[bpf-next,v3,0/4] allow variable-offset stack acces

Message

Andrei Matei Feb. 7, 2021, 1:10 a.m. UTC

Before this patch, variable offset access to the stack was dissalowed
for regular instructions, but was allowed for "indirect" accesses (i.e.
helpers). This patch removes the restriction, allowing reading and
writing to the stack through stack pointers with variable offsets. This
makes stack-allocated buffers more usable in programs, and brings stack
pointers closer to other types of pointers.
    
The motivation is being able to use stack-allocated buffers for data
manipulation. When the stack size limit is sufficient, allocating
buffers on the stack is simpler than per-cpu arrays, or other
alternatives.

V2 -> V3

- var-offset writes mark all the stack slots in range as initialized, so
  that future reads are not rejected.
- rewrote the C test to not use uprobes, as per Andrii's suggestion.
- addressed other review comments from Alexei.

V1 -> V2

- add support for var-offset stack writes, in addition to reads
- add a C test
- made variable offset direct reads no longer destroy spilled registers
  in the access range
- address review nits



Andrei Matei (4):
  bpf: allow variable-offset stack access
  selftest/bpf: adjust expected verifier errors
  selftest/bpf: verifier tests for var-off access
  selftest/bpf: add test for var-offset stack access

 include/linux/bpf.h                           |   5 +
 include/linux/bpf_verifier.h                  |   3 +-
 kernel/bpf/verifier.c                         | 657 ++++++++++++++----
 .../selftests/bpf/prog_tests/stack_var_off.c  |  36 +
 .../selftests/bpf/progs/test_stack_var_off.c  |  56 ++
 .../selftests/bpf/verifier/basic_stack.c      |   2 +-
 tools/testing/selftests/bpf/verifier/calls.c  |   4 +-
 .../testing/selftests/bpf/verifier/const_or.c |   4 +-
 .../bpf/verifier/helper_access_var_len.c      |  12 +-
 .../testing/selftests/bpf/verifier/int_ptr.c  |   6 +-
 .../selftests/bpf/verifier/raw_stack.c        |  10 +-
 .../selftests/bpf/verifier/stack_ptr.c        |  22 +-
 tools/testing/selftests/bpf/verifier/unpriv.c |   2 +-
 .../testing/selftests/bpf/verifier/var_off.c  | 115 ++-
 14 files changed, 748 insertions(+), 186 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/stack_var_off.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_stack_var_off.c

Comments

Alexei Starovoitov Feb. 10, 2021, 7:14 p.m. UTC | #1

On Sat, Feb 6, 2021 at 5:11 PM Andrei Matei <[e-mail]> wrote:
>
> Before this patch, variable offset access to the stack was dissalowed
> for regular instructions, but was allowed for "indirect" accesses (i.e.
> helpers). This patch removes the restriction, allowing reading and
> writing to the stack through stack pointers with variable offsets. This
> makes stack-allocated buffers more usable in programs, and brings stack
> pointers closer to other types of pointers.
>
> The motivation is being able to use stack-allocated buffers for data
> manipulation. When the stack size limit is sufficient, allocating
> buffers on the stack is simpler than per-cpu arrays, or other
> alternatives.
>
> V2 -> V3
>
> - var-offset writes mark all the stack slots in range as initialized, so
>   that future reads are not rejected.
> - rewrote the C test to not use uprobes, as per Andrii's suggestion.
> - addressed other review comments from Alexei.

I've fixed up Andrii's nits in patch 4,
then moved skel__attach after test_pid init and applied to bpf-next.

I've played with a few other ways to do var stack access in C and all
looked good.
Thanks a lot for making the verifier smarter.