - start...). Whereas:

	bytes_sg_total = end - offset;

looked easier to follow: total bytes are the distance between 'end' and
'offset'.

-- 
Stefano

Re: [PATCH bpf-next] bpf: remove useless variable

[Daniel Borkmann]

On 12/14/2018 06:03 PM, Martin Lau wrote:
> On Thu, Dec 13, 2018 at 10:44:57PM +0100, Andrea Claudi wrote:
>> bytes is initialized to end - start at the beginning of this function,
>> and is never changed. Remove it making the code a bit more readable.
>>
>> Suggested-by: Stefano Brivio <sbrivio@redhat.com>
>> Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
> The change looks correct.
> I found the original code more intuitive to read though.
> Daniel/John, thoughts?

Change is correct, but I kind of agree with Martin that it seems to
make it slightly less intuitive to follow the logic. Probably just
personal preference. I think the change is mainly about the first
part since 'msg->data + bytes' is perhaps slightly more obvious than
'msg->data + end - start', so that second chunk was kind of a necessity
due to getting rid of bytes var. The first part is on finding the
right scatterlist ring entry based on the user provided data /start/.
And the /offset/ is the accumulation of the sg entries in the skmsg
we needed to walk to get to this point. So the /start - offset/ is
the headroom of the last skmsg entry. This and /bytes/ is needed for
checking whether it's single or multi skmsg element. 'end - offset'
feels a bit less intuitive to follow here given the previous logic.

Thanks,
Daniel

>> ---
>>  net/core/filter.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index aa274679965d..f31a0de14216 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -2170,8 +2170,8 @@ static const struct bpf_func_proto bpf_msg_cork_bytes_proto = {
>>  BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
>>  	   u32, end, u64, flags)
>>  {
>> -	u32 len = 0, offset = 0, copy = 0, poffset = 0, bytes = end - start;
>>  	u32 first_sge, last_sge, i, shift, bytes_sg_total;
>> +	u32 len = 0, offset = 0, copy = 0, poffset = 0;
>>  	struct scatterlist *sge;
>>  	u8 *raw, *to, *from;
>>  	struct page *page;
>> @@ -2196,7 +2196,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
>>  	/* The start may point into the sg element so we need to also
>>  	 * account for the headroom.
>>  	 */
>> -	bytes_sg_total = start - offset + bytes;
>> +	bytes_sg_total = end - offset;
>>  	if (!msg->sg.copy[i] && bytes_sg_total <= len)
>>  		goto out;
>>  
>> @@ -2279,7 +2279,7 @@ BPF_CALL_4(bpf_msg_pull_data, struct sk_msg *, msg, u32, start,
>>  		      msg->sg.end - shift;
>>  out:
>>  	msg->data = sg_virt(&msg->sg.data[first_sge]) + start - offset;
>> -	msg->data_end = msg->data + bytes;
>> +	msg->data_end = msg->data + end - start;
>>  	return 0;
>>  }
>>  
>> -- 
>> 2.19.2