From: Nathan Chancellor <nathan@kernel.org>
To: subashab@codeaurora.org, patchwork-bot+netdevbpf@kernel.org
Cc: stranche@codeaurora.org, davem@davemloft.net, kuba@kernel.org,
ndesaulniers@google.com, sharathv@codeaurora.org,
netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
clang-built-linux@googlegroups.com
Subject: Re: [PATCH net-next] net: ethernet: rmnet: Restructure if checks to avoid uninitialized warning
Date: Fri, 4 Jun 2021 09:54:02 -0700 [thread overview]
Message-ID: <2145e27f-c8b3-ef4b-793a-841cb2f7e60f@kernel.org> (raw)
In-Reply-To: <1f6f8246f0cd477c0b1e2b88b4ec825a@codeaurora.org>
Hi Subash,
On 6/3/2021 10:15 PM, subashab@codeaurora.org wrote:
> On 2021-06-03 16:40, patchwork-bot+netdevbpf@kernel.org 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
prev parent reply other threads:[~2021-06-04 16:54 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-03 17:34 [PATCH net-next] net: ethernet: rmnet: Restructure if checks to avoid uninitialized warning Nathan Chancellor
2021-06-03 22:40 ` patchwork-bot+netdevbpf
2021-06-04 5:15 ` subashab
2021-06-04 16:54 ` Nathan Chancellor [this message]
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