linux-kernel - Re: [UPDATED PATCH] fix memory corruption from misinterpreted bad_inode

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Message-ID: <20070104235226.GA17561@ftp.linux.org.uk>
Date:	Thu, 4 Jan 2007 23:52:26 +0000
From:	Al Viro <viro@....linux.org.uk>
To:	Mitchell Blank Jr <mitch@...oth.com>
Cc:	Linus Torvalds <torvalds@...l.org>,
	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [UPDATED PATCH] fix memory corruption from misinterpreted bad_inode_ops return values

On Thu, Jan 04, 2007 at 03:21:06PM -0800, Mitchell Blank Jr wrote:
> Linus Torvalds wrote:
> > Well, that probably would work, but it's also true that returning a 64-bit 
> > value on a 32-bit platform really _does_ depend on more than the size.
> 
> Yeah, obviously this is restricted to the signed-integer case.  My point
> was just that you could have the compiler figure out which variant to pick
> for loff_t automatically.
> 
> > "let's not play tricks with function types at all".
> 
> I think I agree.  The real (but harder) fix for the wasted space issue
> would be to get the toolchain to automatically combine functions that
> end up compiling into identical assembly.

Can't do.

int f(void)
{
	return 0;
}

int g(void)
{
	return 0;
}

int is_f(int (*p)(void))
{
	return p == f;
}

main()
{
	printf("%d %d\n", is_f(f), is_f(g));
}

would better produce
1 0
for anything resembling a sane C compiler.  Comparing pointers to
functions for equality is a well-defined operation and it's not
to be messed with.

You _can_ compile g into jump to f, but that's it.  And that, AFAICS,
is what gcc does.
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