RE: [PATCH] cifs: smbd: Calculate the correct maximum packet size for segmented SMBDirect send/receive

[Long Li <longli@microsoft.com>]

>Subject: Re: [PATCH] cifs: smbd: Calculate the correct maximum packet size
>for segmented SMBDirect send/receive
>
>On Thu, Mar 26, 2020 at 12:43 PM Long Li <longli@microsoft.com> wrote:
>>
>> >Subject: Re: [PATCH] cifs: smbd: Calculate the correct maximum packet
>> >size for segmented SMBDirect send/receive
>> >
>> >Long Li,
>> >
>> >Mostly looks good  but the magic constant 49 I think is wrong.
>> >49 is the structure size of the read/write request header but for
>> >these sizes, if they are odd it just means that the header contain a variable
>sized blob.
>> >I.e. the size is 48 bytes (for the fixed part of the header) + a
>> >variable part which in this case are the ChannelInfo blobs.
>>
>> Agreed. Using a magic number is not a good idea.
>>
>> I was looking at request size definitions in "fs/cifs/smb2pdu.c",
>> they are defined at static const int
>> smb2_req_struct_sizes[NUMBER_OF_SMB2_COMMANDS]
>>
>> for read and write, the sizes are:
>>         /* SMB2_READ */ 49,
>>         /* SMB2_WRITE */ 49,
>>
>> They are used to calculate command header sizes, and fill in the request
>headers. Maybe I should use this variable to get the size?
>
>Nah, those values only specify what values must be set as the structure size in
>the actual packet headers.
>But they do not represent the actual sizes.
>All command headers start with a 2 byte StructureSize field where everything
>except the least significant bit indicates the fixed portion of the response
>header and if the least significant bit is set, then it indicates that the header
>contains an additional variable sized part of header,  (the size of which can be
>determined by looking at the individual fields in the fixed portion of the
>header).
>
>So 49 does not indicate that the header is 49 bytes in size. It means the header
>contains a 48 byte fixed part that is present in every such command headers
>but it also contains 0 or more bytes of additional header.
>
>So see these numbers as "these are the values we must put in the
>StructureSize for the header"  but they do not represent the actual size of the
>header. (well, except if the value is even, then it does represent the header
>size.)
>
>
>So for SMB2_READ and SMB2_WRITE commands, If we do not use RDMA,
>then Channel will be set to 0 and we will not have any ReadWriteChannelInfo
>data.
>In this case the READ/WRITE header size is always 48  (==49 & 0xfffe)
>
>If we are using RDMA, then we still have the 48 byte normal header but we
>also have 1 or more SMB_DIRECT_BUFFER_DESCRIPTOR_V1 structures there.
>So for that case the maximum header size would be 48  + <max number of
>structures> * sizeof(SMB_DIRECT_BUFFER_DESCRIPTOR_V1)

I see, I will send an updated patch.

