Messages in this thread Patch in this message | | | From | Barry Song <> | Date | Thu, 14 Jul 2022 12:23:01 +1200 | Subject | Re: [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
| |