Willem de Bruijn [off-list ref] ezt írta (időpont:
2024. nov. 3., V, 2:27):

Anna Emese Nyiri wrote:

quoted

The Linux socket API currently supports setting SO_PRIORITY at the
socket level, which applies a uniform priority to all packets sent
through that socket. The only exception is IP_TOS; if specified as
ancillary data, the packet does not inherit the socket's priority.
Instead, the priority value is computed when handling the ancillary
data (as implemented in commit <f02db315b8d88>

nit: drop the brackets

quoted

("ipv4: IP_TOS and IP_TTL can be specified as ancillary data")). If
the priority is set via IP_TOS, then skb->priority derives its value
from the rt_tos2priority function, which calculates the priority
based on the value of ipc->tos obtained from IP_TOS. However, if
IP_TOS is not used and the priority has been set through a control
message, skb->priority will take the value provided by that control
message.

The above describes the new situation? There is no way to set
priority to a control message prior to this patch.

quoted

Therefore, when both options are available, the primary
source for skb->priority is the value set via IP_TOS.

Currently, there is no option to set the priority directly from
userspace on a per-packet basis. The following changes allow
SO_PRIORITY to be set through control messages (CMSG), giving
userspace applications more granular control over packet priorities.

This patch enables setting skb->priority using CMSG. If SO_PRIORITY

Duplicate statement. Overall, the explanation can perhaps be
condensed and made more clear.

quoted

is specified as ancillary data, the packet is sent with the priority
value set through sockc->priority_cmsg_value, overriding the

No longer matches the code.

quoted

socket-level values set via the traditional setsockopt() method. This
is analogous to existing support for SO_MARK (as implemented in commit
<c6af0c227a22> ("ip: support SO_MARK cmsg")).

Suggested-by: Ferenc Fejes <redacted>
Signed-off-by: Anna Emese Nyiri <redacted>
---
 include/net/inet_sock.h | 2 +-
 include/net/ip.h        | 3 ++-
 include/net/sock.h      | 4 +++-
 net/can/raw.c           | 2 +-
 net/core/sock.c         | 8 ++++++++
 net/ipv4/ip_output.c    | 7 +++++--
 net/ipv4/raw.c          | 2 +-
 net/ipv6/ip6_output.c   | 3 ++-
 net/ipv6/raw.c          | 2 +-
 net/packet/af_packet.c  | 2 +-
 10 files changed, 25 insertions(+), 10 deletions(-)

diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h
index 56d8bc5593d3..3ccbad881d74 100644
--- a/include/net/inet_sock.h
+++ b/include/net/inet_sock.h

@@ -172,7 +172,7 @@ struct inet_cork {
      u8                      tx_flags;
      __u8                    ttl;
      __s16                   tos;
-     char                    priority;
+     u32                     priority;

Let's check with pahole how this affects struct size and holes.
It likely adds a hole, but unavoidably so.

quoted

      __u16                   gso_size;
      u32                     ts_opt_id;
      u64                     transmit_time;

diff --git a/include/net/ip.h b/include/net/ip.h
index 0e548c1f2a0e..e8f71a191277 100644
--- a/include/net/ip.h
+++ b/include/net/ip.h

@@ -81,7 +81,7 @@ struct ipcm_cookie {
      __u8                    protocol;
      __u8                    ttl;
      __s16                   tos;
-     char                    priority;
+     u32                     priority;

No need for a field in ipcm_cookie, when also present in
sockcm_cookie. As SO_PRIORITY is not limited to IP, sockcm_cookie is
the right location.

I think there could be a problem if the priority is set by IP_TOS for
some reason, and then also via cmsg. The latter value may overwrite
it. In the ip_setup_cork() function, there is therefore a check for
the value cork->tos != -1 to give priority to the value set by IP_TOS.
And that's why I thought that there should be a priority field in both
ipcm_cookie and sockcm_cookie. The priority field already existed in
ipcm_cookie, I didn't add it. I just changed the type.

If cmsg IP_TOS is present, that can overridde ipc->sockc.priority with
rt_tos2priority.

Interesting that this override by IP_TOS seems to be IPV4 only. There
is no equivalent call to rt_tos2priority when setting IPV6_TCLASS.

quoted

      __u16                   gso_size;
 };

@@ -96,6 +96,7 @@ static inline void ipcm_init_sk(struct ipcm_cookie *ipcm,
      ipcm_init(ipcm);

      ipcm->sockc.mark = READ_ONCE(inet->sk.sk_mark);
+     ipcm->sockc.priority = READ_ONCE(inet->sk.sk_priority);
      ipcm->sockc.tsflags = READ_ONCE(inet->sk.sk_tsflags);
      ipcm->oif = READ_ONCE(inet->sk.sk_bound_dev_if);
      ipcm->addr = inet->inet_saddr;

diff --git a/include/net/sock.h b/include/net/sock.h
index 7464e9f9f47c..316a34d6c48b 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h

@@ -1814,13 +1814,15 @@ struct sockcm_cookie {
      u32 mark;
      u32 tsflags;
      u32 ts_opt_id;
+     u32 priority;
 };

 static inline void sockcm_init(struct sockcm_cookie *sockc,
                             const struct sock *sk)
 {
      *sockc = (struct sockcm_cookie) {
-             .tsflags = READ_ONCE(sk->sk_tsflags)
+             .tsflags = READ_ONCE(sk->sk_tsflags),
+             .priority = READ_ONCE(sk->sk_priority),
      };
 }

diff --git a/net/can/raw.c b/net/can/raw.c
index 255c0a8f39d6..46e8ed9d64da 100644
--- a/net/can/raw.c
+++ b/net/can/raw.c

@@ -962,7 +962,7 @@ static int raw_sendmsg(struct socket *sock, struct msghdr *msg, size_t size)
      }

      skb->dev = dev;
-     skb->priority = READ_ONCE(sk->sk_priority);
+     skb->priority = sockc.priority;
      skb->mark = READ_ONCE(sk->sk_mark);
      skb->tstamp = sockc.transmit_time;

diff --git a/net/core/sock.c b/net/core/sock.c
index 5ecf6f1a470c..d5586b9212dd 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c

@@ -2941,6 +2941,14 @@ int __sock_cmsg_send(struct sock *sk, struct cmsghdr *cmsg,
      case SCM_RIGHTS:
      case SCM_CREDENTIALS:
              break;
+     case SO_PRIORITY:
+             if (cmsg->cmsg_len != CMSG_LEN(sizeof(u32)))
+                     return -EINVAL;
+             if (sk_set_prio_allowed(sk, *(u32 *)CMSG_DATA(cmsg))) {
+                     sockc->priority = *(u32 *)CMSG_DATA(cmsg);
+                     break;
+             }
+             return -EPERM;

nit: invert to make the error case the (speculated as unlikely) branch
and have the common path unindented.

quoted

      default:
              return -EINVAL;
      }