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Subject[PATCH 2/2] usb: gadget: f fs: buffer data from ‘oversized’ OUT requests
Date
f_fs rounds up read(2) requests to a multiple of a max packet size
which means that host may provide more data than user has space for.
So far, the excess data has been silently ignored.

This introduces a buffer for a tail of such requests so that they are
returned on next read instead of being ignored.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
---
drivers/usb/gadget/function/f_fs.c | 130 +++++++++++++++++++++++++++++++------
1 file changed, 109 insertions(+), 21 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index e26a6b4..08a1ac2 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -130,6 +130,12 @@ struct ffs_epfile {

struct dentry *dentry;

+ /*
+ * Buffer for holding data from partial reads which may happen since
+ * we’re rounding user read requests to a multiple of a max packet size.
+ */
+ struct ffs_buffer *read_buffer; /* P: epfile->mutex */
+
char name[5];

unsigned char in; /* P: ffs->eps_lock */
@@ -138,6 +144,12 @@ struct ffs_epfile {
unsigned char _pad;
};

+struct ffs_buffer {
+ size_t length;
+ char *data;
+ char storage[];
+};
+
/* ffs_io_data structure ***************************************************/

struct ffs_io_data {
@@ -667,6 +679,11 @@ static ssize_t ffs_copy_to_iter(void *data, int data_len, struct iov_iter *iter)
* Was the buffer aligned in the first place, no such problem would
* happen.
*
+ * Data may be dropped only in AIO reads. Synchronous reads are handled
+ * by splitting a request into multiple parts. This splitting may still
+ * be a problem though so it’s likely best to align the buffer
+ * regardless of it being AIO or not..
+ *
* This only affects OUT endpoints, i.e. reading data with a read(2),
* aio_read(2) etc. system calls. Writing data to an IN endpoint is not
* affected.
@@ -717,6 +734,56 @@ static void ffs_epfile_async_io_complete(struct usb_ep *_ep,
schedule_work(&io_data->work);
}

+/* Assumes epfile->mutex is held. */
+static ssize_t __ffs_epfile_read_buffered(struct ffs_epfile *epfile,
+ struct iov_iter *iter)
+{
+ struct ffs_buffer *buf = epfile->read_buffer;
+ ssize_t ret;
+ if (!buf)
+ return 0;
+
+ ret = copy_to_iter(buf->data, buf->length, iter);
+ if (buf->length == ret) {
+ kfree(buf);
+ epfile->read_buffer = NULL;
+ } else if (unlikely(iov_iter_count(iter))) {
+ ret = -EFAULT;
+ } else {
+ buf->length -= ret;
+ buf->data += ret;
+ }
+ return ret;
+}
+
+/* Assumes epfile->mutex is held. */
+static ssize_t __ffs_epfile_read_data(struct ffs_epfile *epfile,
+ void *data, int data_len,
+ struct iov_iter *iter)
+{
+ struct ffs_buffer *buf;
+
+ ssize_t ret = copy_to_iter(data, data_len, iter);
+ if (likely(data_len == ret))
+ return ret;
+
+ if (unlikely(iov_iter_count(iter)))
+ return -EFAULT;
+
+ /* See ffs_copy_to_iter for more context. */
+ pr_warn("functionfs read size %d > requested size %zd, splitting request into multiple reads.",
+ data_len, ret);
+
+ data_len -= ret;
+ buf = kmalloc(sizeof(*buf) + data_len, GFP_KERNEL);
+ buf->length = data_len;
+ buf->data = buf->storage;
+ memcpy(buf->storage, data + ret, data_len);
+ epfile->read_buffer = buf;
+
+ return ret;
+}
+
static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
{
struct ffs_epfile *epfile = file->private_data;
@@ -746,21 +813,40 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
if (halt && epfile->isoc)
return -EINVAL;

