bpf: fix bpf_d_path() helper prototype #6462
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Commit 37cce22 ("bpf: verifier: Refactor helper access type tracking") started distinguishing read vs write accesses performed by helpers. The second argument of bpf_d_path() is a pointer to a buffer that the helper fills with the resulting path. However, its prototype currently uses ARG_PTR_TO_MEM without MEM_WRITE. Before 37cce22, helper accesses were conservatively treated as potential writes, so this mismatch did not cause issues. Since that commit, the verifier may incorrectly assume that the buffer contents are unchanged across the helper call and base its optimizations on this wrong assumption. This can lead to misbehaviour in BPF programs that read back the buffer, such as prefix comparisons on the returned path. Fix this by marking the second argument of bpf_d_path() as ARG_PTR_TO_MEM | MEM_WRITE so that the verifier correctly models the write to the caller-provided buffer. Fixes: 37cce22 ("bpf: verifier: Refactor helper access type tracking") Co-developed-by: Zesen Liu <[email protected]> Signed-off-by: Zesen Liu <[email protected]> Co-developed-by: Peili Gao <[email protected]> Signed-off-by: Peili Gao <[email protected]> Co-developed-by: Haoran Ni <[email protected]> Signed-off-by: Haoran Ni <[email protected]> Signed-off-by: Shuran Liu <[email protected]> Reviewed-by: Matt Bobrowski <[email protected]>
Add a regression test for bpf_d_path() to cover incorrect verifier assumptions caused by an incorrect function prototype. The test attaches to the fallocate hook, calls bpf_d_path() and verifies that a simple prefix comparison on the returned pathname behaves correctly after the fix in patch 1. It ensures the verifier does not assume the buffer remains unwritten. Co-developed-by: Zesen Liu <[email protected]> Signed-off-by: Zesen Liu <[email protected]> Co-developed-by: Peili Gao <[email protected]> Signed-off-by: Peili Gao <[email protected]> Co-developed-by: Haoran Ni <[email protected]> Signed-off-by: Haoran Ni <[email protected]> Signed-off-by: Shuran Liu <[email protected]>
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Pull request for series with
subject: bpf: fix bpf_d_path() helper prototype
version: 4
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=1030360