[PATCH master] fs: /dev/mem: handle copy at offset 0 correctly

[Ahmad Fatoum]

Despite that /dev/mem has a size of 0, the check preventing
out-of-bounds access works for /dev/mem as well, because of unsigned
wrap around. Corner case is when offset is zero. There will be no wrap
around and

  count = min(count, 0 - 0) = 0

Leading to unexpected behavior with e.g.

  memcmp -s /dev/mem 0

on systems without MMU. Fix this.

Signed-off-by: Ahmad Fatoum <ahmad at a3f.at>
---
 fs/devfs-core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/devfs-core.c b/fs/devfs-core.c
index 3715e543e632..2d016e0e4861 100644
--- a/fs/devfs-core.c
+++ b/fs/devfs-core.c
@@ -573,7 +573,10 @@ ssize_t mem_copy(struct device_d *dev, void *dst, const void *src,
 	if (!dev || dev->num_resources < 1)
 		return -1;
 
-	count = size = min(count, resource_size(&dev->resource[0]) - offset);
+	if (resource_size(&dev->resource[0]) > 0 || offset != 0)
+		count = min(count, resource_size(&dev->resource[0]) - offset);
+
+	size = count;
 
 	/* no rwsize specification given. Do whatever memcpy likes best */
 	if (!rwsize) {
-- 
2.33.0