From: Nick Owens <mischief@9.offblast.org>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Subject: Re: [9fans] 8l -e
Date: Tue, 27 May 2014 14:30:18 -0700 [thread overview]
Message-ID: <20140527213018.GQ3763@iota.offblast.org> (raw)
In-Reply-To: <0e05f8946ce2cff4a4ad7d2e044b984d@ladd.quanstro.net>
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On Tue, May 27, 2014 at 04:16:24PM -0400, erik quanstrom wrote:
> > you can see there is a JMP over _tracein and a RET before _traceout.
> > what gives?
>
> ah, that's the magic! the idea is to be able to enable and disable these tracepoints
> at runtime in a multiprocessor environment without any locking.
>
> - erik
>
ok. i'm beginning to understand better. is there a specific use case,
such as the kernel or userland?
i didn't see anything like a tool that could poke nops into the right
places. i started to write an acid function to put the nops in one
named function, and then i realized that the ret can appear several
times in one function and i would need to search for and patch them
out. but only the *first* ret, not second, e.g.:
setvar+0x3a 0x0000adf5 RET <--- should be NOP
setvar+0x3b 0x0000adf6 CALL _traceout(SB)
setvar+0x40 0x0000adfb RET <--- should not be NOP
i was able to patch the JMP, and the RET but only in the case where it
appears at the bottom of a function.
defn traceon(fn){
bound = fnbound(fn);
// nop first jmp
*(bound[0]) = 0x90\b;
*(bound[0]+1) = 0x90\b;
// and the ret
// XXX should search for ret
*(bound[1]-7) = 0x90\b;
}
maybe these were not the droids i was looking for.
my real goal is to make timing statistics for function calls in a program.
perhaps this goal is better fulfilled simply by prof!
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next prev parent reply other threads:[~2014-05-27 21:30 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-27 20:10 Nick Owens
2014-05-27 20:16 ` erik quanstrom
2014-05-27 21:30 ` Nick Owens [this message]
2014-05-27 21:33 ` erik quanstrom
2015-07-08 2:43 ` michaelian ennis
2015-07-08 2:44 ` michaelian ennis
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