Messages in this thread Patch in this message | | | From | Phil Auld <> | Subject | [PATCH v3] drivers/base/node.c: fix userspace break from using bin_attributes for cpumap and cpulist | Date | Wed, 13 Jul 2022 14:38:55 -0400 |
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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)); 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)); /** * struct node_access_nodes - Access class device to hold user visible -- 2.31.1
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