Re: [Qemu-devel] [PATCH 7/8] io: file descriptor not initialized in qio_channel_command_new_spawn()

[Eric Blake]

On 08/31/2018 10:36 AM, Liam Merwick wrote:
> On 30/08/2018 17:18, Eric Blake wrote:
> > On 08/30/2018 10:47 AM, Liam Merwick wrote:
> > > Incorrect checking of flags could result in uninitialized
> > > file descriptor being used.

> Looking at it again, the very minor optimisation of converting the 2nd 
> 'if' to an 'else if' has the useful side-effect of appeasing the static 
> analysis tool.

I never figured out what the tool precisely thought was wrong in the 
first place. Can you paste the output of the tool to see exactly what it 
analyzed as the potential flaw?  Perhaps the analyzer was trying to see 
what would happen if a caller submitting the fourth value (3 on systems 
where O_RDONLY is 0, 0 on systems where O_RDONLY is 1) and the code not 
behaving in that setup, even though we know that all callers only submit 
the three valid values and never the fourth invalid value?  Or maybe 
it's a weakness where we have made dependent assumptions but in 
independent branches, in such a way that we know it will never fail but 
the analyzer doesn't?

> --- a/io/channel-command.c
> +++ b/io/channel-command.c
> @@ -61,8 +61,7 @@ qio_channel_command_new_spawn(const char *const argv[],
>
>       if (flags == O_RDONLY) {
>           stdinnull = true;
> -    }
> -    if (flags == O_WRONLY) {
> +    } else if (flags == O_WRONLY) {

But this sort of change is acceptable, especially if it does shut up the 
analyzer, and done with a commit message mentioning that it is not a 
semantic change but does make it easier to use the static checker to 
find real problems by getting rid of noise.

>           stdoutnull = true;
>       }
>
> Regards,
> Liam

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