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Re: [Qemu-devel] [PATCH] nbd/server: Silence gcc false positive

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH] nbd/server: Silence gcc false positive
Date: Fri, 22 Jun 2018 10:26:42 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

On 06/22/2018 07:58 AM, Eric Blake wrote:
The code has a while() loop that always initialized 'end', and
the loop always executes at least once (as evidenced by the assert()
just prior to the loop).  But some versions of gcc still complain
that 'end' is used uninitialized, so silence them.

Signed-off-by: Eric Blake <address@hidden>

Peter, do you want to apply this directly as a build fix, or shall
I submit a pull request?

Phooey. I'm obviously under a lot of stress. In my rush to fix the build problem, I introduced a logic error that breaks NBD dirty bitmaps. :(

  nbd/server.c | 3 ++-
  1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/nbd/server.c b/nbd/server.c
index 274604609f4..50ac8bfafc6 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1937,7 +1937,7 @@ static unsigned int bitmap_to_extents(BdrvDirtyBitmap 
*bitmap, uint64_t offset,
                                        unsigned int nb_extents,
                                        bool dont_fragment)
-    uint64_t begin = offset, end;
+    uint64_t begin = offset, end = offset;
      uint64_t overall_end = offset + *length;
      unsigned int i = 0;
      BdrvDirtyBitmapIter *it;
@@ -1977,6 +1977,7 @@ static unsigned int bitmap_to_extents(BdrvDirtyBitmap 
*bitmap, uint64_t offset,


+    assert(offset > end);
      *length = end - offset;

The assertion is backwards; that should be 'offset < end'. :(

Sending another fix. And putting on the brown paper bag of shame.

And bummer that neither 'make check' nor iotests exposes the breakage. My RFC hack for adding x-block-status to the client would be useful for that purpose.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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