selftests/bpf: #define LOCAL_LABEL_LEN for jit_disasm_helpers.c

Extract local label length as a #define directive and
elaborate why 'i % MAX_LOCAL_LABELS' expression is needed
for local labels array initialization.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240823080644.263943-4-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/tools/testing/selftests/bpf/jit_disasm_helpers.c b/tools/testing/selftests/bpf/jit_disasm_helpers.c
index 1b0f1fd267c050e8cebe5ae6f8f94db84838c2cf..febd6b12e372db9ee9aca985e81a8605e7d8fdf3 100644 (file)
--- a/tools/testing/selftests/bpf/jit_disasm_helpers.c
+++ b/tools/testing/selftests/bpf/jit_disasm_helpers.c
@@ -16,6 +16,11 @@
  */
 #define MAX_LOCAL_LABELS 32
 
+/* Local labels are encoded as 'L42', this requires 4 bytes of storage:
+ * 3 characters + zero byte
+ */
+#define LOCAL_LABEL_LEN 4
+
 static bool llvm_initialized;
 
 struct local_labels {
@@ -23,7 +28,7 @@ struct local_labels {
        __u32 prog_len;
        __u32 cnt;
        __u32 pcs[MAX_LOCAL_LABELS];
-       char names[MAX_LOCAL_LABELS][4];
+       char names[MAX_LOCAL_LABELS][LOCAL_LABEL_LEN];
 };
 
 static const char *lookup_symbol(void *data, uint64_t ref_value, uint64_t *ref_type,
@@ -118,8 +123,14 @@ static int disasm_one_func(FILE *text_out, uint8_t *image, __u32 len)
        }
        qsort(labels.pcs, labels.cnt, sizeof(*labels.pcs), cmp_u32);
        for (i = 0; i < labels.cnt; ++i)
-               /* use (i % 100) to avoid format truncation warning */
-               snprintf(labels.names[i], sizeof(labels.names[i]), "L%d", i % 100);
+               /* gcc is unable to infer upper bound for labels.cnt and assumes
+                * it to be U32_MAX. U32_MAX takes 10 decimal digits.
+                * snprintf below prints into labels.names[*],
+                * which has space only for two digits and a letter.
+                * To avoid truncation warning use (i % MAX_LOCAL_LABELS),
+                * which informs gcc about printed value upper bound.
+                */
+               snprintf(labels.names[i], sizeof(labels.names[i]), "L%d", i % MAX_LOCAL_LABELS);
 
        /* now print with labels */
        labels.print_phase = true;