bpf, riscv: Clear fetch destination on faulting arena atomic

A RMW atomic on an arena pointer is converted to BPF_PROBE_ATOMIC and
gets an exception table entry, but that entry records no destination
register to clear unless the instruction is a load-acquire today. That
is right for a plain BPF_{ADD,AND,OR,XOR}, which only writes memory,
but an RMW carrying BPF_FETCH also reads the old value into a register:
src_reg for BPF_{ADD,AND,OR,XOR} | BPF_FETCH and BPF_XCHG, and r0 for
BPF_CMPXCHG. emit_atomic_rmw() emits it that way, e.g.:

  [...]
  case BPF_XCHG:
          ctx->ex_insn_off = ctx->ninsns;
          emit(is64 ? rv_amoswap_d(rs, rs, rd, 1, 1) :
               rv_amoswap_w(rs, rs, rd, 1, 1), ctx);
  [...]

Thus, a fault over an unmapped arena page ex_handler_bpf() jumps over
the access but leaves rs untouched, and the program resumes with
whatever it held before the atomic instead of the 0 that every other
BPF_PROBE_* access delivers. Fill the exception table entry in from
bpf_atomic_load_reg(), which returns the BPF register an atomic reads
the memory operand into or -1 when it has none. A load-acquire ends up
with the same register it gets today, it just goes through the helper.
Unlike x86-64 and arm64, riscv64 does not report arena violations from
its exception handler, so there is no access direction to correct here,
only the missing register clear.

Fixes: fb7cefabae ("riscv, bpf: Add support arena atomics for RV64")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Link: https://patch.msgid.link/20260811131600.506721-2-daniel@iogearbox.net
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

arch/riscv/net/bpf_jit_comp64.c

@@ -1992,10 +1992,19 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 			ret = emit_atomic_rmw(rd, rs, insn, ctx);
 
 		/* ret can be 1 (skip-zext); extable entry still needs to be added */
-		if (ret >= 0)
-			ret = add_exception_handler(insn,
-				bpf_atomic_is_load_acq(insn) ? rd : REG_DONT_CLEAR_MARKER,
-				ctx) ?: ret;
+		if (ret >= 0) {
+			/*
+			 * A load-acquire reads into dst_reg, and a read-modify-write
+			 * carrying BPF_FETCH reads the old value into src_reg, or into
+			 * r0 for a BPF_CMPXCHG. Clear that register on fault, the
+			 * remaining atomics have no destination register.
+			 */
+			int load_reg = bpf_atomic_load_reg(insn);
+
+			ret = add_exception_handler(insn, load_reg < 0 ?
+					REG_DONT_CLEAR_MARKER : regmap[load_reg],
+					ctx) ?: ret;
+		}
 
 		if (ret)
 			return ret;