+ /* We will be using request and read_buffer */
+ ret = ffs_mutex_lock(&epfile->mutex, file->f_flags & O_NONBLOCK);
+ if (unlikely(ret))
+ goto error;
+
/* Allocate & copy */
if (!halt) {
+ struct usb_gadget *gadget;
+
+ /*
+ * Do we have buffered data from previous partial read? Check
+ * that for synchronous case only because we do not have
+ * facility to ‘wake up’ a pending asynchronous read and push
+ * buffered data to it which we would need to make things behave
+ * consistently.
+ */
+ if (!io_data->aio && io_data->read) {
+ ret = __ffs_epfile_read_buffered(epfile, &io_data->data);
+ if (ret)
+ goto error_mutex;
+ }
+
/*
* if we _do_ wait above, the epfile->ffs->gadget might be NULL
* before the waiting completes, so do not assign to 'gadget'
* earlier
*/
- struct usb_gadget *gadget = epfile->ffs->gadget;
- size_t copied;
+ gadget = epfile->ffs->gadget;

spin_lock_irq(&epfile->ffs->eps_lock);
/* In the meantime, endpoint got disabled or changed. */
if (epfile->ep != ep) {
- spin_unlock_irq(&epfile->ffs->eps_lock);
- return -ESHUTDOWN;
+ ret = -ESHUTDOWN;
+ goto error_lock;
}
data_len = iov_iter_count(&io_data->data);
/*
@@ -772,22 +858,17 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
spin_unlock_irq(&epfile->ffs->eps_lock);

data = kmalloc(data_len, GFP_KERNEL);
- if (unlikely(!data))
- return -ENOMEM;
- if (!io_data->read) {
- copied = copy_from_iter(data, data_len, &io_data->data);
- if (copied != data_len) {
- ret = -EFAULT;
- goto error;
- }
+ if (unlikely(!data)) {
+ ret = -ENOMEM;
+ goto error_mutex;
+ }
+ if (!io_data->read &&
+ copy_from_iter(data, data_len, &io_data->data) != data_len) {
+ ret = -EFAULT;
+ goto error_mutex;
}
}

- /* We will be using request */
- ret = ffs_mutex_lock(&epfile->mutex, file->f_flags & O_NONBLOCK);
- if (unlikely(ret))
- goto error;
-
spin_lock_irq(&epfile->ffs->eps_lock);

if (epfile->ep != ep) {
@@ -843,8 +924,8 @@ static ssize_t ffs_epfile_io(struct file *file, struct ffs_io_data *io_data)
if (interrupted)
ret = -EINTR;
else if (io_data->read && ep->status > 0)
- ret = ffs_copy_to_iter(data, ep->status,
- &io_data->data);
+ ret = __ffs_epfile_read_data(epfile, data, ep->status,
+ &io_data->data);
else
ret = ep->status;
goto error_mutex;
@@ -1012,6 +1093,8 @@ ffs_epfile_release(struct inode *inode, struct file *file)

ENTER();

+ kfree(epfile->read_buffer);
+ epfile->read_buffer = NULL;
ffs_data_closed(epfile->ffs);

return 0;
@@ -1637,19 +1720,24 @@ static void ffs_func_eps_disable(struct ffs_function *func)
unsigned count = func->ffs->eps_count;
unsigned long flags;

- spin_lock_irqsave(&func->ffs->eps_lock, flags);
do {
+ if (epfile)
+ mutex_lock(&epfile->mutex);
+ spin_lock_irqsave(&func->ffs->eps_lock, flags);
/* pending requests get nuked */
if (likely(ep->ep))
usb_ep_disable(ep->ep);
++ep;
+ spin_unlock_irqrestore(&func->ffs->eps_lock, flags);

if (epfile) {
epfile->ep = NULL;
+ kfree(epfile->read_buffer);
+ epfile->read_buffer = NULL;
+ mutex_unlock(&epfile->mutex);
++epfile;
}
} while (--count);
- spin_unlock_irqrestore(&func->ffs->eps_lock, flags);
}

static int ffs_func_eps_enable(struct ffs_function *func)
--
2.8.0.rc3.226.g39d4020
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