[PATCH bpf-next v2] bpf, mips: No need to use min() to get MAX_TAIL_CALL_CNT

[Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>]

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@xxxxxxxxxxxxx>
Suggested-by: Johan Almbladh <johan.almbladh@xxxxxxxxxxxxxxxxx>
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---

v2: Add BUILD_BUG_ON(MAX_TAIL_CALL_CNT > 0xffff) with a comment
    suggested by Daniel and Johan, thank you.

 arch/mips/net/bpf_jit_comp32.c | 10 +++++++++-
 arch/mips/net/bpf_jit_comp64.c | 10 +++++++++-
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/mips/net/bpf_jit_comp32.c b/arch/mips/net/bpf_jit_comp32.c
index 83c975d..ace5db3 100644
--- a/arch/mips/net/bpf_jit_comp32.c
+++ b/arch/mips/net/bpf_jit_comp32.c
@@ -1377,11 +1377,19 @@ void build_prologue(struct jit_context *ctx)
 	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);
 
 	/*
diff --git a/arch/mips/net/bpf_jit_comp64.c b/arch/mips/net/bpf_jit_comp64.c
index 6475828..0e7c1bd 100644
--- a/arch/mips/net/bpf_jit_comp64.c
+++ b/arch/mips/net/bpf_jit_comp64.c
@@ -548,11 +548,19 @@ void build_prologue(struct jit_context *ctx)
 	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 === */
 
-- 
2.1.0