linux-kernel - Re: [PATCH] NFS/flexfiles: Replace one-element array with flexible-array member

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Message-Id: <4CB2BE7E-5979-4424-A331-A5162190A102@toblux.com>
Date: Sat, 17 Aug 2024 23:17:21 +0200
From: Thorsten Blum <thorsten.blum@...lux.com>
To: Trond Myklebust <trondmy@...merspace.com>
Cc: "kees@...nel.org" <kees@...nel.org>,
 "bcodding@...hat.com" <bcodding@...hat.com>,
 "anna@...nel.org" <anna@...nel.org>,
 "josef@...icpanda.com" <josef@...icpanda.com>,
 "jlayton@...nel.org" <jlayton@...nel.org>,
 "kolga@...app.com" <kolga@...app.com>,
 "gustavoars@...nel.org" <gustavoars@...nel.org>,
 "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH] NFS/flexfiles: Replace one-element array with
 flexible-array member

On 17. Aug 2024, at 22:40, Trond Myklebust <trondmy@...merspace.com> wrote:
> On Sat, 2024-08-17 at 16:20 +0200, Thorsten Blum wrote:
>> Replace the deprecated one-element array with a modern flexible-array
>> member in the struct nfs4_flexfile_layoutreturn_args.
>> 
>> Adjust the struct size accordingly.
>> 
>> There are no binary differences after this conversion.
>> 
>> Link: https://github.com/KSPP/linux/issues/79
>> Signed-off-by: Thorsten Blum <thorsten.blum@...lux.com>
>> ---
>> fs/nfs/flexfilelayout/flexfilelayout.c | 2 +-
>> fs/nfs/flexfilelayout/flexfilelayout.h | 2 +-
>> 2 files changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c
>> b/fs/nfs/flexfilelayout/flexfilelayout.c
>> index 39ba9f4208aa..fc698fa9aaea 100644
>> --- a/fs/nfs/flexfilelayout/flexfilelayout.c
>> +++ b/fs/nfs/flexfilelayout/flexfilelayout.c
>> @@ -2224,7 +2224,7 @@ ff_layout_prepare_layoutreturn(struct
>> nfs4_layoutreturn_args *args)
>>  struct nfs4_flexfile_layoutreturn_args *ff_args;
>>  struct nfs4_flexfile_layout *ff_layout =
>> FF_LAYOUT_FROM_HDR(args->layout);
>> 
>> - ff_args = kmalloc(sizeof(*ff_args), nfs_io_gfp_mask());
>> + ff_args = kmalloc(struct_size(ff_args, pages, 1),
>> nfs_io_gfp_mask());
>>  if (!ff_args)
>>  goto out_nomem;
>>  ff_args->pages[0] = alloc_page(nfs_io_gfp_mask());
>> diff --git a/fs/nfs/flexfilelayout/flexfilelayout.h
>> b/fs/nfs/flexfilelayout/flexfilelayout.h
>> index f84b3fb0dddd..a269753f9a46 100644
>> --- a/fs/nfs/flexfilelayout/flexfilelayout.h
>> +++ b/fs/nfs/flexfilelayout/flexfilelayout.h
>> @@ -115,7 +115,7 @@ struct nfs4_flexfile_layoutreturn_args {
>>  struct nfs42_layoutstat_devinfo
>> devinfo[FF_LAYOUTSTATS_MAXDEV];
>>  unsigned int num_errors;
>>  unsigned int num_dev;
>> - struct page *pages[1];
>> + struct page *pages[];
>> };
>> 
>> static inline struct nfs4_flexfile_layout *
> 
> NACK. There is no advantage to using a flexible array here. Indeed,
> you're replacing something that is correctly dimensioned (we only ever
> use 1 page), and that can be easily bounds checked by the compiler with
> something that has neither property.

I see. Why is it an array then and not just a pointer to a struct?

Thanks,
Thorsten