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Date:   Thu, 14 Jul 2022 12:32:17 -0700
From:   Yury Norov <yury.norov@gmail.com>
To:     Phil Auld <pauld@redhat.com>
Cc:     linux-kernel@vger.kernel.org,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Barry Song <21cnbao@gmail.com>,
        Tian Tao <tiantao6@hisilicon.com>
Subject: Re: [PATCH v4 RESEND] drivers/base: fix userspace break from using
 bin_attributes for cpumap and cpulist
Message-ID: <YtBvQUzRn5idNA5J@yury-laptop>
References: <20220714183021.2924119-1-pauld@redhat.com>
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In-Reply-To: <20220714183021.2924119-1-pauld@redhat.com>
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On Thu, Jul 14, 2022 at 02:30:21PM -0400, Phil Auld 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
> (8192 * 5). In order to get near that you'd need a system with every other CPU on one
> node or something similar. e.g. (0,2,4,8, ... ).

My python says:
        >>> len(str(list(range(0, 8194, 2))).replace(" ", ''))
        19931

Where the list looks like:
        [0,2,4,6,8,10,...,8190,8192]
        
So excluding open and close braces, max length of the cpu list is
19929 bytes, which is almost3 times smaller than your estimation
(8192 * 7 = 57344).


For NR_CPUS == 16x8192:
        >>> len(str(list(range(0, 8194 * 16, 2))).replace(" ", '')) - 2
        403308
        >>> 8192 * 16 * 7
        917504

For NR_CPUS == 128x8192:
        >>> len(str(list(range(0, 8194 * 128, 2))).replace(" ", '')) - 2
        3639774
        >>> 8192 * 16 * 7
        7340032

Looks like it 2x overestimates for large lists, and 4x for standard
256-bit mask. Before, it was possible to fit ~1800 cpus into 4k page,
after - 585.

> To simplify the math and support
> larger NR_CPUS in the future we are using NR_CPUS * 7. We also set it to a min of
> PAGE_SIZE to retain the older behavior for smaller NR_CPUS. The cpumap file wants to
> be something like NR_CPUS/4 + NR_CPUS/32, for the ","s so for simplicity we are using
> NR_CPUS/2.

This again overestimates almost twice. In this case, NR_CPUS * 9/32 - 1
is a precise value, if I didn't screw up. Why don't you just use it?