Thanks,
Long

>
>
>regards
>ronnie sahlberg
>
>
>
>>
>> >
>> >So we should probably add to smb2pdu.h a #define
>> >MAX_SMB2_READWRITE_RESPONSE_SIZE 48 and use this in the
>calculations.
>> >Then we need to add the maximum size we will use for ChannelInfo.
>> >
>> >Maybe we should have a define also for the
>MAX_SMB2_CHANNEL_INFO_SIZE
>> >
>> >regards
>> >ronnie sahlberg
>> >
>> >On Thu, Mar 26, 2020 at 5:31 AM longli--- via samba-technical <samba-
>> >technical@lists.samba.org> wrote:
>> >>
>> >> From: Long Li <longli@microsoft.com>
>> >>
>> >> The packet size needs to take account of SMB2 header size and
>> >> possible encryption header size. This is only done when signing is
>> >> used and it is for RDMA send/receive, not read/write.
>> >>
>> >> Also remove the dead SMBD code in smb2_negotiate_r(w)size.
>> >>
>> >> Signed-off-by: Long Li <longli@microsoft.com>
>> >> ---
>> >>  fs/cifs/smb2ops.c   | 38 ++++++++++++++++----------------------
>> >>  fs/cifs/smbdirect.c |  3 +--
>> >>  2 files changed, 17 insertions(+), 24 deletions(-)
>> >>
>> >> diff --git a/fs/cifs/smb2ops.c b/fs/cifs/smb2ops.c index
>> >> 4c0922596467..9043d34eef43 100644
>> >> --- a/fs/cifs/smb2ops.c
>> >> +++ b/fs/cifs/smb2ops.c
>> >> @@ -332,16 +332,6 @@ smb2_negotiate_wsize(struct cifs_tcon *tcon,
>> >struct smb_vol *volume_info)
>> >>         /* start with specified wsize, or default */
>> >>         wsize = volume_info->wsize ? volume_info->wsize :
>> >CIFS_DEFAULT_IOSIZE;
>> >>         wsize = min_t(unsigned int, wsize, server->max_write);
>> >> -#ifdef CONFIG_CIFS_SMB_DIRECT
>> >> -       if (server->rdma) {
>> >> -               if (server->sign)
>> >> -                       wsize = min_t(unsigned int,
>> >> -                               wsize, server->smbd_conn-
>>max_fragmented_send_size);
>> >> -               else
>> >> -                       wsize = min_t(unsigned int,
>> >> -                               wsize, server->smbd_conn->max_readwrite_size);
>> >> -       }
>> >> -#endif
>> >>         if (!(server->capabilities & SMB2_GLOBAL_CAP_LARGE_MTU))
>> >>                 wsize = min_t(unsigned int, wsize,
>> >> SMB2_MAX_BUFFER_SIZE);
>> >>
>> >> @@ -360,8 +350,15 @@ smb3_negotiate_wsize(struct cifs_tcon *tcon,
>> >> struct smb_vol *volume_info)  #ifdef CONFIG_CIFS_SMB_DIRECT
>> >>         if (server->rdma) {
>> >>                 if (server->sign)
>> >> +                       /*
>> >> +                        * Account for SMB2 data transfer packet header
>> >> +                        * SMB2_READ/SMB2_WRITE (49) and possible encryption
>> >> +                        * headers
>> >> +                        */
>> >>                         wsize = min_t(unsigned int,
>> >> -                               wsize, server->smbd_conn-
>>max_fragmented_send_size);
>> >> +                               wsize,
>> >> +                               server->smbd_conn->max_fragmented_send_size -
>> >> +                                       49 - sizeof(struct
>> >> + smb2_transform_hdr));
>> >>                 else
>> >>                         wsize = min_t(unsigned int,
>> >>                                 wsize,
>> >> server->smbd_conn->max_readwrite_size);
>> >> @@ -382,16 +379,6 @@ smb2_negotiate_rsize(struct cifs_tcon *tcon,
>> >> struct
>> >smb_vol *volume_info)
>> >>         /* start with specified rsize, or default */
>> >>         rsize = volume_info->rsize ? volume_info->rsize :
>CIFS_DEFAULT_IOSIZE;
>> >>         rsize = min_t(unsigned int, rsize, server->max_read);
>> >> -#ifdef CONFIG_CIFS_SMB_DIRECT
>> >> -       if (server->rdma) {
>> >> -               if (server->sign)
>> >> -                       rsize = min_t(unsigned int,
>> >> -                               rsize, server->smbd_conn-
>>max_fragmented_recv_size);
>> >> -               else
>> >> -                       rsize = min_t(unsigned int,
>> >> -                               rsize, server->smbd_conn->max_readwrite_size);
>> >> -       }
>> >> -#endif
>> >>
>> >>         if (!(server->capabilities & SMB2_GLOBAL_CAP_LARGE_MTU))
>> >>                 rsize = min_t(unsigned int, rsize,
>> >> SMB2_MAX_BUFFER_SIZE); @@ -411,8 +398,15 @@
>> >> smb3_negotiate_rsize(struct cifs_tcon *tcon, struct smb_vol
>> >> *volume_info)
>> >#ifdef CONFIG_CIFS_SMB_DIRECT
>> >>         if (server->rdma) {
>> >>                 if (server->sign)
>> >> +                       /*
>> >> +                        * Account for SMB2 data transfer packet header
>> >> +                        * SMB2_READ/SMB2_WRITE (49) and possible encryption
>> >> +                        * headers
>> >> +                        */
>> >>                         rsize = min_t(unsigned int,
>> >> -                               rsize, server->smbd_conn-
>>max_fragmented_recv_size);
>> >> +                               rsize,
>> >> +                               server->smbd_conn->max_fragmented_recv_size -
>> >> +                                       49 - sizeof(struct
>> >> + smb2_transform_hdr));
>> >>                 else
>> >>                         rsize = min_t(unsigned int,
>> >>                                 rsize,
>> >> server->smbd_conn->max_readwrite_size);
>> >> diff --git a/fs/cifs/smbdirect.c b/fs/cifs/smbdirect.c index
>> >> eb1e40af9f3a..0327b575ab87 100644
>> >> --- a/fs/cifs/smbdirect.c
>> >> +++ b/fs/cifs/smbdirect.c
>> >> @@ -2097,8 +2097,7 @@ int smbd_send(struct TCP_Server_Info *server,
>> >>         for (i = 0; i < num_rqst; i++)
>> >>                 remaining_data_length += smb_rqst_len(server,
>> >> &rqst_array[i]);
>> >>
>> >> -       if (remaining_data_length + sizeof(struct smbd_data_transfer) >
>> >> -               info->max_fragmented_send_size) {
>> >> +       if (remaining_data_length > info->max_fragmented_send_size)
>> >> + {
>> >>                 log_write(ERR, "payload size %d > max size %d\n",
>> >>                         remaining_data_length, info->max_fragmented_send_size);
>> >>                 rc = -EINVAL;
>> >> --
>> >> 2.17.1
>> >>
>> >>