[PATCH V2] sysfs: Add sysfs_emit and sysfs_emit_at to format sysfs output

[Joe Perches <joe@perches.com>]

Output defects can exist in sysfs content using sprintf and snprintf.

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

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

Add a generic sysfs_emit_at function that can be used in multiple
call situations that also ensures that no overrun is done.

Signed-off-by: Joe Perches <joe@perches.com>
---

V2: Simplify sysfs_emit and add sysfs_emit_at
    Include Documentation change

 Documentation/filesystems/sysfs.rst |  8 ++---
 fs/sysfs/file.c                     | 49 +++++++++++++++++++++++++++++
 include/linux/sysfs.h               | 15 +++++++++
 3 files changed, 67 insertions(+), 5 deletions(-)

diff --git a/Documentation/filesystems/sysfs.rst b/Documentation/filesystems/sysfs.rst
index ab0f7795792b..d44249050f4a 100644
--- a/Documentation/filesystems/sysfs.rst
+++ b/Documentation/filesystems/sysfs.rst
@@ -242,12 +242,10 @@ Other notes:
   is 4096.
 
 - show() methods should return the number of bytes printed into the
-  buffer. This is the return value of scnprintf().
+  buffer.
 
-- show() must not use snprintf() when formatting the value to be
-  returned to user space. If you can guarantee that an overflow
-  will never happen you can use sprintf() otherwise you must use
-  scnprintf().
+- show() should only use sysfs_emit() or sysfs_emit_at() when formatting
+  the value to be returned to user space.
 
 - store() should return the number of bytes used from the buffer. If the
   entire buffer has been used, just return the count argument.
diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index eb6897ab78e7..e8c6d20bab8e 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -707,3 +707,52 @@ 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.
+ *	@fmt:	format
+ *	@...:	optional arguments to @format
+ *
+ *
+ * Returns number of characters written to @buf.
+ */
+int sysfs_emit(char *buf, const char *fmt, ...)
+{
+	va_list args;
+	int len;
+
+	va_start(args, fmt);
+	len = vscnprintf(buf, PAGE_SIZE, fmt, args);
+	va_end(args);
+
+	return len;
+}
+EXPORT_SYMBOL_GPL(sysfs_emit);
+
+/**
+ *	sysfs_emit_at - scnprintf equivalent, aware of PAGE_SIZE buffer.
+ *	@buf:	start of PAGE_SIZE buffer.
+ *	@at:	offset in @buf to start write in bytes
+ *		@at must be >= 0 && < PAGE_SIZE
+ *	@fmt:	format
+ *	@...:	optional arguments to @fmt
+ *
+ *
+ * Returns number of characters written starting at &@buf[@at].
+ */
+int sysfs_emit_at(char *buf, int at, const char *fmt, ...)
+{
+	va_list args;
+	int len;
+
+	if (WARN(at < 0 || at >= PAGE_SIZE, "invalid sysfs_emit_at: %d\n", at))
+		return 0;
+
+	va_start(args, fmt);
+	len = vscnprintf(buf + at, PAGE_SIZE - at, fmt, args);
+	va_end(args);
+
+	return len;
+}
+EXPORT_SYMBOL_GPL(sysfs_emit_at);
diff --git a/include/linux/sysfs.h b/include/linux/sysfs.h
index 34e84122f635..2caa34c1ca1a 100644
--- a/include/linux/sysfs.h
+++ b/include/linux/sysfs.h
@@ -329,6 +329,10 @@ 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(2, 3)
+int sysfs_emit(char *buf, const char *fmt, ...);
+__printf(3, 4)
+int sysfs_emit_at(char *buf, int at, const char *fmt, ...);
 
 #else /* CONFIG_SYSFS */
 
@@ -576,6 +580,17 @@ static inline int sysfs_group_change_owner(struct kobject *kobj,
 	return 0;
 }
 
+__printf(2, 3)
+static inline int sysfs_emit(char *buf, const char *fmt, ...)
+{
+	return 0;
+}
+
+__printf(3, 4)
+static inline int sysfs_emit_at(char *buf, int at, const char *fmt, ...)
+{
+	return 0;
+}
 #endif /* CONFIG_SYSFS */
 
 static inline int __must_check sysfs_create_file(struct kobject *kobj,
-- 
2.26.0