[RFC,bpf-next,v2,0/3] compiler attribute: define __user as __attribute__((btf_type_tag("user")))

Message

Yonghong Song Nov. 18, 2021, 6:48 p.m. UTC

Latest clang compiler supports a type attribute like
  __attribute__((btf_type_tag("<arbitrary string>")))
which can be used in places like __user/__rcu.
With the above attribute, clang was able to preserve the "<arbitrary string>"
info in dwarf and subsequently BTF. For example __user/__rcu
attribute can be preserved in BTF, which currently is loaded inside
the kernel. Such information can then be used by verifier to check
bpf program memory access conforms to the access attribute.
Please see Patches 1 and 2 for details.

This is a RFC patch as it depends on pahole patch and
a new pahole version. The following is pahole patch link:
  https://lore.kernel.org/bpf/[e-mail]/
Second, for bpf verifier use of this new __user tag information,
I only implemented support to check function parameter
dereference. More work will be needed to check other
non function parameter memory accesses.

Changelog:
  RFC v1 -> RFC v2:
   - kernel test bot reported a compilation error. Fix it.

Yonghong Song (3):
  compiler_types: define __user as __attribute__((btf_type_tag("user")))
  bpf: reject program if a __user tagged memory accessed in kernel way
  selftests/bpf: add a selftest with __user tag

 include/linux/bpf.h                           |  1 +
 include/linux/bpf_verifier.h                  |  1 +
 include/linux/btf.h                           |  5 ++++
 include/linux/compiler_types.h                |  2 ++
 kernel/bpf/btf.c                              | 13 ++++++++--
 kernel/bpf/verifier.c                         | 17 ++++++++++---
 lib/Kconfig.debug                             |  5 ++++
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  9 +++++++
 .../selftests/bpf/prog_tests/btf_tag.c        | 23 ++++++++++++++++++
 .../selftests/bpf/progs/btf_type_tag_user.c   | 24 +++++++++++++++++++
 10 files changed, 95 insertions(+), 5 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/btf_type_tag_user.c