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Subject[PATCH 08/13] cgroup: update the meaning of cftype->max_write_len
Date
cftype->max_write_len is used to extend the maximum size of writes.
It's interpreted in such a way that the actual maximum size is one
less than the specified value. The default size is defined by
CGROUP_LOCAL_BUFFER_SIZE. Its interpretation is quite confusing - its
value is decremented by 1 and then compared for equality with max
size, which means that the actual default size is
CGROUP_LOCAL_BUFFER_SIZE - 2, which is 62 chars.

There's no point in having a limit that low. Update its definition so
that it means the actual string length sans termination and anything
below PAGE_SIZE-1 is treated as PAGE_SIZE-1.

.max_write_len for "release_agent" is updated to PATH_MAX-1 and
cgroup_release_agent_write() is updated so that the redundant strlen()
check is removed and it uses strlcpy() instead of strcpy().
.max_write_len initializations in blk-throttle.c and cfq-iosched.c are
no longer necessary and removed. The one in cpuset is kept unchanged
as it's an approximated value to begin with.

This will also make transition to kernfs smoother.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
block/blk-throttle.c | 4 ----
block/cfq-iosched.c | 3 ---
include/linux/cgroup.h | 5 +++--
kernel/cgroup.c | 18 ++++++++----------
4 files changed, 11 insertions(+), 19 deletions(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 1474c3a..861c363 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1425,28 +1425,24 @@ static struct cftype throtl_files[] = {
.private = offsetof(struct throtl_grp, bps[READ]),
.seq_show = tg_print_conf_u64,
.write_string = tg_set_conf_u64,
- .max_write_len = 256,
},
{
.name = "throttle.write_bps_device",
.private = offsetof(struct throtl_grp, bps[WRITE]),
.seq_show = tg_print_conf_u64,
.write_string = tg_set_conf_u64,
- .max_write_len = 256,
},
{
.name = "throttle.read_iops_device",
.private = offsetof(struct throtl_grp, iops[READ]),
.seq_show = tg_print_conf_uint,
.write_string = tg_set_conf_uint,
- .max_write_len = 256,
},
{
.name = "throttle.write_iops_device",
.private = offsetof(struct throtl_grp, iops[WRITE]),
.seq_show = tg_print_conf_uint,
.write_string = tg_set_conf_uint,
- .max_write_len = 256,
},
{
.name = "throttle.io_service_bytes",
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 744833b..4611879 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1838,7 +1838,6 @@ static struct cftype cfq_blkcg_files[] = {
.flags = CFTYPE_ONLY_ON_ROOT,
.seq_show = cfqg_print_leaf_weight_device,
.write_string = cfqg_set_leaf_weight_device,
- .max_write_len = 256,
},
{
.name = "weight",
@@ -1853,7 +1852,6 @@ static struct cftype cfq_blkcg_files[] = {
.flags = CFTYPE_NOT_ON_ROOT,
.seq_show = cfqg_print_weight_device,
.write_string = cfqg_set_weight_device,
- .max_write_len = 256,
},
{
.name = "weight",
@@ -1866,7 +1864,6 @@ static struct cftype cfq_blkcg_files[] = {
.name = "leaf_weight_device",
.seq_show = cfqg_print_leaf_weight_device,
.write_string = cfqg_set_leaf_weight_device,
- .max_write_len = 256,
},
{
.name = "leaf_weight",
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 884d367..1e4f9e2 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -398,8 +398,9 @@ struct cftype {
umode_t mode;

/*
- * If non-zero, defines the maximum length of string that can
- * be passed to write_string; defaults to 64
+ * The maximum length of string, excluding trailing nul, that can
+ * be passed to write_string. If < PAGE_SIZE-1, PAGE_SIZE-1 is
+ * assumed.
*/
size_t max_write_len;

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 5935c2f..1534542 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2211,13 +2211,14 @@ static int cgroup_procs_write(struct cgroup_subsys_state *css,
static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
struct cftype *cft, const char *buffer)
{
- BUILD_BUG_ON(sizeof(css->cgroup->root->release_agent_path) < PATH_MAX);
- if (strlen(buffer) >= PATH_MAX)
- return -EINVAL;
+ struct cgroupfs_root *root = css->cgroup->root;
+
+ BUILD_BUG_ON(sizeof(root->release_agent_path) < PATH_MAX);
if (!cgroup_lock_live_group(css->cgroup))
return -ENODEV;
spin_lock(&release_agent_path_lock);
- strcpy(css->cgroup->root->release_agent_path, buffer);
+ strlcpy(root->release_agent_path, buffer,
+ sizeof(root->release_agent_path));
spin_unlock(&release_agent_path_lock);
mutex_unlock(&cgroup_mutex);
return 0;
@@ -2243,20 +2244,17 @@ static int cgroup_sane_behavior_show(struct seq_file *seq, void *v)
return 0;
}

-/* A buffer size big enough for numbers or short strings */
-#define CGROUP_LOCAL_BUFFER_SIZE 64
-
static ssize_t cgroup_file_write(struct file *file, const char __user *userbuf,
size_t nbytes, loff_t *ppos)
{
struct cfent *cfe = __d_cfe(file->f_dentry);
struct cftype *cft = __d_cft(file->f_dentry);
struct cgroup_subsys_state *css = cfe->css;
- size_t max_bytes = cft->max_write_len ?: CGROUP_LOCAL_BUFFER_SIZE - 1;
+ size_t max_bytes = max(cft->max_write_len, PAGE_SIZE);
char *buf;
int ret;

- if (nbytes >= max_bytes)
+ if (nbytes > max_bytes)
return -E2BIG;

buf = kmalloc(nbytes + 1, GFP_KERNEL);
@@ -3917,7 +3915,7 @@ static struct cftype cgroup_base_files[] = {
.flags = CFTYPE_INSANE | CFTYPE_ONLY_ON_ROOT,
.seq_show = cgroup_release_agent_show,
.write_string = cgroup_release_agent_write,
- .max_write_len = PATH_MAX,
+ .max_write_len = PATH_MAX - 1,
},
{ } /* terminate */
};
--
1.8.5.3


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