The potential issue of kprobe_write_ctx+freplace was mentioned in
"bpf: Disallow !kprobe_write_ctx progs tail-calling kprobe_write_ctx progs" [1].

It is true issue, that the test in patch #3 verifies that kprobe_write_ctx=false
kprobe progs can be abused to modify struct pt_regs via kprobe_write_ctx=true
freplace progs.

When struct pt_regs is modified, bpf_prog_test_run_opts() gets -EFAULT instead
of 0.

test_kprobe_write_ctx:FAIL:bpf_prog_test_run_opts unexpected error: -14 (errno 14)

We will disallow freplace on kprobe programs with different kprobe_write_ctx
values.

However, xdp_has_frags does not cause such severe issue.

The issue of xdp_has_frags+freplace is that it is able to break the backwards
compatibility of XDP.

At the commit f45d5b6ce2e8 ("bpf: generalise tail call map compatibility check"),
the backwards compatibility was protected against tail calls, but it missed the
freplace case, which is also able to extend XDP progs. If xdp_has_frags=true
freplace progs are allowed to attach to xdp_has_frags=false XDP progs, the
backwards compatibility is broken.

We will disallow freplace on XDP programs with different xdp_has_frags values,
like the way in bpf_prog_map_compatible().

Links:
[1] https://lore.kernel.org/bpf/CAP01T74w4KVMn9bEwpQXrk+bqcUxzb6VW1SQ_QvNy0A4EY-9Jg@mail.gmail.com/ (local)

Leon Hwang (3):
  bpf: Disallow freplace on XDP with mismatched xdp_has_frags values
  bpf: Disallow freplace on kprobe with mismatched kprobe_write_ctx
    values
  selftests/bpf: Add tests to verify freplace compatibility

 kernel/bpf/verifier.c                         |  12 ++
 .../bpf/prog_tests/freplace_compatible.c      | 124 ++++++++++++++++++
 .../selftests/bpf/progs/freplace_compatible.c |  40 ++++++
 3 files changed, 176 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/freplace_compatible.c
 create mode 100644 tools/testing/selftests/bpf/progs/freplace_compatible.c

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2.53.0