arm64: bpf: lift restriction on last instruction

Earlier implementation assumed last instruction is BPF_EXIT.
Since this is no longer a restriction in eBPF, we remove this
limitation.

Per Alexei Starovoitov [1]:
> classic BPF has a restriction that last insn is always BPF_RET.
> eBPF doesn't have BPF_RET instruction and this restriction.
> It has BPF_EXIT insn which can appear anywhere in the program
> one or more times and it doesn't have to be last insn.

[1] https://lkml.org/lkml/2014/11/27/2

Fixes: e54bcde3d69d ("arm64: eBPF JIT compiler")
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: Zi Shen Lim <zlim.lnx@gmail.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 41f1e3e2ea24..edba042b2325 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -60,7 +60,7 @@ struct jit_ctx {
         const struct bpf_prog *prog;
         int idx;
         int tmp_used;
-        int body_offset;
+        int epilogue_offset;
         int *offset;
         u32 *image;
 };
@@ -130,8 +130,8 @@ static void jit_fill_hole(void *area, unsigned int size)
 
 static inline int epilogue_offset(const struct jit_ctx *ctx)
 {
-        int to = ctx->offset[ctx->prog->len - 1];
-        int from = ctx->idx - ctx->body_offset;
+        int to = ctx->epilogue_offset;
+        int from = ctx->idx;
 
         return to - from;
 }
@@ -463,6 +463,8 @@ emit_cond_jmp:
         }
         /* function return */
         case BPF_JMP | BPF_EXIT:
+                /* Optimization: when last instruction is EXIT,
+                   simply fallthrough to epilogue. */
                 if (i == ctx->prog->len - 1)
                         break;
                 jmp_offset = epilogue_offset(ctx);
@@ -685,11 +687,13 @@ void bpf_int_jit_compile(struct bpf_prog *prog)
 
         /* 1. Initial fake pass to compute ctx->idx. */
 
-        /* Fake pass to fill in ctx->offset. */
+        /* Fake pass to fill in ctx->offset and ctx->tmp_used. */
         if (build_body(&ctx))
                 goto out;
 
         build_prologue(&ctx);
+
+        ctx.epilogue_offset = ctx.idx;
         build_epilogue(&ctx);
 
         /* Now we know the actual image size. */
@@ -706,7 +710,6 @@ void bpf_int_jit_compile(struct bpf_prog *prog)
 
         build_prologue(&ctx);
 
-        ctx.body_offset = ctx.idx;
         if (build_body(&ctx)) {
                 bpf_jit_binary_free(header);
                 goto out;