tools, bpf_asm: Warn when jumps are out of range

When compiling larger programs with bpf_asm, it's possible to
accidentally exceed jt/jf range, in which case it won't complain, but
rather silently emit a truncated offset, leading to a "happy debugging"
situation.

Add a warning to help detecting such issues. It could be made an error
instead, but this might break compilation of existing code (which might
be working by accident).

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191107100349.88976-1-iii@linux.ibm.com

diff --git a/tools/bpf/bpf_exp.y b/tools/bpf/bpf_exp.y
index 56ba1de..8d48e89 100644 (file)
--- a/tools/bpf/bpf_exp.y
+++ b/tools/bpf/bpf_exp.y
@@ -545,6 +545,16 @@ static void bpf_reduce_k_jumps(void)
        }
 }
 
+static uint8_t bpf_encode_jt_jf_offset(int off, int i)
+{
+       int delta = off - i - 1;
+
+       if (delta < 0 || delta > 255)
+               fprintf(stderr, "warning: insn #%d jumps to insn #%d, "
+                               "which is out of range\n", i, off);
+       return (uint8_t) delta;
+}
+
 static void bpf_reduce_jt_jumps(void)
 {
        int i;
@@ -552,7 +562,7 @@ static void bpf_reduce_jt_jumps(void)
        for (i = 0; i < curr_instr; i++) {
                if (labels_jt[i]) {
                        int off = bpf_find_insns_offset(labels_jt[i]);
-                       out[i].jt = (uint8_t) (off - i -1);
+                       out[i].jt = bpf_encode_jt_jf_offset(off, i);
                }
        }
 }
@@ -564,7 +574,7 @@ static void bpf_reduce_jf_jumps(void)
        for (i = 0; i < curr_instr; i++) {
                if (labels_jf[i]) {
                        int off = bpf_find_insns_offset(labels_jf[i]);
-                       out[i].jf = (uint8_t) (off - i - 1);
+                       out[i].jf = bpf_encode_jt_jf_offset(off, i);
                }
        }
 }