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[PATCH 5/8] staging: comedi: allow buffer wraparound in comedi_write()

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`comedi_write()` copies data from the user buffer to the acquisition
data buffer, which is cyclic, using a single call to `copy_from_user()`.
It currently avoids having to deal with wraparound of the cyclic buffer
by limiting the amount it copies (and the amount returned to the user).
Change it to deal with the wraparound using two calls to
`copy_from_user()` if necessary.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
---
 drivers/staging/comedi/comedi_fops.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
index 8c784c4..4dd4289 100644
--- a/drivers/staging/comedi/comedi_fops.c
+++ b/drivers/staging/comedi/comedi_fops.c
@@ -2346,6 +2346,7 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 	add_wait_queue(&async->wait_head, &wait);
 	while (count == 0 && !retval) {
 		unsigned runflags;
+		unsigned int wp, n1, n2;
 
 		set_current_state(TASK_INTERRUPTIBLE);
 
@@ -2360,9 +2361,6 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 		/* Allocate all free buffer space. */
 		comedi_buf_write_alloc(s, async->prealloc_bufsz);
 		m = comedi_buf_write_n_allocated(s);
-		/* Avoid buffer wraparound. */
-		if (async->buf_write_ptr + m > async->prealloc_bufsz)
-			m = async->prealloc_bufsz - async->buf_write_ptr;
 		n = min_t(size_t, m, nbytes);
 
 		if (n == 0) {
@@ -2388,8 +2386,14 @@ static ssize_t comedi_write(struct file *file, const char __user *buf,
 			continue;
 		}
 
-		m = copy_from_user(async->prealloc_buf + async->buf_write_ptr,
-				   buf, n);
+		wp = async->buf_write_ptr;
+		n1 = min(n, async->prealloc_bufsz - wp);
+		n2 = n - n1;
+		m = copy_from_user(async->prealloc_buf + wp, buf, n1);
+		if (m)
+			m += n2;
+		else if (n2)
+			m = copy_from_user(async->prealloc_buf, buf + n1, n2);
 		if (m) {
 			n -= m;
 			retval = -EFAULT;
-- 
2.6.2

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[PATCH 0/8] staging: comedi: some comedi_write() changes Ian Abbott <abbotti@mev.co.uk> - 2015-11-18 19:00 +0100
  [PATCH 1/8] staging: comedi: rearrange comedi_write() code Ian Abbott <abbotti@mev.co.uk> - 2015-11-18 19:00 +0100
    RE: [PATCH 1/8] staging: comedi: rearrange comedi_write() code Hartley Sweeten <HartleyS@visionengravers.com> - 2015-11-19 18:10 +0100
  [PATCH 5/8] staging: comedi: allow buffer wraparound in comedi_write() Ian Abbott <abbotti@mev.co.uk> - 2015-11-18 19:00 +0100
  [PATCH 8/8] staging: comedi: check for more errors for zero-length write Ian Abbott <abbotti@mev.co.uk> - 2015-11-18 19:00 +0100
  [PATCH 4/8] staging: comedi: avoid bad truncation of a size_t in comedi_write() Ian Abbott <abbotti@mev.co.uk> - 2015-11-18 19:00 +0100
  [PATCH 6/8] staging: comedi: return error on "write" if no command set up Ian Abbott <abbotti@mev.co.uk> - 2015-11-18 19:00 +0100
  [PATCH 7/8] staging: comedi: simplify returned errors for comedi_write() Ian Abbott <abbotti@mev.co.uk> - 2015-11-18 19:00 +0100
  RE: [PATCH 0/8] staging: comedi: some comedi_write() changes Hartley Sweeten <HartleyS@visionengravers.com> - 2015-11-19 18:40 +0100

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