[bpf-next,0/2] Fix attaching fentry/fexit/fmod_ret/lsm to modules

Message

Viktor Malik Nov. 28, 2022, 7:26 a.m. UTC

While working on bpftrace support for BTF in modules [1], I noticed that
the verifier behaves incorrectly when attaching to fentry of multiple
functions of the same name located in different modules (or in vmlinux).

The reason for this is that if the target program is not specified, the
verifier will search kallsyms for the trampoline address to attach to.
The entire kallsyms is always searched, not respecting the module in
which the function to attach to is located.

This patch fixes the above issue by extracting the module name from the
BTF of the attachment target (which must be specified) and by doing the
search in kallsyms of the correct module.

This also adds a new test in test_progs which tries to attach a program
to fentry of two functions of the same name, one located in vmlinux and
the other in bpf_testmod. Prior to the fix, the verifier would always
use the vmlinux function address as the target trampoline, attempting to
attach two functions to the same trampoline (which is prohibited).

[1] https://github.com/iovisor/bpftrace/pull/2315

Viktor Malik (2):
  bpf: Fix attaching fentry/fexit/fmod_ret/lsm to modules
  bpf/selftests: Test fentry attachment to shadowed functions

 include/linux/btf.h                           |   1 +
 kernel/bpf/btf.c                              |   5 +
 kernel/bpf/verifier.c                         |   9 +-
 net/bpf/test_run.c                            |   5 +
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |   7 +
 .../bpf/prog_tests/module_attach_shadow.c     | 120 ++++++++++++++++++
 6 files changed, 146 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/module_attach_shadow.c