netdev - Re: [PATCH] ipv6: sr: update the struct ipv6_sr

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Message-ID: <1a10815f-8830-e71b-a323-36566befc99a@solarflare.com>
Date:   Tue, 14 Nov 2017 14:14:01 +0000
From:   Edward Cree <ecree@...arflare.com>
To:     David Miller <davem@...emloft.net>, <amsalam20@...il.com>
CC:     <david.lebrun@...ouvain.be>, <netdev@...r.kernel.org>
Subject: Re: [PATCH] ipv6: sr: update the struct ipv6_sr_hdr

On 14/11/17 12:37, David Miller wrote:
> From: Ahmed Abdelsalam <amsalam20@...il.com>
> Date: Sun, 12 Nov 2017 21:37:01 +0100
> 
>> diff --git a/include/uapi/linux/seg6.h b/include/uapi/linux/seg6.h
>> index 2f6fb0d..3f4b3ab 100644
>> --- a/include/uapi/linux/seg6.h
>> +++ b/include/uapi/linux/seg6.h
>> @@ -26,9 +26,9 @@ struct ipv6_sr_hdr {
>>  	__u8	hdrlen;
>>  	__u8	type;
>>  	__u8	segments_left;
>> -	__u8	first_segment;
>> +	__u8	last_entry;
> 
> This is user ABI and cannot be changed.
> 
> Sorry, folks should have considered these issues when the SR
> changes were submitted.  This field must keep the name 'first_segment'
> forever.

Surely renaming struct fields only changes the API, not the ABI?
Binaries compiled against the old struct definition will still behave the
 same (AFAICT the patch doesn't change how these fields are used), while
 sources being recompiled (so they care about the name change) can be
 changed.