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From:   Ard Biesheuvel <ard.biesheuvel@linaro.org>
Date:   Mon, 8 Oct 2018 19:30:23 +0200
Message-ID: <CAKv+Gu-08i6QEahyYhBtCBmpOYNuxirgdsgGvRw+Y0DX3+DVNQ@mail.gmail.com>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Matthew Helsley <mhelsley@vmware.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        Andrew Lutomirski <luto@kernel.org>
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On 8 October 2018 at 19:25, Andy Lutomirski <luto@amacapital.net> wrote:
> On Mon, Oct 8, 2018 at 9:40 AM Peter Zijlstra <peterz@infradead.org> wrot=
e:
>>
>> On Mon, Oct 08, 2018 at 09:29:56AM -0700, Andy Lutomirski wrote:
>> >
>> >
>> > > On Oct 8, 2018, at 8:57 AM, Peter Zijlstra <peterz@infradead.org> wr=
ote:
>> > >
>> > > On Mon, Oct 08, 2018 at 01:33:14AM -0700, Andy Lutomirski wrote:
>> > >>> Can't we hijack the relocation records for these functions before =
they
>> > >>> get thrown out in the (final) link pass or something?
>> > >>
>> > >> I could be talking out my arse here, but I thought we could do this=
,
>> > >> too, then changed my mind.  The relocation records give us the
>> > >> location of the call or jump operand, but they don=E2=80=99t give t=
he address
>> > >> of the beginning of the instruction.
>> > >
>> > > But that's like 1 byte before the operand, right? We could even doub=
le check
>> > > this by reading back that byte and ensuring it is in fact 0xE8 (CALL=
).
>> > >
>> > > AFAICT there is only the _1_ CALL encoding, and that is the 5 byte: =
E8 <PLT32>,
>> > > so if we have the PLT32 location, we also have the instruction locat=
ion. Or am
>> > > I missing something?
>> >
>> > There=E2=80=99s also JMP and Jcc, any of which can be used for rail ca=
lls, but
>> > those are also one byte. I suppose GCC is unlikely to emit a prefixed
>> > form of any of these. So maybe we really can assume they=E2=80=99re al=
l one
>> > byte.
>>
>> Oh, I had not considered tail calls..
>>
>> > But there is a nasty potential special case: anything that takes the
>> > function=E2=80=99s address. This includes jump tables, computed gotos,=
 and
>> > plain old function pointers. And I suspect that any of these could
>> > have one of the rather large number of CALL/JMP/Jcc bytes before the
>> > relocation by coincidence.
>>
>> We can have objtool verify the CALL/JMP/Jcc only condition. So if
>> someone tries to take the address of a patchable function, it will error
>> out.
>
> I think we should just ignore the sites that take the address and
> maybe issue a warning.  After all, GCC can create them all by itself.
> We'll always have a plain wrapper function, and I think we should just
> not patch code that takes its address.  So we do, roughly:
>
> void default_foo(void);
>
> GLOBAL(foo)
>   jmp *current_foo(%rip)
> ENDPROC(foo)
>
> And code that does:
>
> foo();
>
> as a call, a tail call, a conditional tail call, etc, gets discovered
> by objtool + relocation processing or whatever and gets patched.  (And
> foo() itself gets patched, too, as a special case.  But we patch foo
> itself at some point during boot to turn it into a direct JMP.  Doing
> it this way means that the whole mechanism works from very early
> boot.)

Does that mean that architectures could opt out of doing the whole
objtool + relocation processing thing, and instead take the hit of
going through the trampoline for all calls?

> And anything awful like:
>
> switch(whatever) {
> case 0:
>   foo();
> };
>
> that gets translated to a jump table and gets optimized such that it
> jumps straight to foo just gets left alone, since it still works.
> It's just a bit suboptimial.  Similarly, code that does:
>
> void (*ptr)(void);
> ptr =3D foo;
>
> gets a bona fide pointer to foo(), and any calls through the pointer
> land on foo() and jump to the current selected foo with only a single
> indirect branch / retpoline.
>
> Does this seem reasonable?  Is there a reason we should make it more
> restrictive?