hdparm: reduce possibility of numeric overflow in -T.

do_time                                              386     410     +24
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/0 up/down: 24/0)               Total: 24 bytes

1 files changed, 14 insertions, 6 deletions

diff --git a/miscutils/hdparm.c b/miscutils/hdparm.c
index 8a7204f29..1f85644d6 100644
--- a/miscutils/hdparm.c
+++ b/miscutils/hdparm.c
@@ -1375,10 +1375,13 @@ static void print_timing(unsigned long m, unsigned elapsed_us)
         printf("%5lu MB in %u.%02u seconds = %lu kB/s\n",
                 m, sec, hs,
                 /* Trying to not overflow 32-bit arith in m * CONST
-                 * by keeping CONST not so big. + 1 prevents div-by-0. */
-                (m * (1024 * 1000000 / (64*1024))) / (elapsed_us / (64*1024) + 1)
-                // ~= (m * 1024 * 1000000) / elapsed_ms
-                // = (m * 1024) / (elapsed_ms / 1000000)
+                 * by keeping CONST not so big. But elapsed_us / CONST2 
+                 * also should not introduce big errors. Currently,
+                 * 16000us is ~1.6% of 1 second.
+                 * "+ 1" prevents div-by-0. */
+                (m * (1024 * 1000000 / (16*1024))) / (elapsed_us / (16*1024) + 1)
+                // ~= (m * 1024 * 1000000) / elapsed_us
+                // = (m * 1024) / (elapsed_us / 1000000)
                 // = kb / elapsed_sec
         );
 }
@@ -1418,6 +1421,11 @@ static void do_time(int cache /*,int fd*/)
 
         /* Now do the timing */
         iterations = 0;
+        /* Max time to run (small for cache, avoids getting
+         * huge total_MB which can overlow on print_timing) */
+        elapsed2 = 510000; /* cache */
+        if (!cache)
+                elapsed2 = 3000000; /* not cache */
         start = monotonic_us();
         do {
                 if (cache)
@@ -1425,11 +1433,11 @@ static void do_time(int cache /*,int fd*/)
                 read_big_block(buf);
                 elapsed = (unsigned)monotonic_us() - start;
                 ++iterations;
-        } while (elapsed < 3000000 && iterations < max_iterations);
+        } while (elapsed < elapsed2 && iterations < max_iterations);
         total_MB = (unsigned long)iterations * TIMING_BUF_MB;
         //printf(" elapsed:%u iterations:%u ", elapsed, iterations);
         if (cache) {
-                /* Now remove the lseek() and monotonic_us() overheads
+                /* Cache: remove lseek() and monotonic_us() overheads
                  * from elapsed */
                 start = monotonic_us();
                 do {