netdev - Re: [PATCH] mac80211: initialize variable have_higher_than

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Message-ID: <3db47d49-68fb-c286-b237-bfce1cb0ff08@redhat.com>
Date:   Wed, 29 Dec 2021 11:26:00 -0800
From:   Tom Rix <trix@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     johannes@...solutions.net, davem@...emloft.net, kuba@...nel.org,
        nathan@...nel.org, linville@...driver.com,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mac80211: initialize variable have_higher_than_11mbit


On 12/28/21 10:55 AM, Nick Desaulniers wrote:
> On Fri, Dec 24, 2021 at 6:01 AM Tom Rix <trix@...hat.com> wrote:
>>
>> On 12/23/21 12:30 PM, Nick Desaulniers wrote:
>>> On Thu, Dec 23, 2021 at 8:29 AM <trix@...hat.com> wrote:
>>>> From: Tom Rix <trix@...hat.com>
>>>>
>>>> Clang static analysis reports this warnings
>>>>
>>>> mlme.c:5332:7: warning: Branch condition evaluates to a
>>>>     garbage value
>>>>       have_higher_than_11mbit)
>>>>       ^~~~~~~~~~~~~~~~~~~~~~~
>>>>
>>>> have_higher_than_11mbit is only set to true some of the time in
>>>> ieee80211_get_rates() but is checked all of the time.  So
>>>> have_higher_than_11mbit needs to be initialized to false.
>>> LGTM. There's only one caller of ieee80211_get_rates() today; if there
>>> were others, they could make a similar mistake in the future. An
>>> alternate approach: ieee80211_get_rates() could unconditionally write
>>> false before the loop that could later write true. Then call sites
>>> don't need to worry about this conditional assignment. Perhaps that
>>> would be preferable? If not:
>> The have_higher_than_11mbit variable had previously be initialized to false.
>>
>> The commit 5d6a1b069b7f moved the variable without initializing.
> I'm not disagreeing with that.
>
> My point is that these sometimes uninitialized warnings you're
> finding+fixing with clang static analyzer are demonstrating a
> recurring pattern with code.
>
> When _not_ using the static analyzer, -Wuninitialized and
> -Wsometimes-uninitialized work in Clang by building a control flow
> graph, but they only analyze a function locally.
>
> For example, consider the following code:
> ```
> _Bool is_thursday(void);
> void hello(int);
>
> void init (int* x) {
>    if (is_thursday())
>      *x = 1;
> }
>
> void foo (void) {
>    int x;
>    init(&x);
>    hello(x);
> }
> ```
> (Clang+GCC today will warn on the above; x is considered to "escape"
> the scope of foo as init could write the address of x to a global.
> Instead clang's static analyzer will take the additional time to
> analyze the callee.  But here's a spooky question: what happens when
> init is in another translation unit? IIRC, the static analyzer doesn't
> do cross TU analysis; I could be wrong though, I haven't run it in a
> while.)
>
> My point is that you're sending patches initializing x, when I think
> it might be nicer to instead have functions like init always write a
> value (unconditionally, rather than conditionally).  That way other
> callers of init don't have to worry about sometimes initialized
> variables.

The variable is passed to only to the static function ieee80211_get_rates().

Tom

>
>> Tom
>>
>>> Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
>>>
>>>> Fixes: 5d6a1b069b7f ("mac80211: set basic rates earlier")
>>>> Signed-off-by: Tom Rix <trix@...hat.com>
>>>> ---
>>>>    net/mac80211/mlme.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/mac80211/mlme.c b/net/mac80211/mlme.c
>>>> index 51f55c4ee3c6e..766cbbc9c3a72 100644
>>>> --- a/net/mac80211/mlme.c
>>>> +++ b/net/mac80211/mlme.c
>>>> @@ -5279,7 +5279,7 @@ static int ieee80211_prep_connection(struct ieee80211_sub_if_data *sdata,
>>>>            */
>>>>           if (new_sta) {
>>>>                   u32 rates = 0, basic_rates = 0;
>>>> -               bool have_higher_than_11mbit;
>>>> +               bool have_higher_than_11mbit = false;
>>>>                   int min_rate = INT_MAX, min_rate_index = -1;
>>>>                   const struct cfg80211_bss_ies *ies;
>>>>                   int shift = ieee80211_vif_get_shift(&sdata->vif);
>>>> --
>>>> 2.26.3
>>>>
>