[PATCH] sysfs: Add sysfs_emit to replace sprintf to PAGE_SIZE buffers.

[Joe Perches]

sprintf does not know the PAGE_SIZE maximum of the temporary buffer
used for outputting sysfs content requests and it's possible to
overrun the buffer length.

Add a generic sysfs_emit mechanism that knows that the size of the
temporary buffer and ensures that no overrun is done.

Signed-off-by: Joe Perches <j...@perches.com>
---
 fs/sysfs/file.c       | 30 ++++++++++++++++++++++++++++++
 include/linux/sysfs.h |  8 ++++++++
 2 files changed, 38 insertions(+)

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index eb6897ab78e7..06a13bbd7080 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -707,3 +707,33 @@ int sysfs_change_owner(struct kobject *kobj, kuid_t kuid, 
kgid_t kgid)
        return 0;
 }
 EXPORT_SYMBOL_GPL(sysfs_change_owner);
+
+/**
+ *     sysfs_emit - scnprintf equivalent, aware of PAGE_SIZE buffer.
+ *     @buf:   start of PAGE_SIZE buffer.
+ *     @pos:   current position in buffer
+ *              (pos - buf) must always be < PAGE_SIZE
+ *     @fmt:   format
+ *     @...:   arguments to format
+ *
+ *
+ * Returns number of characters written at pos.
+ */
+int sysfs_emit(char *buf, char *pos, const char *fmt, ...)
+{
+       va_list args;
+       bool bad_pos = pos < buf;
+       bool bad_len = (pos - buf) >= PAGE_SIZE;
+       int len;
+
+       if (WARN(bad_pos || bad_len, "(pos < buf):%d (pos >= PAGE_SIZE):%d\n",
+                bad_pos, bad_len))
+               return 0;
+
+       va_start(args, fmt);
+       len = vscnprintf(pos, PAGE_SIZE - (pos - buf), fmt, args);
+       va_end(args);
+
+       return len;
+}
+EXPORT_SYMBOL_GPL(sysfs_emit);
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index 34e84122f635..5a21d3d30016 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -329,6 +329,8 @@ int sysfs_groups_change_owner(struct kobject *kobj,
 int sysfs_group_change_owner(struct kobject *kobj,
                             const struct attribute_group *groups, kuid_t kuid,
                             kgid_t kgid);
+__printf(3, 4)
+int sysfs_emit(char *buf, char *pos, const char *fmt, ...);
 
 #else /* CONFIG_SYSFS */
 
@@ -576,6 +578,12 @@ static inline int sysfs_group_change_owner(struct kobject 
*kobj,
        return 0;
 }
 
+__printf(3, 4)
+static inline int sysfs_emit(char *buf, char *pos, const char *fmt, ...)
+{
+       return 0;
+}
+
 #endif /* CONFIG_SYSFS */
 
 static inline int __must_check sysfs_create_file(struct kobject *kobj,
-- 
2.26.0