linux-kernel - [PATCH bpf 0/2] riscv, bpf: Properly sign-extend return values

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Date:   Wed,  4 Oct 2023 14:07:04 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, Pu Lehui <pulehui@...wei.com>
Cc:     Björn Töpel <bjorn@...osinc.com>,
        linux-kernel@...r.kernel.org, Luke Nelson <luke.r.nels@...il.com>,
        Xi Wang <xi.wang@...il.com>, linux-riscv@...ts.infradead.org
Subject: [PATCH bpf 0/2] riscv, bpf: Properly sign-extend return values

From: Björn Töpel <bjorn@...osinc.com>

The RISC-V architecture does not expose sub-registers, and hold all
32-bit values in a sign-extended format [1] [2]:

  | The compiler and calling convention maintain an invariant that all
  | 32-bit values are held in a sign-extended format in 64-bit
  | registers. Even 32-bit unsigned integers extend bit 31 into bits
  | 63 through 32. Consequently, conversion between unsigned and
  | signed 32-bit integers is a no-op, as is conversion from a signed
  | 32-bit integer to a signed 64-bit integer.

While BPF, on the other hand, exposes sub-registers, and use
zero-extension (similar to arm64/x86).

This has led to some subtle bugs, where a BPF JITted program has not
sign-extended the a0 register (return value in RISC-V land), passed
the return value up the kernel, e.g.:

  | int from_bpf(void);
  |
  | long foo(void)
  | {
  |    return from_bpf();
  | }

This series fixes this issue by keeping a pair of return value
registers; a0 (RISC-V ABI, sign-extended), a5 (BPF, zero-extended).

The following test_progs now pass, which were previously broken:

  | 13      bpf_cookie
  | 19      bpf_mod_race
  | 68      deny_namespace
  | 119     libbpf_get_fd_by_id_opts
  | 135     lookup_key
  | 137     lsm_cgroup
  | 284     test_lsm

Björn

Björn Töpel (2):
  riscv, bpf: Sign-extend return values
  riscv, bpf: Track both a0 (RISC-V ABI) and a5 (BPF) return values

 arch/riscv/net/bpf_jit_comp64.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

base-commit: 9077fc228f09c9f975c498c55f5d2e882cd0da59
-- 
2.39.2