Re: [PATCH net-next] net: ethernet: rmnet: Restructure if checks to avoid uninitialized warning

[Nathan Chancellor]

Hi Subash,

On 6/3/2021 10:15 PM, subashab@xxxxxxxxxxxxxx wrote:
> On 2021-06-03 16:40, patchwork-bot+netdevbpf@xxxxxxxxxx wrote:
> > Hello:
> >
> > This patch was applied to netdev/net-next.git (refs/heads/master):
> >
> > On Thu,  3 Jun 2021 10:34:10 -0700 you wrote:
> > > Clang warns that proto in rmnet_map_v5_checksum_uplink_packet() might be
> > > used uninitialized:
> > >
> > > drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:283:14: warning:
> > > variable 'proto' is used uninitialized whenever 'if' condition is false
> > > [-Wsometimes-uninitialized]
> > >                 } else if (skb->protocol == htons(ETH_P_IPV6)) {
> > >                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:295:36: note:
> > > uninitialized use occurs here
> > >                 check = rmnet_map_get_csum_field(proto, trans);
> > >                                                  ^~~~~
> > > drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:283:10: note:
> > > remove the 'if' if its condition is always true
> > >                 } else if (skb->protocol == htons(ETH_P_IPV6)) {
> > >                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > > drivers/net/ethernet/qualcomm/rmnet/rmnet_map_data.c:270:11: note:
> > > initialize the variable 'proto' to silence this warning
> > >                 u8 proto;
> > >                         ^
> > >                          = '\0'
> > > 1 warning generated.
> > >
> > > [...]
> >
> > Here is the summary with links:
> >   - [net-next] net: ethernet: rmnet: Restructure if checks to avoid
> > uninitialized warning
> >     https://git.kernel.org/netdev/net-next/c/118de6106735
> >
> > You are awesome, thank you!
> > --
> > Deet-doot-dot, I am a bot.
> > https://korg.docs.kernel.org/patchwork/pwbot.html
>
> Hi Nathan
>
> Can you tell why CLANG detected this error.
> Does it require a bug fix.

As far as I understand it, clang does not remember the conditions of 
previous if statements when generating this warning. Basically:

void bar(int x)
{
}

int foo(int a, int b)
{
	int x;

	if (!a && !b)
		goto out;

	if (a)
		x = 1;
	else if (b)
		x = 2;

	bar(x);

out:
	return 0;
}

clang will warn that x is uninitialized when neither of the second if 
statement's conditions are true, even though we as humans know that is 
not possible due to the first if statement. I am guessing this has 
something to do with how clang generates its control flow graphs. While 
this is a false positive, I do not personally see this as a bug in the 
compiler. The code is more clear to both the compiler and humans if it 
is written as:

	if (a)
		x = 1;
	else if (b)
		x = 2;
	else
		goto out;

Cheers,
Nathan