[PATCHv3 3/3] lxc: fix a memory leak

[Shi Lei]

On 2020-12-18 at 22:01, John Ferlan wrote:

>

>Coverity reminds us of the ancient software engineering proverb related

>to being stuck with ownership because you touched the code last :-) - I

>know this patch didn't cause the problem, but because the code was

>touched Coverity decided to look harder and found another leak.

>

>On 12/16/20 1:01 AM, Shi Lei wrote:

>> In virLXCProcessSetupInterfaceTap, containerVeth needs to be freed on

>> failure.

>> 

>> Signed-off-by: Shi Lei <shi_lei at massclouds.com>

>> ---

>>  src/lxc/lxc_process.c | 4 ++--

>>  1 file changed, 2 insertions(+), 2 deletions(-)

>> 
>> diff --git a/src/lxc/lxc_process.c b/src/lxc/lxc_process.c

>> index 85d0287a..0f7c9295 100644

>> --- a/src/lxc/lxc_process.c

>> +++ b/src/lxc/lxc_process.c

>> @@ -303,7 +303,7 @@ virLXCProcessSetupInterfaceTap(virDomainDefPtr vm,

>>                                 const char *brname)

>>  {

>>      char *parentVeth;

>

>Coverity complains that @parentVeth is leaked too - although it's far

>more opaque and ornery.

>

>Let's assume on input that net->ifname != NULL - that means that in

>virNetDevVethCreate the call to virNetDevGenerateName and the memory in

>@**ifname (e.g. @parentVeth's copy of net->ifname) will be g_free()'d

>when !virNetDevExists(try) succeeds and @*ifname gets the memory from

>@try. In this case @try is a g_strdup_printf of @*ifname. So this code

>essentially free's memory pointed to by net->ifname, but since

>@parentVeth is used in the call net->ifname is *NOT* updated which is a

>second problem that Coverity did not note.

>

>Then let's say the call to virNetDevGenerateName fails for @veth2...

>Since on input @orig1 != NULL (e.g. @parentVeth != NULL), we will not

>VIR_FREE(*veth1); - that's fine given the original assumption, but when

>we return to virLXCProcessSetupInterfaceTap that means net->ifname will

>point at memory that was g_free'd and @parentVeth will not be free'd,

>thus Coverity squawks loud and proud about the resource leak - although

>it doesn't complain about the fact that net->ifname now points to memory

>that was free'd.

>

>As an aside, I see no way on input @*veth2 could not be NULL so in the

>cleanup path the check for @orig2 would seem to be dead code. Although,

>sure future changes could alter that reality.

>

>As a test if I replace all @parentVeth refs w/ net->ifname - then

>Coverity is happy again. I will leave it up to Laine or Shi Lei to

>generate the "real fix" and/or validate that my reading of the logic is

>right or not ;-).

>

>John

Thanks! :-)

As far as I can see, if the original net->ifname != NULL, it should be a user-provided
name and NOT a template (prefix+'%d'). When @parentVeth (as the copy of net->ifname)
is passed into virNetDevGenerateName, this function will leave it unchanged because it is
not a template.

So for now these problems will not happen. But for future, I think it's necessary to fix
virLXCProcessSetupInterfaceTap.

I agree that it's a right way to replace all *parentVeth* with net->ifname in
virLXCProcessSetupInterfaceTap. And the *parentVeth* should be removed.

Shi Lei