5.15-stable review patch. If anyone has any objections, please let me know.
------------------
From: Björn Töpel bjorn@rivosinc.com
[ Upstream commit 2f1b0d3d733169eb11680bfa97c266ae5e757148 ]
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(); | }
Here, a0 would be 0xffff_ffff, instead of the expected 0xffff_ffff_ffff_ffff.
Internally, the RISC-V JIT uses a5 as a dedicated register for BPF return values.
Keep a5 zero-extended, but explicitly sign-extend a0 (which is used outside BPF land). Now that a0 (RISC-V ABI) and a5 (BPF ABI) differs, a0 is only moved to a5 for non-BPF native calls (BPF_PSEUDO_CALL).
Fixes: 2353ecc6f91f ("bpf, riscv: add BPF JIT for RV64G") Signed-off-by: Björn Töpel bjorn@rivosinc.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Link: https://github.com/riscv/riscv-isa-manual/releases/download/riscv-isa-releas... # [2] Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/releases/download/draft... # [2] Link: https://lore.kernel.org/bpf/20231004120706.52848-2-bjorn@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- arch/riscv/net/bpf_jit_comp64.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c index 5fc1386bf311d..3f471884816af 100644 --- a/arch/riscv/net/bpf_jit_comp64.c +++ b/arch/riscv/net/bpf_jit_comp64.c @@ -220,7 +220,7 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx) emit_addi(RV_REG_SP, RV_REG_SP, stack_adjust, ctx); /* Set return value. */ if (!is_tail_call) - emit_mv(RV_REG_A0, RV_REG_A5, ctx); + emit_addiw(RV_REG_A0, RV_REG_A5, 0, ctx); emit_jalr(RV_REG_ZERO, is_tail_call ? RV_REG_T3 : RV_REG_RA, is_tail_call ? 4 : 0, /* skip TCC init */ ctx); @@ -875,7 +875,8 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx, if (ret) return ret;
- emit_mv(bpf_to_rv_reg(BPF_REG_0, ctx), RV_REG_A0, ctx); + if (insn->src_reg != BPF_PSEUDO_CALL) + emit_mv(bpf_to_rv_reg(BPF_REG_0, ctx), RV_REG_A0, ctx); break; } /* tail call */