[PATCH bpf-next v3 0/3] bpf: allow void* cast using bpf_rdonly_cast()

[Eduard Zingerman <eddyz87@gmail.com>]

Currently, pointers returned by `bpf_rdonly_cast()` have a type of
"pointer to btf id", and only casts to structure types are allowed.
Access to memory pointed to by these pointers is done through
`BPF_PROBE_{MEM,MEMSX}` instructions and does not produce errors on
invalid memory access.

This patch set extends `bpf_rdonly_cast()` to allow casts to an
equivalent of 'void *', effectively replacing
`bpf_probe_read_kernel()` calls in situations where access to
individual bytes or integers is necessary.

The mechanism was suggested and explored by Andrii Nakryiko in [1].

To help with detecting support for this feature, an
`enum bpf_features` is added with intended usage as follows:

  if (bpf_core_enum_value_exists(enum bpf_features,
                                 BPF_FEAT_RDONLY_CAST_TO_VOID))
    ...

[1] https://github.com/anakryiko/linux/tree/bpf-mem-cast

Changelog:

v2: https://lore.kernel.org/bpf/20250625000520.2700423-1-eddyz87@gmail.com/
v2 -> v3:
- dropped direct numbering for __MAX_BPF_FEAT.

v1: https://lore.kernel.org/bpf/20250624191009.902874-1-eddyz87@gmail.com/
v1 -> v2:
- renamed BPF_FEAT_TOTAL to __MAX_BPF_FEAT and moved patch introducing
  bpf_features enum to the start of the series (Alexei);
- dropped patch #3 allowing optout from CAP_SYS_ADMIN drop in
  prog_tests/verifier.c, use a separate runner in prog_tests/*
  instead.

Eduard Zingerman (3):
  bpf: add bpf_features enum
  bpf: allow void* cast using bpf_rdonly_cast()
  selftests/bpf: check operations on untrusted ro pointers to mem

 kernel/bpf/verifier.c                         |  79 ++++++++--
 .../bpf/prog_tests/mem_rdonly_untrusted.c     |   9 ++
 .../bpf/progs/mem_rdonly_untrusted.c          | 136 ++++++++++++++++++
 3 files changed, 212 insertions(+), 12 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/mem_rdonly_untrusted.c
 create mode 100644 tools/testing/selftests/bpf/progs/mem_rdonly_untrusted.c

-- 
2.47.1