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From:   Barry Song <21cnbao@gmail.com>
Date:   Wed, 13 Jul 2022 11:18:59 +1200
Message-ID: <CAGsJ_4xG0az1-g8DWL-mEv_cF3ZBMe6j87m_cxeL9abvxGNW=g@mail.gmail.com>
Subject: Re: [PATCH] drivers/base/node.c: fix userspace break from using
 bin_attributes for cpumap and cpulist
To:     Phil Auld <pauld@redhat.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki : --cc=" <rafael@kernel.org>,
        Tian Tao <tiantao6@hisilicon.com>,
        Barry Song <song.bao.hua@hisilicon.com>
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On Wed, Jul 13, 2022 at 9:58 AM Phil Auld <pauld@redhat.com> wrote:
>
> Using bin_attributes with a 0 size causes fstat and friends to return that 0 size.
> This breaks userspace code that retrieves the size before reading the file. Rather
> than reverting 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size
> limitation of cpumap ABI") let's put in a size value at compile time. Use direct
> comparison and a worst-case maximum to ensure compile time constants. For cpulist the
> max is on the order of NR_CPUS * (ceil(log10(NR_CPUS)) + 1) which for 8192 is 40960.
> In order to get near that you'd need a system with every other CPU on one node or
> something similar. e.g. (0,2,4,... 1024,1026...). We set it to a min of PAGE_SIZE
> to retain the older behavior. For cpumap, PAGE_SIZE is plenty big.
>
> On an 80 cpu 4-node system (NR_CPUS == 8192)
>
> before:
>
> -r--r--r--. 1 root root 0 Jul 12 14:08 /sys/devices/system/node/node0/cpulist
> -r--r--r--. 1 root root 0 Jul 11 17:25 /sys/devices/system/node/node0/cpumap

it is a fundamental problem of bin_attr, isn't it? when we don't know the
exact size of an attribute, and this size might become more than one
PAGE_SIZE, we use bin_attr to break the limitation. but the fact is that
we really don't know or it is really hard to know the actual size of the
attribute.

>
> after:
>
> -r--r--r--. 1 root root 40960 Jul 12 16:48 /sys/devices/system/node/node0/cpulist
> -r--r--r--. 1 root root  4096 Jul 12 15:50 /sys/devices/system/node/node0/cpumap

if we finally set a size which might be improper, it seems we defeat the
purpose we start to move to bin_attr?

>
> Fixes: 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size limitation of cpumap ABI")
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: "Rafael J. Wysocki" <rafael@kernel.org>
> Signed-off-by: Phil Auld <pauld@redhat.com>
> ---
>  drivers/base/node.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 0ac6376ef7a1..291c69671f23 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -45,7 +45,7 @@ static inline ssize_t cpumap_read(struct file *file, struct kobject *kobj,
>         return n;
>  }
>
> -static BIN_ATTR_RO(cpumap, 0);
> +static BIN_ATTR_RO(cpumap, PAGE_SIZE);

PAGE_SIZE is probably big enough, will we still calculate to get it rather than
hard coding?

>
>  static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
>                                    struct bin_attribute *attr, char *buf,
> @@ -66,7 +66,7 @@ static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
>         return n;
>  }
>
> -static BIN_ATTR_RO(cpulist, 0);
> +static BIN_ATTR_RO(cpulist, (((NR_CPUS * 5) > PAGE_SIZE) ? NR_CPUS *5 : PAGE_SIZE));

I am still not sure why it is NR_CPUS * 5. Is 5 bytes big enough to
describe the number
of cpu id? technically it seems not,  for example,  for cpuid=100000,
we need at least 6
bytes.

BTW, my silly question is that what if we set the size to MAXIMUM int?
Will it fix
the userspace fsstat?

>
>  /**
>   * struct node_access_nodes - Access class device to hold user visible
> --
> 2.31.1
>

Thanks
Barry