linux-kernel - Re: [RFC v2 2/7] tables.h: add linker table support

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Message-ID: <56CCE9B6.4080000@zytor.com>
Date:	Tue, 23 Feb 2016 15:22:30 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Luis R. Rodriguez" <mcgrof@...nel.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Michael Brown <mcb30@...e.org>,
	Juergen Gross <jgross@...e.com>,
	Ming Lei <ming.lei@...onical.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arnd Bergmann <arnd@...db.de>, linux-arch@...r.kernel.org,
	Russell King <linux@....linux.org.uk>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	jbaron@...mai.com, ananth@...ibm.com,
	anil.s.keshavamurthy@...el.com, David Miller <davem@...emloft.net>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	andriy.shevchenko@...ux.intel.com,
	David Woodhouse <dwmw2@...radead.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	linux-security-module <linux-security-module@...r.kernel.org>
Subject: Re: [RFC v2 2/7] tables.h: add linker table support

On 02/23/2016 03:08 PM, Luis R. Rodriguez wrote:
> 
> OK yes I see the issue now. I've modified this to use const, and
> retested the kprobe patch and it works well still. kprobe would not
> use LINKTABLE_TEXT, instead it uses its own macro, however users of
> LINKTABLE_TEXT would then have const declared. The implications are
> that you *can* declare structs so long as everything is const.
> 

No, such structures belong in .rodata not in .text.

I have been thinking about it somewhat, and for text we really have text
"ranges" rather than tables.  The big difference between the two are:

1. priority ordering doesn't make any sense for ranges.
2. ranges can be hierarchial, that is, range "bar" can be entirely
   inside range "foo".
3. ranges aren't typed (although in C, that pretty much means they are
   "char" or "unsigned char" as there really isn't any way to define an
   "array of void".)
4. the only useful operator on a range is "is address X inside this
   range"; this operator is likely *not* useful for a table, since
   if you have to ever invoke it you are probably doing something very
   wrong.

For this to work, we need strings such that they will always sort in the
appropriate order with the bracket symbols around subranges.  I need to
think about this a few minutes before I actually say anything about it...

	-hpa