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Re: [libvirt] [PATCH 3/4] qemu: add missing break in qemuDomainDeviceCalculatePCIConnectFlags



On Thu, Feb 23, 2017 at 10:46:14AM +0000, Daniel P. Berrange wrote:
On Thu, Feb 23, 2017 at 11:38:40AM +0100, Martin Kletzander wrote:
On Thu, Feb 23, 2017 at 09:48:48AM +0000, Daniel P. Berrange wrote:
> On Wed, Feb 22, 2017 at 09:19:15PM +0100, Martin Kletzander wrote:
> > On Wed, Feb 22, 2017 at 02:44:01PM -0500, Laine Stump wrote:
> > > On 02/22/2017 12:52 PM, Daniel P. Berrange wrote:
> > > > One of the conditions in qemuDomainDeviceCalculatePCIConnectFlags
> > > > was missing a break that could result it in falling through to
> > > > an incorrect codepath.
> > >
> > > Actually that's not true. Every codepath of the preceding case ends with
> > > a "return blah".  This is true for the entire function - every case of
> > > every switch in the function ends with "return blah". The entire purpose
> > > of the function is to determine the flags value, and there are no
> > > resources that need cleaning up before returning, so as soon as the
> > > value is determined, it immediately returns.
> > >
> > > I suppose it could be rewritten to change all of those into "ret = blah;
> > > break;", then "return ret;" at the end, but it seemed safer to return
> > > immediately than to trust that no new code would be added later in the
> > > function (and also it's more compact)
> > >
> > > I wonder if this is just a more extreme case of the logic in whatever
> > > check necessitated that I add an extra "return 0" at the very end of the
> > > function. (that happens even in gcc 6.x; at an earlier point when the
> > > function was simpler, that wasn't needed, but after some additions it
> > > started producing the "control reaches end of function that requires a
> > > return value" or whatever that warning is, and the only way to eliminate
> > > it was with the extra return 0.)
> > >
> > > If adding the break shuts up the warning, then I guess ACK, but it would
> > > probably be better if 1) gcc fixed their incorrect warning, or 2) we
> > > switched the entire function to use the less-compact "ret = blah;
> > > break;" style instead of returning directly, so there wasn't a single
> > > stray break sitting in the middle.
> > >
> >
> > I would say NACK since 1) is the correct option (at least for now),
> > there is no reason for adding more lines of code that don't make sense
> > just because of a compiler version that was not released yet, or does
> > not even have a release plan yet.
>
> GCC 7 *is* released - and has even had a bug fix release too, so ignoring
> this is not an option. In any case, as Eric mentions this is a genuine
> bug in our code since we can fall out of the inner switch if the input
> variable contains a value that doesn't map to an named enum value.
>

Where did you get the package/tarball?  I don't see anything in the
release page [1].  On the other hand, when I checked it yesterday, I
looked and the development timeline [2] and I thought it's 2016
apparently because when I see the dates now it makes sense that the
release should be around the corner.  Anyway, even if they did not
update the release page, on snapshot ftp [3] there is not even a release
candidate.

I didn't use any tarball - just what's in Fedora which is
gcc-7.0.1-0.9.fc26.x86_64

Fedora dist-git says the tarball is gcc-7.0.1-20170219.tar.bz2

Odd that its not on the download page though as that's a clearly a
release version number, not a git snapshot or pre-release version.


The timestamp indicates it's just a snapshot.  GCC doesn't do x.0
releases since gcc-5, I believe, so unless it's 7.1 it's not an official
release.

I remember others not being happy when we were doing workarounds for
packages that downstream distros just decided to package out of VCS or
snapshots.  I don't feel it's right either and I thought you're on that
side as well.  Anyway, if it really was released, I am OK with this
going in.

Regardless of whether its a release or pre-release, this is a clear
bug in the code that needs fixing - its just not a workaround for a
compiler. As such I've pushed this series.


As Laine said as well, there is no bug in the code.  All the codepaths
in that switch case end with return.  And it *is* clearly a bug in the
compiler as there are other cases (e.g. VIR_DOMAIN_CONTROLLER_TYPE_SCSI)
that behave the same, there is no break after it and the compiler is
clearly OK with all the other ones.

Regards,
Daniel
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