A production BPF program had the following code produced by LLVM.
r0 = 1024;
r1 = ...; // r1 = trusted_or_null_(id=1)
r3 = r1; // r3 = trusted_or_null_(id=1) r1 = trusted_or_null_(id=1)
r3 += r0; // r3 = trusted_or_null_(id=1, off=1024)
if r1 == 0 goto pc+X;
After cb4158ce8ec8 ("bpf: Mark raw_tp arguments with PTR_MAYBE_NULL"),
the production BPF program began throwing a warning in the verifier
because for the code above, when unmarking null mark from r1, the
verifier will notice another register r3 with same id but off != 0,
which is unexpected, since offset modification on PTR_MAYBE_NULL is not
permitted, but the aforementioned commit relaxed that restriction to
preserve compatibility with non-NULL raw_tp args.
Another production program hit a case where generic code it was calling
into would perform a NULL check, while the program knows and is written
with the knowledge that the raw_tp arg can never be NULL.
In earlier versions before the raw_tp change, verifier would never walk
the path where raw_tp arg was seen as scalar zero, but now it will,
hence code in the program that operates on the raw_tp arg later on will
fail on dereferencing a scalar.
Provide a fix to suppress the warning for raw_tp args, and not mark NULL
checked raw_tp args as scalars. We will follow up with a more generic
fix to handle such patterns for all pointer types in the verifier, which
currently involves playing whack-a-mole with suppressing such LLVM
optimizations and reworking BPF programs to avoid verifier errors.
Changelog:
----------
v2 -> v3
v2: https://lore.kernel.org/bpf/20241205223152.2434683-1-memxor@xxxxxxxxx
* Add Acked-by for Patch 1
* Add fix for scalar dereference issue
* Roll both fixes into one, as second fix undoes first
* Fix nits
v1 -> v2
v1: https://lore.kernel.org/bpf/20241204024154.21386-1-memxor@xxxxxxxxx
* Fix eager unmarking bug (Eduard)
* Generalize approach, always unmark NULL when off == 0 is checked
* Make NULL check noop if operand has off != 0
* Do not reset id when treating as noop
* Trim comment (Alexei)
* Adjust selftests
Kumar Kartikeya Dwivedi (3):
bpf: Suppress warning for non-zero off raw_tp arg NULL check
bpf: Do not mark NULL-checked raw_tp arg as scalar
selftests/bpf: Add raw_tp tests for PTR_MAYBE_NULL marking
kernel/bpf/verifier.c | 44 +++++++--
.../selftests/bpf/prog_tests/raw_tp_null.c | 6 ++
.../selftests/bpf/progs/raw_tp_null_fail.c | 90 +++++++++++++++++++
3 files changed, 133 insertions(+), 7 deletions(-)
create mode 100644 tools/testing/selftests/bpf/progs/raw_tp_null_fail.c
base-commit: 5a6ea7022ff4d2a65ae328619c586d6a8909b48b
--
2.43.5
