bpf: x86: fix epilogue generation for eBPF programs

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.
Fix eBPF JIT to emit epilogue when first BPF_EXIT is seen
and all other BPF_EXIT instructions will be emitted as jump.

Since jump offset to epilogue is computed as:
jmp_offset = ctx->cleanup_addr - addrs[i]
we need to change type of cleanup_addr to signed to compute the offset as:
(long long) ((int)20 - (int)30)
instead of:
(long long) ((unsigned int)20 - (int)30)

Fixes: 622582786c9e ("net: filter: x86: internal BPF JIT")
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 2 deletions

diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
index 3f627345d51c..7e90244c84e3 100644
--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -178,7 +178,7 @@ static void jit_fill_hole(void *area, unsigned int size)
 }
 
 struct jit_context {
-        unsigned int cleanup_addr; /* epilogue code offset */
+        int cleanup_addr; /* epilogue code offset */
         bool seen_ld_abs;
 };
 
@@ -192,6 +192,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
         struct bpf_insn *insn = bpf_prog->insnsi;
         int insn_cnt = bpf_prog->len;
         bool seen_ld_abs = ctx->seen_ld_abs | (oldproglen == 0);
+        bool seen_exit = false;
         u8 temp[BPF_MAX_INSN_SIZE + BPF_INSN_SAFETY];
         int i;
         int proglen = 0;
@@ -854,10 +855,11 @@ common_load:
                         goto common_load;
 
                 case BPF_JMP | BPF_EXIT:
-                        if (i != insn_cnt - 1) {
+                        if (seen_exit) {
                                 jmp_offset = ctx->cleanup_addr - addrs[i];
                                 goto emit_jmp;
                         }
+                        seen_exit = true;
                         /* update cleanup_addr */
                         ctx->cleanup_addr = proglen;
                         /* mov rbx, qword ptr [rbp-X] */