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Re: [PATCH 1/1] x86: fix text_poke
- From: Jeremy Fitzhardinge <jeremy at goop dot org>
- To: Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>
- Cc: Linus Torvalds <torvalds at linux-foundation dot org>, "H. Peter Anvin" <hpa at zytor dot com>, Andi Kleen <andi at firstfloor dot org>, Ingo Molnar <mingo at elte dot hu>, Jiri Slaby <jirislaby at gmail dot com>, David Miller <davem at davemloft dot net>, zdenek dot kabelac at gmail dot com, rjw at sisk dot pl, paulmck at linux dot vnet dot ibm dot com, akpm at linux-foundation dot org, linux-ext4 at vger dot kernel dot org, herbert at gondor dot apana dot org dot au, penberg at cs dot helsinki dot fi, clameter at sgi dot com, linux-kernel at vger dot kernel dot org, pageexec at freemail dot hu, "Frank Ch. Eigler" <fche at redhat dot com>, systemtap at sources dot redhat dot com
- Date: Fri, 25 Apr 2008 16:13:59 -0700
- Subject: Re: [PATCH 1/1] x86: fix text_poke
- References: <20080425163035.GE9503@Krystal> <481209F2.4050908@zytor.com> <20080425170929.GA16180@Krystal> <20080425183748.GB16180@Krystal> <48123C9B.9020306@zytor.com> <20080425203717.GB25950@Krystal> <481241DC.3070601@zytor.com> <alpine.LFD.1.10.0804251349510.2779@woody.linux-foundation.org> <20080425211205.GC25950@Krystal> <alpine.LFD.1.10.0804251456410.2779@woody.linux-foundation.org> <20080425230028.GC31226@Krystal>
Mathieu Desnoyers wrote:
This idea has been considered a few years ago at OLS in the tracing BOF
if I remember well. The results were this : First, there is no way to
guarantee that no code path, nor any return address from any function,
interrupt, sleeping thread, will return to the "old" version of the
function. Nor is it possible to determine when a quiescent state is
reached. Therefore, we couldn't see how we can do the teardown.
Does that matter? The new function is semantically identical to the old
one, and the old code will remain in place. If there's still users in
the old function it may take a while for them to get flushed out (and
won't be traced in the meantime), but you have to expect some missed
events if you're shoving any kind of dynamic marker into the code. The
main problem is if there's something still depending on the first 5
bytes of the function (most likely if there's a loop head somewhere near
the top of the function).
Updating the markers would mean you'd leave a trail of old versions
hanging around as modules, but that's not a huge cost...
The second point is dependency between execution flow and variables. If
we don't do a complete copy of the variables (which I don't see how we
can do atomically), we will have to share the variables between the old
and the new copies of the functions. However, some variables might
encode information about the execution flow of the program and depend on
the actual address at which the code is linked (function pointers for
instance). Stuff like "goto *addr" would also break.
Obviously you'd only pick up new callers of the function, which would
mean that they'd pick up the new versions of those function-local
things. Though you'd need to make sure that the new versions of the
function are using the old version's static variables...
Then dealing with multiple code patching infrastructures (kprobes,
alternatives, paravirt) would become hellish. If a kprobe is planted in
the original version of the function, we have to insert it in the new
version... and the teardown of the old function is still a problem.
The module machinery already deals with patching paravirt and
alternatives into loaded modules. Your bespoke module would get dealt
with like any other module.
J