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SubjectRe: [PATCH v3] drivers/base/node.c: fix userspace break from using bin_attributes for cpumap and cpulist
On Thu, Jul 14, 2022 at 6:38 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
> (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,... 1024,1026...). To simplify the math and
> support larger NR_CPUS we are using NR_CPUS * 7 to support a future with much larger NR_CPUS.
> 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.
>
> 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
>
> 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")
> 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 | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/base/node.c b/drivers/base/node.c
> index 0ac6376ef7a1..89c932a1d8ca 100644
> --- a/drivers/base/node.c
> +++ b/drivers/base/node.c
> @@ -45,7 +45,11 @@ static inline ssize_t cpumap_read(struct file *file, struct kobject *kobj,
> return n;
> }
>
> -static BIN_ATTR_RO(cpumap, 0);
> +/* Report a valid max size for this file to avoid breaking userspace. We use NR_CPUS/2 as
> + * a simplification of NR_CPUS/8 + NR_CPUS/32. Use PAGE_SIZE as a minimum for smaller
> + * configurations.
> + */
> +static BIN_ATTR_RO(cpumap, (((NR_CPUS >> 1) > PAGE_SIZE) ? NR_CPUS >> 1 : PAGE_SIZE));

the code should be fine. but the comment seems to be wrong?

/$ cat /sys/devices/system/node/node0/cpumap
00000000,00000000,00000000,000000ff

4 cpus need one byte in hex, 32 cpus need a comma.
for 32cpus, we totally need 9 bytes.

Based on your comment, you get 32/8+32/32=5.
should be NR_CPUS/4 ?

>
> static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
> struct bin_attribute *attr, char *buf,
> @@ -66,7 +70,15 @@ static inline ssize_t cpulist_read(struct file *file, struct kobject *kobj,
> return n;
> }
>
> -static BIN_ATTR_RO(cpulist, 0);
> +/* Report a valid maximum size for this file since 0 breaks userspace, which
> + * may use the size from fstat to allocate a read buffer.
> + * The value 7 is a hardcoded version of ceil(log10(NR_CPUS)) + 1 for future values
> + * of NR_CPUS that may be upto 2 orders of magnitude larger than 8192.
> + * In a worst case system every other cpu is on one of two nodes. This leads to
> + * a file like "0,2,4,6,8...1024,...8190,...". Use PAGE_SIZE as a minimum for smaller
> + * NR_CPUS.
> +*/
> +static BIN_ATTR_RO(cpulist, (((NR_CPUS * 7) > PAGE_SIZE) ? NR_CPUS * 7 : PAGE_SIZE));
>

It seems to be very sufficient. At least, my poor math tells me 7
bytes can describe cpu id like
"100000," and up to "999999,"
but it is still hard for me to understand the comments :-)

btw, we have a lot of other places which might need this, such as
drivers/base/topology.c

so perhaps we can move them to some common place,

#define cpu_bitmap_bytes (((NR_CPUS >> 1) > PAGE_SIZE) ? NR_CPUS >> 1
: PAGE_SIZE)
#define cpu_list_bytes (((NR_CPUS * 7) > PAGE_SIZE) ? NR_CPUS * 7 : PAGE_SIZE)

is include/linux/cpumask.h a good place for it?

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

Thanks
Barry
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