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
