[PATCH 6.1 052/131] riscv, bpf: Sign-extend return values

Greg Kroah-Hartman
2023-10-16T08:56:09Z

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-release-056b6ff-2023-10-02/unpriv-isa-asciidoc.pdf # [2]
Link: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/releases/download/draft-20230929-e5c800e661a53efe3c2678d71a306323b60eb13b/riscv-abi.pdf # [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 69ebab81d9352..8f5d3c57d58ad 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -236,7 +236,7 @@ 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);
@@ -1038,7 +1038,8 @@ 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 */
--
2.40.1