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References: <20220902074706.2211252-1-floridsleeves@gmail.com> <20220906215918.3fe20cca@gandalf.local.home>
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From:   Li Zhong <floridsleeves@gmail.com>
Date:   Wed, 7 Sep 2022 01:40:08 -0700
Message-ID: <CAMEuxRqUsKeqtNu1+cwW0a5o_qt56+O7B3EE2sBzsAc9gKhnsQ@mail.gmail.com>
Subject: Re: [PATCH v1] arch/x86/kernel: check the return value of insn_decode_kernel()
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org, peterz@infradead.org,
        jpoimboe@kernel.org, jbaron@akamai.com, ardb@kernel.org,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com
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On Tue, Sep 6, 2022 at 6:58 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Fri,  2 Sep 2022 00:47:06 -0700
> Li Zhong <floridsleeves@gmail.com> wrote:
>
> > @@ -20,9 +20,10 @@
> >  int arch_jump_entry_size(struct jump_entry *entry)
> >  {
> >       struct insn insn = {};
> > +     int ret;
> >
> > -     insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
> > -     BUG_ON(insn.length != 2 && insn.length != 5);
> > +     ret = insn_decode_kernel(&insn, (void *)jump_entry_code(entry));
>
> It's highly unlikely that length will be 2 or 5 if ret is not zero (as it
> is initialized to zero going into this function).
>

In this case, I think maybe just BUG_ON(ret<0) is enough. However, the code
that decides the value of insn->length could be modified in the future. And
there are other variables insn->next_byte and insn->kaddr that are related to
insn->length, which could also be modified. So I guess we can preserve all the
assertion conditions to guarantee all assumptions are satisfied.

> > +     BUG_ON(ret < 0 || insn.length != 2 && insn.length != 5);
>
> In any case, you need parenthesis around the && condition.
>

Thanks. Will add this in v2 patch.

> -- Steve
>
>
> >
> >       return insn.length;