linux-kernel - Re: [PATCH 3.12-rc7] KVM: Fix modprobe failure for kvm_intel/kvm

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Date:	Wed, 30 Oct 2013 07:23:59 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tim Gardner <tim.gardner@...onical.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Gleb Natapov <gleb@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	KVM list <kvm@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 3.12-rc7] KVM: Fix modprobe failure for kvm_intel/kvm_amd

On Wed, Oct 30, 2013 at 07:31:05PM +0530, Raghavendra K T wrote:
> On 10/30/2013 01:03 AM, Linus Torvalds wrote:
> > On Tue, Oct 29, 2013 at 12:27 PM, Raghavendra K T
> > <raghavendra.kt@...ux.vnet.ibm.com> wrote:
> >>
> >> Could one solution be cascading actual error
> >> that is lost in fs/debugfs/inode.c:__create_file(), so that we could
> >> take correct action in case of failure of debugfs_create_dir()?
> >>
> >> (ugly side is we increase total number of params for __create_file to
> >> 6). or I hope there could be some better solution.
> >
> > The solution to this would be to simply return an error-pointer. See
> > <linux/err.h>. That's what we do for most complex subsystems that
> > return a pointer to a struct: rather than returning "NULL" as an
> > error, return the actual error number encoded in the pointer itself.
> 
> Thank you Linus. Yes, this would have been ideal.
> 
> >
> > But that would require every user of debugfs_create_dir() to be
> > updated to check errors using IS_ERR() instead of checking against
> > NULL, and there's quite a few of them.
> >
> > So I think just making the error be EEXIST is a simpler solution right now.
> >
> 
> Agree.
> I had below patch, and just before sending as formal mail I saw
> Paolo's pull request which already took care of it.
> ---8<---
> 
> 

> >From ac5d9f038c273f27bea7a54aab6af79b57f56317 Mon Sep 17 00:00:00 2001
> From: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> Date: Wed, 30 Oct 2013 18:59:46 +0530
> Subject: [PATCH] Return EEXIST on debugfs_create_dir failure in kvm
> 
> As quoted by Linus,  EFAULT means "user passed in an invalid
> virtual address pointer", which is why the error string is Bad address.
> But when a debugfs directory creation fails, the above error is not valid.
> 
> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> ---
>  I understand that Tim's patch that renames directory to something like
>  kvm-pv would solve kvm-amd/kvm-intel modules insertion problem. 
>  This patch is to address error code  change complained by Linus.
> 
>  arch/x86/kernel/kvm.c | 2 +-
>  virt/kvm/kvm_main.c   | 3 ++-
>  2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index a0e2a8a..e475fdb 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -622,7 +622,7 @@ static int __init kvm_spinlock_debugfs(void)
>  
>  	d_kvm = kvm_init_debugfs();
>  	if (d_kvm == NULL)
> -		return -ENOMEM;
> +		return -EEXIST;

Why even error out at all here?  You should never care about debugfs
file creation return values, so why pass any error back up the stack?

greg k-h
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