linux-kernel - Re: linux-next: add utrace tree

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Date:	Tue, 26 Jan 2010 10:00:15 -0500
From:	"Frank Ch. Eigler" <fche@...hat.com>
To:	Tom Tromey <tromey@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kyle Moffett <kyle@...fetthome.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Peter Zijlstra <peterz@...radead.org>,
	Fr??d??ric Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-next@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	utrace-devel@...hat.com, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: linux-next: add utrace tree

Hi -

On Mon, Jan 25, 2010 at 02:05:54PM -0700, Tom Tromey wrote:
> [...]
> Nevertheless, if the Linux kernel were to present a new user-space API,
> and if it had an advantage over ptrace, then we would port GDB to use
> it.  There are other platforms where, IIRC, we now use some /proc thing
> instead of ptrace.
> 
> There are definitely things we would like from such an API.  Here's a
> few I can think of immediately, there are probably others.
> 
> * Use an fd, not SIGCHLD+wait, to report inferior state changes to gdb.
>   [...] Relatedly, don't mess with the inferior's parentage.

This is satisfied by the gdbstub prototype.

> * Support "displaced stepping" in the kernel [...]

I believe this is tantamount to hardware breakpoint support, which is
already present (via optional uprobes).

> * Support some kind of breakpoint expression in the kernel; this would
>   improve performance of conditional breakpoints.  Perhaps the existing
>   gdb agent expressions could be used.

This is in the todo list.


And that "KILLER FEATURE" of running strace plus gdb on the same
process?  It *already works* with the gdbstub, and unmodified strace +
gdb, thanks to utrace multiplexing process control.  It is still
artificially restricted in many ways, but this sort of thing is ready
for testing:

% process &
[1] 9999
% strace -o FILE -p 9999 &
% gdb process
(gdb) target remote /proc/9999/gdb
(gdb) backtrace 
(gdb) cont
(gdb) ^D
%
[process continues]
% cat FILE
[...]
% kill 9999


- FChE
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