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

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Message-Id: <E1H2s10-0001MH-Kk@be1.lrz>
Date:	Fri, 05 Jan 2007 17:33:58 +0100
From:	Bodo Eggert <7eggert@....de>
To:	Eric Sandeen <sandeen@...hat.com>, Andrew Morton <akpm@...l.org>,
	Al Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...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

Eric Sandeen <sandeen@...hat.com> wrote:
> Andrew Morton wrote:

>> +++ a/fs/bad_inode.c

>> -static int return_EIO(void)
>> +static long return_EIO(void)

> What about ops that return loff_t (64 bits) on 32-bit arches and stuff
> it into 2 registers....

*If* it uses an additional register for the high bits, it will set e.g.:
EDX << 32 | EAX == (s64) -EIO
 and therefore
EAX == -EIO // < -MAXLONGINT-1
EDX == -1

EAX will be the return register for s32. Therefore you can use one function
for both cases on i386:

long long f()
{
        return -42;
}

long      (*l )() = (void*)f; // hide warning
long long (*ll)() =        f;

int main(){
        printf("%ld %lld\n", l(), ll());
}

> I'm still not convinced that this is the best place to be clever :)

ACK, not too clever, but not too stupid, too. Having #ifdef I386 etc.
isn't nice, and something like this shouldn't be arch-specific.
OTOH, C calling convention allows having a different argument signature,
so you can safely use it. It's a feature.
-- 
Ich danke GMX dafür, die Verwendung meiner Adressen mittels per SPF
verbreiteten Lügen zu sabotieren.

http://david.woodhou.se/why-not-spf.html
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