configfs: implement binary attributes

ConfigFS lacked binary attributes up until now. This patch
introduces support for binary attributes in a somewhat similar
manner of sysfs binary attributes albeit with changes that
fit the configfs usage model.

Problems that configfs binary attributes fix are everything that
requires a binary blob as part of the configuration of a resource,
such as bitstream loading for FPGAs, DTBs for dynamically created
devices etc.

Look at Documentation/filesystems/configfs/configfs.txt for internals
and howto use them.

This patch is against linux-next as of today that contains
Christoph's configfs rework.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
[hch: folded a fix from Geert Uytterhoeven <geert+renesas@glider.be>]
[hch: a few tiny updates based on review feedback]
Signed-off-by: Christoph Hellwig <hch@lst.de>

1 files changed, 48 insertions, 9 deletions

diff --git a/Documentation/filesystems/configfs/configfs.txt b/Documentation/filesystems/configfs/configfs.txt
index af68efdbbfad..e5fe521eea1d 100644
--- a/Documentation/filesystems/configfs/configfs.txt
+++ b/Documentation/filesystems/configfs/configfs.txt
@@ -51,15 +51,27 @@ configfs tree is always there, whether mounted on /config or not.
 An item is created via mkdir(2).  The item's attributes will also
 appear at this time.  readdir(3) can determine what the attributes are,
 read(2) can query their default values, and write(2) can store new
-values.  Like sysfs, attributes should be ASCII text files, preferably
-with only one value per file.  The same efficiency caveats from sysfs
-apply.  Don't mix more than one attribute in one attribute file.
-
-Like sysfs, configfs expects write(2) to store the entire buffer at
-once.  When writing to configfs attributes, userspace processes should
-first read the entire file, modify the portions they wish to change, and
-then write the entire buffer back.  Attribute files have a maximum size
-of one page (PAGE_SIZE, 4096 on i386).
+values.  Don't mix more than one attribute in one attribute file.
+
+There are two types of configfs attributes:
+
+* Normal attributes, which similar to sysfs attributes, are small ASCII text
+files, with a maximum size of one page (PAGE_SIZE, 4096 on i386).  Preferably
+only one value per file should be used, and the same caveats from sysfs apply.
+Configfs expects write(2) to store the entire buffer at once.  When writing to
+normal configfs attributes, userspace processes should first read the entire
+file, modify the portions they wish to change, and then write the entire
+buffer back.
+
+* Binary attributes, which are somewhat similar to sysfs binary attributes,
+but with a few slight changes to semantics.  The PAGE_SIZE limitation does not
+apply, but the whole binary item must fit in single kernel vmalloc'ed buffer.
+The write(2) calls from user space are buffered, and the attributes'
+write_bin_attribute method will be invoked on the final close, therefore it is
+imperative for user-space to check the return code of close(2) in order to
+verify that the operation finished successfully.
+To avoid a malicious user OOMing the kernel, there's a per-binary attribute
+maximum buffer value.
 
 When an item needs to be destroyed, remove it with rmdir(2).  An
 item cannot be destroyed if any other item has a link to it (via
@@ -171,6 +183,7 @@ among other things.  For that, it needs a type.
                 struct configfs_item_operations         *ct_item_ops;
                 struct configfs_group_operations        *ct_group_ops;
                 struct configfs_attribute               **ct_attrs;
+                struct configfs_bin_attribute           **ct_bin_attrs;
         };
 
 The most basic function of a config_item_type is to define what
@@ -201,6 +214,32 @@ be called whenever userspace asks for a read(2) on the attribute.  If an
 attribute is writable and provides a ->store  method, that method will be
 be called whenever userspace asks for a write(2) on the attribute.
 
+[struct configfs_bin_attribute]
+
+        struct configfs_attribute {
+                struct configfs_attribute       cb_attr;
+                void                            *cb_private;
+                size_t                          cb_max_size;
+        };
+
+The binary attribute is used when the one needs to use binary blob to
+appear as the contents of a file in the item's configfs directory.
+To do so add the binary attribute to the NULL-terminated array
+config_item_type->ct_bin_attrs, and the item appears in configfs, the
+attribute file will appear with the configfs_bin_attribute->cb_attr.ca_name
+filename.  configfs_bin_attribute->cb_attr.ca_mode specifies the file
+permissions.
+The cb_private member is provided for use by the driver, while the
+cb_max_size member specifies the maximum amount of vmalloc buffer
+to be used.
+
+If binary attribute is readable and the config_item provides a
+ct_item_ops->read_bin_attribute() method, that method will be called
+whenever userspace asks for a read(2) on the attribute.  The converse
+will happen for write(2). The reads/writes are bufferred so only a
+single read/write will occur; the attributes' need not concern itself
+with it.
+
 [struct config_group]
 
 A config_item cannot live in a vacuum.  The only way one can be created