[DTrace-devel] [PATCH v2 3/4] dtrace: add tcp provider

[Kris Van Hees]

Not sure if this would confuse things or help, but here is an early attempt I
did on implementing a tcp provider:

	kvh/2.0-branch-dev-tcp

>From my recollection it was missing stuff, since it was WIP compared to what
Alan has worked on.

On Wed, Jul 02, 2025 at 08:02:00PM -0400, Eugene Loh wrote:
> On 7/2/25 11:06, Alan Maguire wrote:
> 
> > On 02/07/2025 00:16, Eugene Loh wrote:
> > > On most VMs,
> > >      test/unittest/tcp/tst.ipv4remotetcp.sh
> > >      test/unittest/tcp/tst.ipv4remotetcpstate.sh
> > > xfail due to missing remote.  Are we okay with "shrugging our shoulders"
> > > like that?
> > Yeah, I don't think the remote test is robust enough. Specifically in
> > OCI it seems to always fail. I'd suggest we replace it with creating a
> > network namespace with IP addresses configured on top of veths to
> > simulate the remote case, the codepaths will be the same. I've done this
> > in other test suites and it works well.
> 
> Sounds great (if "we" is "you", haha).
> 
> > > Meanwhile, my one non-OCI VM ran those tests.  The first test passes.
> > > The second one consistently reports
> > >      -tcp:::state-change to time-wait - yes
> > >      +tcp:::state-change to time-wait - no
> > I hit some of these failure during development; adding the
> > fbt::tcp_time_wait:entry probe helped. Is that inlined or something
> > perhaps (grep tcp_time_wait /proc/kallsyms)?
> 
> On the VM in question:
> 
> # grep -w tcp_time_wait /proc/kallsyms
> ffffffff92ad25b0 T tcp_time_wait
> # dtrace -lP fbt |& grep tcp_time_wait
> 49373        fbt           vmlinux                     tcp_time_wait return
> 49372        fbt           vmlinux                     tcp_time_wait entry
> # dtrace -lP rawfbt |& grep tcp_time_wait
> 51079     rawfbt           vmlinux                     tcp_time_wait return
> 51078     rawfbt           vmlinux                     tcp_time_wait entry
> 
> > > and occasionally reports stuff like
> > >      dtrace: error in dt_clause_2 for probe ID 4976 (tcp:vmlinux::send):
> > > invalid address (0x1fc0c0000000000) at BPF pc 287
> > >      dtrace: error in dt_clause_2 for probe ID 4976 (tcp:vmlinux::send):
> > > invalid address (0x225b80000000000) at BPF pc 287
> > > 
> > ah, ok there must be a null deref somewhere. Haven't seen this before;
> > what kernel version/arch is this?
> 
> 5.15.0-300.161.13.el9uek.x86_64
> 
> FWIW, I can comment out all probes in tcp other than:
> 
>         { "send", DTRACE_PROBESPEC_NAME,
> "rawfbt::ip_send_unicast_reply:entry" },
> 
> Then I run
> 
> dtrace -c "$testdir/client.ip.pl tcp $dest $tcpport" -qn 'tcp:::send
> /args[2]->ip_saddr == "'$source'"/ { tcpsend++; }'
> 
> The disassembly shows that I look up args[2] using dt_bvar_args() (including
> checking for a fault).  Then we try to dereference args[2]->ip_saddr.  We
> first check the pointer is non NULL.  Then we call dt_cg_load_scalar() to
> bpf_probe_read() from the desired location.  This call is problematic.
> 
> > > The non-remote tests fail on OL8 UEK6 (x86 and arm).
> > >      dtrace: failed to compile script /dev/stdin:
> > >      ".../build/dlibs/5.2/tcp.d", line 177: failed to resolve type of
> > > inet_ntoa arg#1 (ipaddr_t *):
> > >      Unknown type name
> > > 
> > This is a weird failure; I see it on some systems but not on others.
> > In tcp.d we have
> > 
> > #pragma D depends_on library net.d
> > 
> > which contains the typedef for ipaddr_t ; it seems that's not enough to
> > pull in the typedef reliably. I suspect there is a timing element
> > involved here in when the net.d library is included. Perhaps there is a
> > better way to define ipaddr_t ; would using a builtin typedef in
> > _dtrace_typedefs_32/64 work better perhaps?
> 
> Don't know.
> 
> > > The probe names are
> > >      tcp:ip:*:*        Solaris
> > >      tcp:vmlinux:*:*   DTv1
> > >      tcp:vmlinux::*    with this patch (that is, no more function)
> > > I guess precedents have already been set for other SDT providers;  so,
> > > okay.  Just noting for my own sake.
> > > Meanwhile, the typed args[] have changed in number and type from Solaris> to DTv1 to this patch.  Does that merit discussion?
> > Hmm, that's not intentional (aside from the additional INBOUND/OUTBOUND
> > etc which we use to help inform translation).
> 
> Worth mentioning somewhere?
> 
> > Do you see other changes aside from them? Thanks!
> 
> This is what I have for typed args[] for tcp probes.
> 
> The typed probe arguments for probes
>         accept-[refused|established]
>         connect-[refused|established|request]
>         receive
> are the same as for send.
> 
> The typed probe arguments for state-change may be different.
> 
> So, the typed probe arguments are (wide screen, fixed-width font):
> 
> args[0]:      args[1]:      args[2]:      args[3]: args[4]:     
> args[5]:      args[6]:      args[7]:
> 
>             send Solaris         pktinfo_t *   csinfo_t * ipinfo_t *   
> tcpsinfo_t *  tcpinfo_t *
>             send DTv1            (unknown)     (unknown) (unknown)    
> (unknown)     (unknown)     (unknown) int           int
>             send DTv2            pktinfo_t *   csinfo_t * ipinfo_t *   
> tcpsinfo_t *  tcpinfo_t *   int tcplsinfo_t * int
> 
>             state-change Solaris void          csinfo_t * void         
> tcpsinfo_t *  void          tcplsinfo_t *
>             state-change DTv1    (unknown)     (unknown) (unknown)    
> (unknown)     (unknown)     (unknown) int           int
>             state-change DTv2    void      *   csinfo_t * void     *   
> tcpsinfo_t *  void      *   void * tcplsinfo_t * int
> 
> Here, "DTv1" refers to legacy DTrace on Linux.  I guess we can ignore that. 
> By "DTv2" I mean your patch.  For state-change, Solaris calls some things
> "void" (not "void *") and tcplsinfo_t* moves from args[5] to args[6].