bpf, riscv: Fix extable handling for arena load_acquire

emit_atomic_ld_st() returns 1 to have build_body() skip the zext after
a sub-word load_acquire. The caller does "ret = ret ?:
add_exception_handler(...)", which skips add_exception_handler() on any
non-zero ret, so the extable entry is missing and a faulting
PROBE_ATOMIC load_acquire oopses.

REG_DONT_CLEAR_MARKER leaves rd stale on fault, and the verifier still
thinks the load overwrote it, so a program can leak it through a map.

Check ret >= 0 before calling add_exception_handler(), and pass rd for
LOAD_ACQ so the fault zeroes rd like a PROBE_MEM load. Return ret
unchanged for the zext skip.

Fixes: fb7cefabae ("riscv, bpf: Add support arena atomics for RV64")
Suggested-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Feng Jiang <jiangfeng@kylinos.cn>
Reviewed-by: Pu Lehui <pulehui@huawei.com>
Reviewed-by: Björn Töpel <bjorn@kernel.org>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/bpf/20260720-bpf-riscv-fix-extable-v4-1-165c0b3b07d5@kylinos.cn
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

arch/riscv/net/bpf_jit_comp64.c

@@ -1986,7 +1986,12 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
 		else
 			ret = emit_atomic_rmw(rd, rs, insn, ctx);
 
-		ret = ret ?: add_exception_handler(insn, REG_DONT_CLEAR_MARKER, ctx);
+		/* ret can be 1 (skip-zext); extable entry still needs to be added */
+		if (ret >= 0)
+			ret = add_exception_handler(insn,
+				insn->imm == BPF_LOAD_ACQ ? rd : REG_DONT_CLEAR_MARKER,
+				ctx) ?: ret;
+
 		if (ret)
 			return ret;
 		break;