From: Andy Lutomirski <luto@amacapital.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: 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>,
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>,
Ard Biesheuvel <ard.biesheuvel@linaro.org>,
Andrew Lutomirski <luto@kernel.org>
Subject: Re: [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function"
Date: Mon, 8 Oct 2018 10:25:51 -0700 [thread overview]
Message-ID: <CALCETrWuGh=1n2bEE9B0eeyzvnaD4y_Sx6MB+r9B5aFXzg8qBw@mail.gmail.com> (raw)
In-Reply-To: <20181008163953.GD5663@hirez.programming.kicks-ass.net>
On Mon, Oct 8, 2018 at 9:40 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> 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> wrote:
> > >
> > > 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’t give the address
> > >> of the beginning of the instruction.
> > >
> > > But that's like 1 byte before the operand, right? We could even double 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 location. Or am
> > > I missing something?
> >
> > There’s also JMP and Jcc, any of which can be used for rail calls, 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’re all one
> > byte.
>
> Oh, I had not considered tail calls..
>
> > But there is a nasty potential special case: anything that takes the
> > function’s 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.) 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 = 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?
next prev parent reply other threads:[~2018-10-08 17:26 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-06 1:51 [POC][RFC][PATCH 0/2] PROOF OF CONCEPT: Dynamic Functions (jump functions) Steven Rostedt
2018-10-06 1:51 ` [POC][RFC][PATCH 1/2] jump_function: Addition of new feature "jump_function" Steven Rostedt
2018-10-06 2:00 ` Steven Rostedt
2018-10-06 2:02 ` Steven Rostedt
2018-10-06 2:03 ` Steven Rostedt
2018-10-06 15:15 ` Steven Rostedt
2018-10-06 12:12 ` Peter Zijlstra
2018-10-06 13:39 ` Steven Rostedt
2018-10-06 15:13 ` Andy Lutomirski
2018-10-06 15:16 ` Steven Rostedt
2018-10-08 7:21 ` Peter Zijlstra
2018-10-08 8:33 ` Andy Lutomirski
2018-10-08 15:57 ` Peter Zijlstra
2018-10-08 16:29 ` Andy Lutomirski
2018-10-08 16:39 ` Steven Rostedt
2018-10-08 16:39 ` Peter Zijlstra
2018-10-08 17:25 ` Andy Lutomirski [this message]
2018-10-08 17:30 ` Ard Biesheuvel
2018-10-08 17:42 ` Andy Lutomirski
2018-10-08 17:44 ` Jiri Kosina
2018-10-08 17:45 ` Ard Biesheuvel
2018-10-08 17:47 ` Andy Lutomirski
2018-10-09 2:17 ` Josh Poimboeuf
2018-10-09 3:57 ` Steven Rostedt
2018-10-10 17:52 ` Josh Poimboeuf
2018-10-10 18:03 ` Andy Lutomirski
2018-10-10 18:16 ` Josh Poimboeuf
2018-10-10 18:17 ` Josh Poimboeuf
2018-10-10 21:13 ` Andy Lutomirski
2018-10-11 3:07 ` Josh Poimboeuf
2018-10-11 12:52 ` Josh Poimboeuf
2018-10-11 16:20 ` Andy Lutomirski
2018-10-10 18:33 ` Josh Poimboeuf
2018-10-10 18:56 ` Steven Rostedt
2018-10-10 20:16 ` Josh Poimboeuf
2018-10-10 20:57 ` Andy Lutomirski
2018-10-08 16:31 ` Steven Rostedt
2018-10-08 11:30 ` Ard Biesheuvel
2018-10-09 3:44 ` Masami Hiramatsu
2018-10-09 3:55 ` Steven Rostedt
2018-10-09 16:04 ` Masami Hiramatsu
2018-10-09 8:59 ` David Laight
2018-10-06 1:51 ` [POC][RFC][PATCH 2/2] tracepoints: Implement it with dynamic functions Steven Rostedt
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