[PATCH] LoongArch: BPF: Fix off by one error in build_prologue()

[Hengqi Chen <hengqi.chen@gmail.com>]

Vincent reported that running BPF progs with tailcalls on LoongArch
causes kernel hard lockup. Debugging the issues shows that the JITed
image missing a jirl instruction at the end of the epilogue.

There are two passes in JIT compile, the first pass set the flags and
the second pass generates JIT code based on those flags. With BPF progs
mixing bpf2bpf and tailcalls, build_prologue() generates N insns in the
first pass and then generates N+1 insns in the second pass. This makes
epilogue_offset off by one and we will jump to some unexpected insn and
cause lockup. Fix this by inserting a nop insn.

Fixes: 5dc615520c4d ("LoongArch: Add BPF JIT support")
Fixes: bb035ef0cc91 ("LoongArch: BPF: Support mixing bpf2bpf and tailcalls")
Reported-by: Vincent Li <vincent.mc.li@gmail.com>
Closes: https://lore.kernel.org/loongarch/CAK3+h2w6WESdBN3UCr3WKHByD7D6Q_Ve1EDAjotVrnx6Or_c8g@mail.gmail.com/
Closes: https://lore.kernel.org/bpf/CAK3+h2woEjG_N=-XzqEGaAeCmgu2eTCUc7p6bP4u8Q+DFHm-7g@mail.gmail.com/
Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
---
 arch/loongarch/include/asm/inst.h | 5 +++++
 arch/loongarch/net/bpf_jit.c      | 2 ++
 2 files changed, 7 insertions(+)

diff --git a/arch/loongarch/include/asm/inst.h b/arch/loongarch/include/asm/inst.h
index 3089785ca97e..d61b356170fe 100644
--- a/arch/loongarch/include/asm/inst.h
+++ b/arch/loongarch/include/asm/inst.h
@@ -695,6 +695,11 @@ static inline void emit_jirl(union loongarch_instruction *insn,
 	insn->reg2i16_format.rj = rj;
 }
 
+static inline void emit_nop(union loongarch_instruction *insn)
+{
+	insn->word = INSN_NOP;
+}
+
 #define DEF_EMIT_REG2BSTRD_FORMAT(NAME, OP)				\
 static inline void emit_##NAME(union loongarch_instruction *insn,	\
 			       enum loongarch_gpr rd,			\
diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index ea357a3edc09..2346c0b55043 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -142,6 +142,8 @@ static void build_prologue(struct jit_ctx *ctx)
 	 */
 	if (seen_tail_call(ctx) && seen_call(ctx))
 		move_reg(ctx, TCC_SAVED, REG_TCC);
+	else
+		emit_insn(ctx, nop);
 
 	ctx->stack_size = stack_adjust;
 }
-- 
2.43.5