From mboxrd@z Thu Jan 1 00:00:00 1970 From: "'Marcelo Ricardo Leitner'" Date: Mon, 23 Jan 2017 12:36:28 +0000 Subject: Re: [PATCHv3 net-next 3/4] sctp: add support for generating stream reconf add incoming/outgoing stre Message-Id: <20170123123628.GX3781@localhost.localdomain> List-Id: References: <0d8d8e5482205ead64a2cfd2d414e918ce5602dd.1484845510.git.lucien.xin@gmail.com> <063D6719AE5E284EB5DD2968C1650D6DB026900F@AcuExch.aculab.com> <20170120163921.GU3781@localhost.localdomain> <063D6719AE5E284EB5DD2968C1650D6DB026B8E0@AcuExch.aculab.com> In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6DB026B8E0@AcuExch.aculab.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: David Laight Cc: 'Xin Long' , network dev , "linux-sctp@vger.kernel.org" , Neil Horman , Vlad Yasevich , "davem@davemloft.net" On Mon, Jan 23, 2017 at 12:26:12PM +0000, David Laight wrote: > From: Marcelo Ricardo Leitner > Sent: 20 January 2017 16:39 > > To: David Laight > > On Fri, Jan 20, 2017 at 02:50:01PM +0000, David Laight wrote: > > > From: Xin Long > > > > Sent: 19 January 2017 17:19 > > > > This patch is to define Add Incoming/Outgoing Streams Request > > > > Parameter described in rfc6525 section 4.5 and 4.6. They can > > > > be in one same chunk trunk as rfc6525 section 3.1-7 describes, > > > > so make them in one function. > > > ... > > > > +struct sctp_strreset_addstrm { > > > > + sctp_paramhdr_t param_hdr; > > > > + __u32 request_seq; > > > > + __u16 number_of_streams; > > > > + __u16 reserved; > > > > +} __packed; > > > ... > > > > + addstrm.param_hdr.type = SCTP_PARAM_RESET_ADD_OUT_STREAMS; > > > > + addstrm.param_hdr.length = htons(size); > > > > + addstrm.number_of_streams = htons(out); > > > > + addstrm.request_seq = htonl(asoc->strreset_outseq); > > > > + addstrm.reserved = 0; > > > > + > > > > + sctp_addto_chunk(retval, size, &addstrm); > > > > > > Since you allocate the sctp_strreset_addstrm structure on stack > > > there is no requirement for it to be packed. > > > > It shouldn't matter that it's allocated on stack. Why should it? > > We need it to be packed as this is a header that will be sent out to > > another peer, so there can't be any padding on it. > > That isn't what __packed means. > It means that the compiler must assume that the structure can be > misaligned in memory and must use byte memory accesses on systems > that fault misaligned memory accesses. That's a side-effect of it. https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#Common-Type-Attributes "This attribute, attached to struct or union type definition, specifies that each member (other than zero-width bit-fields) of the structure or union is placed to minimize the memory required. " So, no padding. A field just after the other, which is what we want on a network header. Marcelo