On 08/31/2018 11:19 AM, Liam Merwick wrote:
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?
Thanks.
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?
The specific error it reported was
Error: File Invalid
File Descriptor not Initialized [file-desc-not-init]:
The value <unknown> is not initialized as a file descriptor.
at line 91 of io/channel-command.c in function
'qio_channel_command_new_spawn'.
resource not initialized when flags != 0 at line 62
flags is not O_RDONLY,
and flags != 1 at line 65
flags is not O_WRONLY,
and stdinnull is false at line 69
and stdoutnull is false at line 69.
so yes, both stdinnull and stdoutnull will be false at this point.
Based on our earlier masking, that means flags is either O_RDWR (2) or
bogus (3). The analyzer is then complaining about:
dup2(stdinnull ? devnull : stdinfd[0], STDIN_FILENO);
So either it is complaining about devnull being uninitialized (which it
is when flags is O_RDWR - but in that situation, we know stdinnull is
false which also implies that the left half of ?: is unreachable and
therefore shouldn't matter) or it is complaining about stdinfd[0] being
uninitialized (but we can't reach here without having gone through the
earlier (!stdinnull && pipe(stdinfd)), and again, given that stdinnull
is false, that is initialized). The fact that it lists '<unknown>'
rather than the actual (sub-)expression that it claims is uninitialized
doesn't help. So yeah, I'm seriously confused as to why this false
positive is being reported, or why the change from 'if/if' to 'if/else
if' shuts it up.
I've been staring at the code and can see no reason why it isn't a false
positive (and I'll let the tool authors know)
Thanks for the followup.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
