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From:   Phil Auld <pauld@redhat.com>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        Barry Song <21cnbao@gmail.com>,
        Tian Tao <tiantao6@hisilicon.com>,
        Yury Norov <yury.norov@gmail.com>
Subject: [PATCH v4 RESEND] drivers/base: fix userspace break from using bin_attributes for cpumap and cpulist
Date:   Thu, 14 Jul 2022 14:30:21 -0400
Message-Id: <20220714183021.2924119-1-pauld@redhat.com>
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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. (0,2,4,8, ... ). 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.

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 
+ * 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