On 2022/11/30 19:38, Björn Töpel wrote:
Pu Lehui <pulehui@xxxxxxxxxxxxxxx> writes:
From: Pu Lehui <pulehui@xxxxxxxxxx>
For BPF_PSEUDO_FUNC instruction, verifier will refill imm with
correct addresses of bpf_calls and then run last pass of JIT.
Since the emit_imm of RV64 is variable-length, which will emit
appropriate length instructions accorroding to the imm, it may
broke ctx->offset, and lead to unpredictable problem, such as
inaccurate jump. So let's fix it with fixed-length imm64 insns.
Ah, nice one! So, the the invariant doesn't hold (the image grow in the
last pass).
Fixes: 69c087ba6225 ("bpf: Add bpf_for_each_map_elem() helper")
This is odd? This can't be the right Fixes-tag...
Only BPF_PSEUDO_FUNC instruction need extra jit pass after refill imm in
jit_subprogs. Others, like bpf helper call, will update ctx->offset in
jit iterations. So the fixes-tag is 69c087ba6225.
Signed-off-by: Pu Lehui <pulehui@xxxxxxxxxx>
---
arch/riscv/net/bpf_jit_comp64.c | 31 ++++++++++++++++++++++++++++++-
1 file changed, 30 insertions(+), 1 deletion(-)
diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index eb99df41fa33..f984d5fa014b 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -139,6 +139,30 @@ static bool in_auipc_jalr_range(s64 val)
val < ((1L << 31) - (1L << 11));
}
+/* Emit fixed-length instructions for 32-bit imm */
+static void emit_fixed_imm32(u8 rd, s32 val, struct rv_jit_context *ctx)
+{
+ s32 upper = (val + (1U << 11)) >> 12;
+ s32 lower = ((val & 0xfff) << 20) >> 20;
+
+ emit(rv_lui(rd, upper), ctx);
+ emit(rv_addi(rd, rd, lower), ctx);
+}
+
+/* Emit fixed-length instructions for 64-bit imm */
+static void emit_fixed_imm64(u8 rd, s64 val, struct rv_jit_context *ctx)
+{
+ /* Compensation for sign-extension of rv_addi */
+ s32 imm_hi = (val + (1U << 31)) >> 32;
+ s32 imm_lo = val;
+
+ emit_fixed_imm32(rd, imm_hi, ctx);
+ emit_fixed_imm32(RV_REG_T1, imm_lo, ctx);
+ emit(rv_slli(rd, rd, 32), ctx);
+ emit(rv_add(rd, rd, RV_REG_T1), ctx);
+}
Hmm, will this really be fixed? We can end up with compressed
instructions, which can then be a non-compressed in the last pass, and
we have the same problem?
The range of valid address for RV64 (sv39 to sv57) are
0xffffffff00000000 to 0xffffffffffffffff, so I think we can do better
than 6 insn, no? My gut feeling (I need to tinker a bit) is that 4
should be sufficient.
Note that worst case for a imm64 load are 8 instructions, but this is
not the general case.
Björn
