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

[Kris Van Hees]

On Thu, Jul 03, 2025 at 11:29:42AM -0400, Kris Van Hees wrote:
> 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

But... it is only on an internal repo since I never finished it :)  But perhaps
it can help with review.  Probably not worth pushing to github because I do not
think what I did is useful anymore in view of Alan's work.

> >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].