linux-kernel - Re: [PATCH 1/2] coda: fix reference counting in coda_file

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Date:   Thu, 22 Apr 2021 14:39:41 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Jan Harkes <jaharkes@...cmu.edu>, linux-kernel@...r.kernel.org,
        linux-unionfs@...r.kernel.org, dri-devel@...ts.freedesktop.org
Cc:     coda@...cmu.edu, miklos@...redi.hu, akpm@...ux-foundation.org,
        jgg@...pe.ca
Subject: Re: [PATCH 1/2] coda: fix reference counting in coda_file_mmap error
 path

Hi Jan,

Am 22.04.21 um 14:27 schrieb Jan Harkes:
> Looks good to me.
>
> I'm also maintaining an out of tree coda module build that people sometimes use, which has workarounds for differences between the various kernel versions.
>
> Do you have a reference to the corresponding mmap_region change? If it is merged already I'll probably be able to find it. Is this mmap_region change expected to be backported to any lts kernels?

That is the following upstream commit in Linus tree:

commit 1527f926fd04490f648c42f42b45218a04754f87
Author: Christian König <christian.koenig@....com>
Date:   Fri Oct 9 15:08:55 2020 +0200

     mm: mmap: fix fput in error path v2

But I don't think we should backport that.

And sorry for the noise. We had so many places which expected different 
behavior that I didn't noticed that two occasions in the fs code 
actually rely on the current behavior.

For your out of tree module you could make the code version independent 
by setting the vma back to the original file in case of an error. That 
should work with both behaviors in mmap_region.

Thanks,
Christian.

>
> Jan
>
> On April 21, 2021 9:20:11 AM EDT, "Christian König" <ckoenig.leichtzumerken@...il.com> wrote:
>> mmap_region() now calls fput() on the vma->vm_file.
>>
>> So we need to drop the extra reference on the coda file instead of the
>> host file.
>>
>> Signed-off-by: Christian König <christian.koenig@....com>
>> Fixes: 1527f926fd04 ("mm: mmap: fix fput in error path v2")
>> CC: stable@...r.kernel.org # 5.11+
>> ---
>> fs/coda/file.c | 6 +++---
>> 1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/coda/file.c b/fs/coda/file.c
>> index 128d63df5bfb..ef5ca22bfb3e 100644
>> --- a/fs/coda/file.c
>> +++ b/fs/coda/file.c
>> @@ -175,10 +175,10 @@ coda_file_mmap(struct file *coda_file, struct
>> vm_area_struct *vma)
>> 	ret = call_mmap(vma->vm_file, vma);
>>
>> 	if (ret) {
>> -		/* if call_mmap fails, our caller will put coda_file so we
>> -		 * should drop the reference to the host_file that we got.
>> +		/* if call_mmap fails, our caller will put host_file so we
>> +		 * should drop the reference to the coda_file that we got.
>> 		 */
>> -		fput(host_file);
>> +		fput(coda_file);
>> 		kfree(cvm_ops);
>> 	} else {
>> 		/* here we add redirects for the open/close vm_operations */