Re: [PATCH 1/2] block/export: Free ignored Error

[Vladimir Sementsov-Ogievskiy]

22.04.2021 17:53, Max Reitz wrote:

When invoking block-export-add with some iothread and
fixed-iothread=false, and changing the node's iothread fails, the error
is supposed to be ignored.

However, it is still stored in *errp, which is wrong.  If a second error
occurs, the "*errp must be NULL" assertion in error_setv() fails:

   qemu-system-x86_64: ../util/error.c:59: error_setv: Assertion
   `*errp == NULL' failed.

So the error from bdrv_try_set_aio_context() must be freed when it is
ignored.

Fixes: f51d23c80af73c95e0ce703ad06a300f1b3d63ef
        ("block/export: add iothread and fixed-iothread options")
Signed-off-by: Max Reitz <mre...@redhat.com>
---
  block/export/export.c | 4 ++++
  1 file changed, 4 insertions(+)

diff --git a/block/export/export.c b/block/export/export.c
index fec7d9f738..ce5dd3e59b 100644
--- a/block/export/export.c
+++ b/block/export/export.c
@@ -68,6 +68,7 @@ static const BlockExportDriver 
*blk_exp_find_driver(BlockExportType type)

BlockExport *blk_exp_add(BlockExportOptions *export, Error **errp)

  {
+    ERRP_GUARD();
      bool fixed_iothread = export->has_fixed_iothread && 
export->fixed_iothread;
      const BlockExportDriver *drv;
      BlockExport *exp = NULL;
@@ -127,6 +128,9 @@ BlockExport *blk_exp_add(BlockExportOptions *export, Error 
**errp)
              ctx = new_ctx;
          } else if (fixed_iothread) {
              goto fail;
+        } else {
+            error_free(*errp);
+            *errp = NULL;
          }
      }

I don't think ERRP_GUARD is needed in this case: we don't need to handle errp 
somehow except for just free if it was set. So we can simply do:

} else if (errp) {
   error_free(*errp);
   *errp = NULL;
}

Let's only check that errp is really set on failure path of 
bdrv_try_set_aio_context():

bdrv_try_set_aio_context() fails iff bdrv_can_set_aio_context() fails, which in 
turn may fail iff bdrv_parent_can_set_aio_context() or 
bdrv_child_can_set_aio_context() fails.

bdrv_parent_can_set_aio_context() has two failure path, on first it set errp by 
hand, and on second it has assertion that errp is set.

bdrv_child_can_set_aio_context() may fail only if nested call to 
bdrv_can_set_aio_context() fails, so recursion is closed.


--
Best regards,
Vladimir