bpf, mips: No need to use min() to get MAX_TAIL_CALL_CNT

MAX_TAIL_CALL_CNT is 33, so min(MAX_TAIL_CALL_CNT, 0xffff) is always
MAX_TAIL_CALL_CNT, it is better to use MAX_TAIL_CALL_CNT directly.

At the same time, add BUILD_BUG_ON(MAX_TAIL_CALL_CNT > 0xffff) with a
comment on why the assertion is there.
Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Suggested-by: Johan Almbladh <johan.almbladh@anyfinetworks.com>
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/1661742309-2320-1-git-send-email-yangtiezhu@loongson.cn

arch/mips/net/bpf_jit_comp32.c

@@ -1376,12 +1376,20 @@ void build_prologue(struct jit_context *ctx)
 	const u8 *fp = bpf2mips32[BPF_REG_FP];
 	int stack, saved, locals, reserved;
 
+	/*
+	 * In the unlikely event that the TCC limit is raised to more
+	 * than 16 bits, it is clamped to the maximum value allowed for
+	 * the generated code (0xffff). It is better fail to compile
+	 * instead of degrading gracefully.
+	 */
+	BUILD_BUG_ON(MAX_TAIL_CALL_CNT > 0xffff);
+
 	/*
 	 * The first two instructions initialize TCC in the reserved (for us)
 	 * 16-byte area in the parent's stack frame. On a tail call, the
 	 * calling function jumps into the prologue after these instructions.
 	 */
-	emit(ctx, ori, MIPS_R_T9, MIPS_R_ZERO, min(MAX_TAIL_CALL_CNT, 0xffff));
+	emit(ctx, ori, MIPS_R_T9, MIPS_R_ZERO, MAX_TAIL_CALL_CNT);
 	emit(ctx, sw, MIPS_R_T9, 0, MIPS_R_SP);
 
 	/*

arch/mips/net/bpf_jit_comp64.c

@@ -547,12 +547,20 @@ void build_prologue(struct jit_context *ctx)
 	u8 zx = bpf2mips64[JIT_REG_ZX];
 	int stack, saved, locals, reserved;
 
+	/*
+	 * In the unlikely event that the TCC limit is raised to more
+	 * than 16 bits, it is clamped to the maximum value allowed for
+	 * the generated code (0xffff). It is better fail to compile
+	 * instead of degrading gracefully.
+	 */
+	BUILD_BUG_ON(MAX_TAIL_CALL_CNT > 0xffff);
+
 	/*
 	 * The first instruction initializes the tail call count register.
 	 * On a tail call, the calling function jumps into the prologue
 	 * after this instruction.
 	 */
-	emit(ctx, ori, tc, MIPS_R_ZERO, min(MAX_TAIL_CALL_CNT, 0xffff));
+	emit(ctx, ori, tc, MIPS_R_ZERO, MAX_TAIL_CALL_CNT);
 
 	/* === Entry-point for tail calls === */