[PATCH bpf-next v3 0/2] bpf: Fix memory access flags in helper prototypes

Zesen Liu
2026-01-20T08:29:32Z

Hi,

This series adds missing memory access flags (MEM_RDONLY or MEM_WRITE) to
several bpf helper function prototypes that use ARG_PTR_TO_MEM but lack the
correct flag. It also adds a new check in verifier to ensure the flag is
specified.

Missing memory access flags in helper prototypes can lead to critical
correctness issues when the verifier tries to perform code optimization.
After commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type
tracking"), the verifier relies on the memory access flags, rather than
treating all arguments in helper functions as potentially modifying the
pointed-to memory.

Using ARG_PTR_TO_MEM alone without flags does not make sense because:

- If the helper does not change the argument, missing MEM_RDONLY causes the
verifier to incorrectly reject a read-only buffer.
- If the helper does change the argument, missing MEM_WRITE causes the
verifier to incorrectly assume the memory is unchanged, leading to
errors in code optimization.

We have already seen several reports regarding this:

- commit ac44dcc788b9 ("bpf: Fix verifier assumptions of bpf_d_path's
output buffer") adds MEM_WRITE to bpf_d_path;
- commit 2eb7648558a7 ("bpf: Specify access type of bpf_sysctl_get_name
args") adds MEM_WRITE to bpf_sysctl_get_name.

This series looks through all prototypes in the kernel and completes the
flags. It also adds check_mem_arg_rw_flag_ok() and wires it into
check_func_proto() to statically restrict ARG_PTR_TO_MEM from appearing
without memory access flags.

Changelog
=========

v3:
- Rebased to bpf-next to address check_func_proto() signature changes, as
suggested by Eduard Zingerman.

v2:
- Add missing MEM_RDONLY flags to protos with ARG_PTR_TO_FIXED_SIZE_MEM.

Thanks,

Zesen Liu

---
Zesen Liu (2):
bpf: Fix memory access flags in helper prototypes
bpf: Require ARG_PTR_TO_MEM with memory flag

kernel/bpf/helpers.c | 2 +-
kernel/bpf/syscall.c | 2 +-
kernel/bpf/verifier.c | 17 +++++++++++++++++
kernel/trace/bpf_trace.c | 6 +++---
net/core/filter.c | 20 ++++++++++----------
5 files changed, 32 insertions(+), 15 deletions(-)
---
base-commit: efad162f5a840ae178e7761c176c49f433c7bb68
change-id: 20251220-helper_proto-fb6e64182467

Best regards,
--
Zesen Liu <ftyghome@gmail.com>