[PATCH bpf-next v2 1/2] bpf, riscv: Add support for signed arena loads

[Chen Pei]

Signed loads from arena memory are currently rejected on riscv64, as
bpf_jit_supports_insn() refuses BPF_MEMSX loads when in_arena is set,
while x86 and arm64 gained support for them in v6.18. Compilers such
as GCC-14 are free to generate signed loads into arena memory, which
breaks loading of otherwise valid BPF programs on riscv64.

Implement BPF_PROBE_MEM32SX support in the RV64 JIT by reusing the
existing arena handling: the arena base (RV_REG_ARENA) is added to
the source register and the load is emitted with sign extension
(lb/lh/lw). Add BPF_PROBE_MEM32SX to the add_exception_handler()
mode gate so that faulting loads get an exception table entry which
clears the destination register and resumes execution. Since
BPF_PROBE_MEM32SX shares its mode value (0xc0) with BPF_ATOMIC, the
gate accepts it only for LDX class instructions so that plain atomic
instructions do not register exception table entries.

Verified by running the arena LDSX selftests (arena_ldsx_disasm,
arena_ldsx_exception, arena_ldsx_s8/s16/s32) and the full
arena_atomics test suite on riscv64 QEMU, all passing.

Signed-off-by: Chen Pei <cp0613 at linux.alibaba.com>
---

Changes in v2:
- Fix extable entry overflow breaking arena_atomics load

 arch/riscv/net/bpf_jit_comp64.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index f9d5347ba966..acc1b16d2e2f 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -777,6 +777,8 @@ static int add_exception_handler(const struct bpf_insn *insn, int dst_reg,
 	if (BPF_MODE(insn->code) != BPF_PROBE_MEM &&
 	    BPF_MODE(insn->code) != BPF_PROBE_MEMSX &&
 	    BPF_MODE(insn->code) != BPF_PROBE_MEM32 &&
+	    !(BPF_MODE(insn->code) == BPF_PROBE_MEM32SX &&
+	      BPF_CLASS(insn->code) == BPF_LDX) &&
 	    BPF_MODE(insn->code) != BPF_PROBE_ATOMIC)
 		return 0;
 
@@ -1902,13 +1904,19 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 	case BPF_LDX | BPF_PROBE_MEM32 | BPF_H:
 	case BPF_LDX | BPF_PROBE_MEM32 | BPF_W:
 	case BPF_LDX | BPF_PROBE_MEM32 | BPF_DW:
+	/* LDX | PROBE_MEM32SX: sign-extending arena load */
+	case BPF_LDX | BPF_PROBE_MEM32SX | BPF_B:
+	case BPF_LDX | BPF_PROBE_MEM32SX | BPF_H:
+	case BPF_LDX | BPF_PROBE_MEM32SX | BPF_W:
 	{
 		bool sign_ext;
 
 		sign_ext = BPF_MODE(insn->code) == BPF_MEMSX ||
-			   BPF_MODE(insn->code) == BPF_PROBE_MEMSX;
+			   BPF_MODE(insn->code) == BPF_PROBE_MEMSX ||
+			   BPF_MODE(insn->code) == BPF_PROBE_MEM32SX;
 
-		if (BPF_MODE(insn->code) == BPF_PROBE_MEM32) {
+		if (BPF_MODE(insn->code) == BPF_PROBE_MEM32 ||
+		    BPF_MODE(insn->code) == BPF_PROBE_MEM32SX) {
 			emit_add(RV_REG_T2, rs, RV_REG_ARENA, ctx);
 			rs = RV_REG_T2;
 		}
@@ -2126,10 +2134,6 @@ bool bpf_jit_supports_insn(struct bpf_insn *insn, bool in_arena)
 			if (insn->imm == BPF_CMPXCHG)
 				return rv_ext_enabled(ZACAS);
 			break;
-		case BPF_LDX | BPF_MEMSX | BPF_B:
-		case BPF_LDX | BPF_MEMSX | BPF_H:
-		case BPF_LDX | BPF_MEMSX | BPF_W:
-			return false;
 		}
 	}
 
-- 
2.50.1