[PATCH v1 1/3] LoongArch: BPF: Fix tail call prologue alignment

[Tiezhu Yang]

Currently, the LoongArch BPF JIT assumes a fixed number of instructions to
skip when performing a tail call, but actually it does not account for the
difference between the main programs and the subprograms. The subprograms
do not initialize the Tail Call Count (TCC), leading to an incorrect jump
offset that skips functional instructions (like stack adjustment).

To fix this issue, add a NOP instruction in the subprogram prologue where
the TCC initialization would normally be. This ensures that both the main
programs and the subprograms have consistent prologue layouts, allowing
tail calls to always land on the correct instruction regardless of the
target program type.

Fixes: c0fcc955ff82 ("LoongArch: BPF: Fix the tailcall hierarchy")
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
 arch/loongarch/net/bpf_jit.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 24913dc7f4e8..aefe5fa1c584 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -149,12 +149,15 @@ static void build_prologue(struct jit_ctx *ctx)
 		emit_insn(ctx, nop);
 
 	/*
-	 * First instruction initializes the tail call count (TCC)
-	 * register to zero. On tail call we skip this instruction,
-	 * and the TCC is passed in REG_TCC from the caller.
+	 * Initialize or align the TCC register slot.
+	 * For main programs, TCC is zeroed. For subprograms, a nop is emitted
+	 * to keep the prologue size consistent, ensuring tail calls skip the
+	 * correct number of instructions.
 	 */
 	if (is_main_prog)
 		emit_insn(ctx, addid, REG_TCC, LOONGARCH_GPR_ZERO, 0);
+	else
+		emit_insn(ctx, nop);
 
 	emit_insn(ctx, addid, LOONGARCH_GPR_SP, LOONGARCH_GPR_SP, -stack_adjust);
 
-- 
2.42.0