linux-kernel - Re: [GIT PULL] usercopy structs for v5.4-rc2

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Date:   Fri, 4 Oct 2019 12:43:30 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] usercopy structs for v5.4-rc2

On Fri, Oct 04, 2019 at 10:53:41AM -0700, Linus Torvalds wrote:
> On Fri, Oct 4, 2019 at 3:42 AM Christian Brauner
> <christian.brauner@...ntu.com> wrote:
> >
> >            The only separate fix we we had to apply
> > was for a warning by clang when building the tests for using the result of
> > an assignment as a condition without parantheses.
> 
> Hmm. That code is ugly, both before and after the fix.
> 
> This just doesn't make sense for so many reasons:
> 
>         if ((ret |= test(umem_src == NULL, "kmalloc failed")))
> 
> where the insanity comes from
> 
>  - why "|=" when you know that "ret" was zero before (and it had to
> be, for the test to make sense)
> 
>  - why do this as a single line anyway?
> 
>  - don't do the stupid "double parenthesis" to hide a warning. Make it
> use an actual comparison if you add a layer of parentheses.
> 
> So
> 
>         if ((x = y))
> 
> is *wrong*. I know the compiler suggests that, but the compiler is
> just being stupid, and the suggestion comes from people who don't have
> any taste.
> 
> If you want to test an assignment, you should just use
> 
>         if ((x = y) != 0)
> 
> instead, at which point it's not syntactic noise mind-games any more,
> but the parenthesis actually make sense.
> 
> However, you had no reason to use an assignment in the conditional in
> the first place.
> 
> IOW, the code should have just been
> 
>         ret = test(umem_src == NULL, "kmalloc failed");
>         if (ret) ...

Yes, I had this as the original fix but I tried to keep the same
intention as the original author. I should have gone with my gut. Sorry
for the ugliness, I'll try to be better in the future.

Cheers,
Nathan