linux-kernel - Re: [GIT PULL] tracing: Fix double free bug

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Message-ID: <CAC_TJvcFkcsFBmAr_nsNOODcjX_wuKKsBgbSKFc5AwAyByufdQ@mail.gmail.com>
Date:   Wed, 17 Nov 2021 16:17:33 -0800
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [GIT PULL] tracing: Fix double free bug

On Wed, Nov 17, 2021 at 4:00 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Wed, 17 Nov 2021 15:38:59 -0800
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
> > On Wed, Nov 17, 2021 at 3:19 PM Steven Rostedt <rostedt@...dmis.org> wrote:
> > >
> > > On error, the operands and the histogram expression are destroyed,
> > > but since the destruction is recursive, do not destroy the operands
> > > if they already belong to the expression that is about to be destroyed.
> >
> > Honestly, this seems horribly ugly.
>
> I guess we have a difference in opinion to what is ugly, as the v1 version
> of Kalesh's patch was closer to yours, and I hated the complexity of having
> to know when to to call what. Because the logic is not that simple.
>
> See https://lore.kernel.org/all/20211117021223.2137117-1-kaleshsingh@google.com/
>
> >
> > The problem seems to be that the "goto error" cases are simply just wrong.
> >
> > Why isn't the fix to make the error cases be the right ones, instead
> > of having one odd error case that then has to do some magic things to
> > not free the wrong things?
> >
> > The patch ends up a bit bigger, mainly because I renamed the different
> > "free" cases, and because I made the non-freeing ones just return the
> > error directly.
>
> I agree with the first part of your patch, which was not the reason for
> being the cause of the bug.
>
> >
> > Something like this (UNTESTED!) patch, IOW?
>
> But the part after the expr is allocated gets a bit more tricky, and that
> is why I requested that logic, namely due to the "combine_consts" case. But
> as the expr->operand[]s are NULL'd and the operand*s are destroyed, I guess
> it's still fine with just freeing the expr if we add an error case there.
>
> Kalesh, care to spin a v3 implementing Linus's solution?

Hi Steve,

Yes I'll resend a new version, more in line to Linus suggestion.

Thanks,
Kalesh
>
> -- Steve