> Add a set of macros for these values to cpumask.h so they can be used in multiple places.
> Apply these to the handful of such files in drivers/base/topology.c as well as node.c.
> 
> On an 80 cpu 4-node sytem (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
> 
> after:
> 
> -r--r--r--. 1 root root 57344 Jul 13 11:32 /sys/devices/system/node/node0/cpulist
> -r--r--r--. 1 root root  4096 Jul 13 11:31 /sys/devices/system/node/node0/cpumap
> 
> CONFIG_NR_CPUS = 16384
> -r--r--r--. 1 root root 114688 Jul 13 14:03 /sys/devices/system/node/node0/cpulist
> -r--r--r--. 1 root root   8192 Jul 13 14:02 /sys/devices/system/node/node0/cpumap
> 
> Fixes: 75bd50fa841 ("drivers/base/node.c: use bin_attribute to break the size limitation of cpumap ABI")
> Fixes: bb9ec13d156 ("topology: 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>
> ---
> 
> v2: Fix cpumap size calculation. Increase multiplier for cpulist size.
> 
> v3: Add comments in code.
> 
> v4: Define constants in cpumask.h. Move comments there. Also fix
> topology.c.
> 
> 
>  drivers/base/node.c     |  4 ++--
>  drivers/base/topology.c | 32 ++++++++++++++++----------------
>  include/linux/cpumask.h | 16 ++++++++++++++++
>  3 files changed, 34 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 0ac6376ef7a1..eb0f43784c2b 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, CPUMAP_FILE_MAX_BYTES);
>  
>  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, CPULIST_FILE_MAX_BYTES);
>  
>  /**
>   * struct node_access_nodes - Access class device to hold user visible
> diff --git a/drivers/base/topology.c b/drivers/base/topology.c
> index ac6ad9ab67f9..89f98be5c5b9 100644
> --- a/drivers/base/topology.c
> +++ b/drivers/base/topology.c
> @@ -62,47 +62,47 @@ define_id_show_func(ppin, "0x%llx");
>  static DEVICE_ATTR_ADMIN_RO(ppin);
>  
>  define_siblings_read_func(thread_siblings, sibling_cpumask);
> -static BIN_ATTR_RO(thread_siblings, 0);
> -static BIN_ATTR_RO(thread_siblings_list, 0);
> +static BIN_ATTR_RO(thread_siblings, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(thread_siblings_list, CPULIST_FILE_MAX_BYTES);
>  
>  define_siblings_read_func(core_cpus, sibling_cpumask);
> -static BIN_ATTR_RO(core_cpus, 0);
> -static BIN_ATTR_RO(core_cpus_list, 0);
> +static BIN_ATTR_RO(core_cpus, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(core_cpus_list, CPULIST_FILE_MAX_BYTES);
>  
>  define_siblings_read_func(core_siblings, core_cpumask);
> -static BIN_ATTR_RO(core_siblings, 0);
> -static BIN_ATTR_RO(core_siblings_list, 0);
> +static BIN_ATTR_RO(core_siblings, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(core_siblings_list, CPULIST_FILE_MAX_BYTES);
>  
>  #ifdef TOPOLOGY_CLUSTER_SYSFS
>  define_siblings_read_func(cluster_cpus, cluster_cpumask);
> -static BIN_ATTR_RO(cluster_cpus, 0);
> -static BIN_ATTR_RO(cluster_cpus_list, 0);
> +static BIN_ATTR_RO(cluster_cpus, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(cluster_cpus_list, CPULIST_FILE_MAX_BYTES);
>  #endif
>  
>  #ifdef TOPOLOGY_DIE_SYSFS
>  define_siblings_read_func(die_cpus, die_cpumask);
> -static BIN_ATTR_RO(die_cpus, 0);
> -static BIN_ATTR_RO(die_cpus_list, 0);
> +static BIN_ATTR_RO(die_cpus, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(die_cpus_list, CPULIST_FILE_MAX_BYTES);
>  #endif
>  
>  define_siblings_read_func(package_cpus, core_cpumask);
> -static BIN_ATTR_RO(package_cpus, 0);
> -static BIN_ATTR_RO(package_cpus_list, 0);
> +static BIN_ATTR_RO(package_cpus, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(package_cpus_list, CPULIST_FILE_MAX_BYTES);
>  
>  #ifdef TOPOLOGY_BOOK_SYSFS
>  define_id_show_func(book_id, "%d");
>  static DEVICE_ATTR_RO(book_id);
>  define_siblings_read_func(book_siblings, book_cpumask);
> -static BIN_ATTR_RO(book_siblings, 0);
> -static BIN_ATTR_RO(book_siblings_list, 0);
> +static BIN_ATTR_RO(book_siblings, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(book_siblings_list, CPULIST_FILE_MAX_BYTES);
>  #endif
>  
>  #ifdef TOPOLOGY_DRAWER_SYSFS
>  define_id_show_func(drawer_id, "%d");
>  static DEVICE_ATTR_RO(drawer_id);
>  define_siblings_read_func(drawer_siblings, drawer_cpumask);
> -static BIN_ATTR_RO(drawer_siblings, 0);
> -static BIN_ATTR_RO(drawer_siblings_list, 0);
> +static BIN_ATTR_RO(drawer_siblings, CPUMAP_FILE_MAX_BYTES);
> +static BIN_ATTR_RO(drawer_siblings_list, CPULIST_FILE_MAX_BYTES);
>  #endif
>  
>  static struct bin_attribute *bin_attrs[] = {
> diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
> index fe29ac7cc469..007acdb462bd 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -1071,4 +1071,20 @@ cpumap_print_list_to_buf(char *buf, const struct cpumask *mask,
>  	[0] =  1UL							\
>  } }
>  
> +/* 
> + * Provide a valid theoretical max size for cpumap ands cpulist sysfs files to 

s/ands/and

> + * avoid breaking userspace which may allocate a buffer based on the size 
> + * reported by e.g. fstat.
> + *
> + * For cpumap NR_CPUS/2 is a simplification of NR_CPUS/4 + NR_CPUS/32. 
> + *
> + * For cpulist 7 is (ceil(log10(NR_CPUS)) + 1) allowing for NR_CPUS to be up to 
> + * 2 orders of magnitude larger than 8192. This covers a worst-case of every 
> + * other cpu being on one of two nodes for a very large NR_CPUS.
> + *
> + *  Use PAGE_SIZE as a minimum for smaller configurations. 
> + */
> +#define CPUMAP_FILE_MAX_BYTES  (((NR_CPUS >> 1) > PAGE_SIZE) ? NR_CPUS >> 1 : PAGE_SIZE)
> +#define CPULIST_FILE_MAX_BYTES  (((NR_CPUS * 7) > PAGE_SIZE) ? NR_CPUS * 7 : PAGE_SIZE)
> +
>  #endif /* __LINUX_CPUMASK_H */
> -- 
> 2.31.1