bpf, mips: Limit to 33 tail calls

All BPF JIT compilers except RISC-V's and MIPS' enforce a 33-tail calls
limit at runtime.  In addition, a test was recently added, in tailcalls2,
to check this limit.

This patch updates the tail call limit in MIPS' JIT compiler to allow
33 tail calls.

Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
Reported-by: Mahshid Khezri <khezri.mahshid@gmail.com>
Signed-off-by: Paul Chaignon <paul.chaignon@orange.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/b8eb2caac1c25453c539248e56ca22f74b5316af.1575916815.git.paul.chaignon@gmail.com

diff --git a/arch/mips/net/ebpf_jit.c b/arch/mips/net/ebpf_jit.c
index 46b76751f3a5fface8c7c2578487a3c043f9c258..3ec69d9cbe88e755e55425779b2c47c761c7a304 100644 (file)
--- a/arch/mips/net/ebpf_jit.c
+++ b/arch/mips/net/ebpf_jit.c
@@ -604,6 +604,7 @@ static void emit_const_to_reg(struct jit_ctx *ctx, int dst, u64 value)
 static int emit_bpf_tail_call(struct jit_ctx *ctx, int this_idx)
 {
        int off, b_off;
+       int tcc_reg;
 
        ctx->flags |= EBPF_SEEN_TC;
        /*
@@ -616,14 +617,14 @@ static int emit_bpf_tail_call(struct jit_ctx *ctx, int this_idx)
        b_off = b_imm(this_idx + 1, ctx);
        emit_instr(ctx, bne, MIPS_R_AT, MIPS_R_ZERO, b_off);
        /*
-        * if (--TCC < 0)
+        * if (TCC-- < 0)
         *     goto out;
         */
        /* Delay slot */
-       emit_instr(ctx, daddiu, MIPS_R_T5,
-                  (ctx->flags & EBPF_TCC_IN_V1) ? MIPS_R_V1 : MIPS_R_S4, -1);
+       tcc_reg = (ctx->flags & EBPF_TCC_IN_V1) ? MIPS_R_V1 : MIPS_R_S4;
+       emit_instr(ctx, daddiu, MIPS_R_T5, tcc_reg, -1);
        b_off = b_imm(this_idx + 1, ctx);
-       emit_instr(ctx, bltz, MIPS_R_T5, b_off);
+       emit_instr(ctx, bltz, tcc_reg, b_off);
        /*
         * prog = array->ptrs[index];
         * if (prog == NULL)