Re: [Libguestfs] [nbdkit PATCH v2] blocksize: Avoid losing aligned writes to RMW race

> Add a test that fails without the change to blocksize.c. With some > carefully timed setups (using the delay filter to stall writes > reaching the plugin, and the plugin itself to stall reads coming back > from the plugin, so that we are reasonably sure of the overall > timeline), we can demonstrate the bug present in the unpatched code, > where an aligned write is lost when it lands in the wrong part of an > unaligned RMW cycle; the fixed code instead shows that the overlapping > operations were serialized. We may need to further tweak the test to > be more reliable even under heavy load, but hopefully 2 seconds per > stall (where a successful test requires 18 seconds) is sufficient for > now. > > Reported-by: Nikolaus Rath <Nikolaus(a)rath.org&gt; > Fixes: 1aadbf9971 ("blocksize: Allow parallel requests", v1.25.3) > --- > > In v2: > - Implement the blocksize fix. > - Drop RFC; I think this is ready, other than determining if we want > to tag it with a CVE number. > - Improve the test: print out more details before assertions, to aid > in debugging if it ever dies under CI resources Apparently there was a bug in the zero callback too (which I had missed): -+ zero='dd if=/dev/zero skip=$4 count=$3 iflag=count_bytes,skip_bytes' \ ++ zero='dd if=/dev/zero of=$tmpdir/img skip=$4 count=$3 \ ++ iflag=count_bytes,skip_bytes' \

After comparing RFC against v2: if you can split the test case to a separate patch (= the second patch in a two-part series), then I'd be happy for you to carry my R-b forward. I don't feel confident enough to review the change to the blocksize filter itself. (My confidence is lacking in my knowledge of nbdkit, not in your patch.)