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From: "me at serhei dot io" <sourceware-bugzilla@sourceware.org>
To: systemtap@sourceware.org
Subject: [Bug bpf/24811] New: stapbpf segfault: nested foreach loops can corrupt sorted key data when limit==0
Date: Mon, 15 Jul 2019 16:51:00 -0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D24811

            Bug ID: 24811
           Summary: stapbpf segfault: nested foreach loops can corrupt
                    sorted key data when limit=3D=3D0
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: bpf
          Assignee: systemtap at sourceware dot org
          Reporter: me at serhei dot io
  Target Milestone: ---

spotted this while working on pr23858 which is a rabbit hole of tricky
behaviour

The existing if (limit =3D=3D 0) goto empty; -> empty: keys.pop_back() code
sequence in bpfinterp.cxx map_get_next_key() seemed fishy to me. Turns out =
you
can cause a segfault with nested foreach loops.

  foreach (k1- in a) { // push sorted data for a=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
    flag =3D flag && k1 =3D=3D (0-a[k1]) // check data for a=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
    foreach (k2- in b) { // push sorted data for b=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
      flag =3D flag && k2 =3D=3D b[k2] // check data for b=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
      foreach (k3- in a limit lim) { // bug -- don't push, pop sorted data =
for
b=20=20=20=20=20=20=20=20=20
        flag =3D 0 // should not be invoked=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20
      } // bug -- after popping, will read sorted data for a on next iterat=
ion=20=20
    } // pop sorted data for b, but with bug pops a=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
  } // pop sorted data for a, but with bug pops <nada> -- segfault?

Filing since I suspect a less convoluted set of loops will cause the same
thing.

--=20
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