qemu-img: Fix overwriting 'ret' before using

This patch moves ret assignment after reporting original error.

We were lucky to pass qemu-iotests 048 (qemu-img compare case) but when
I tried to run with TEST_DIR=/tmp (tmpfs), it fails with a "wrong"
mismatch offset. This fixes two bugs.

In the first if branch, setting ret to 1 before using it makes dead code
in the next line: pnum is never added to mismatch offset even if ret was
0.

In the other if branch, currently the output error is always -4:
  strerror(-4) -> Unknown error -4

Added regression test in case 048.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/qemu-img.c b/qemu-img.c
index bf3fb4f8a6..b6b5644cb6 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1020,10 +1020,10 @@ static int img_compare(int argc, char **argv)
                 }
                 ret = compare_sectors(buf1, buf2, nb_sectors, &pnum);
                 if (ret || pnum != nb_sectors) {
-                    ret = 1;
                     qprintf(quiet, "Content mismatch at offset %" PRId64 "!\n",
                             sectors_to_bytes(
                                 ret ? sector_num : sector_num + pnum));
+                    ret = 1;
                     goto out;
                 }
             }
@@ -1045,9 +1045,9 @@ static int img_compare(int argc, char **argv)
             }
             if (ret) {
                 if (ret < 0) {
-                    ret = 4;
                     error_report("Error while reading offset %" PRId64 ": %s",
                                  sectors_to_bytes(sector_num), strerror(-ret));
+                    ret = 4;
                 }
                 goto out;
             }
@@ -1092,10 +1092,10 @@ static int img_compare(int argc, char **argv)
                                           filename_over, buf1, quiet);
                 if (ret) {
                     if (ret < 0) {
-                        ret = 4;
                         error_report("Error while reading offset %" PRId64
                                      " of %s: %s", sectors_to_bytes(sector_num),
                                      filename_over, strerror(-ret));
+                        ret = 4;
                     }
                     goto out;
                 }