riscv, bpf: Fix possible infinite tailcall when CONFIG_CFI_CLANG is enabled

When CONFIG_CFI_CLANG is enabled, the number of prologue instructions
skipped by tailcall needs to include the kcfi instruction, otherwise the
TCC will be initialized every tailcall is called, which may result in
infinite tailcalls.

Fixes: e63985ec ("bpf, riscv64/cfi: Support kCFI + BPF on riscv64")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Björn Töpel <bjorn@kernel.org>
Link: https://lore.kernel.org/r/20241008124544.171161-1-pulehui@huaweicloud.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

arch/riscv/net/bpf_jit_comp64.c

@@ -18,6 +18,7 @@
 #define RV_MAX_REG_ARGS 8
 #define RV_FENTRY_NINSNS 2
 #define RV_FENTRY_NBYTES (RV_FENTRY_NINSNS * 4)
+#define RV_KCFI_NINSNS (IS_ENABLED(CONFIG_CFI_CLANG) ? 1 : 0)
 /* imm that allows emit_imm to emit max count insns */
 #define RV_MAX_COUNT_IMM 0x7FFF7FF7FF7FF7FF
 
@@ -271,7 +272,8 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
 	if (!is_tail_call)
 		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 ? (RV_FENTRY_NINSNS + 1) * 4 : 0, /* skip reserved nops and TCC init */
+		  /* kcfi, fentry and TCC init insns will be skipped on tailcall */
+		  is_tail_call ? (RV_KCFI_NINSNS + RV_FENTRY_NINSNS + 1) * 4 : 0,
 		  ctx);
 }