[PATCH] IB/hfi1: fix buffer underflow in fault injection code

[Vitaliy Shevtsov <v.shevtsov@maxima.ru>]

[Why]
The fault injection code may have a buffer underflow, which may cause
memory corruption by writing a newline character before the base address of
the array. This can happen if the fault->opcodes bitmap is empty.

Since a file in debugfs is created with an empty bitmap, it is possible to
read the file before any set bits are written to it.

[How]
Fix this by checking that the size variable is greater than zero, otherwise
return zero as the number of bytes read.

Found by Linux Verification Center (linuxtesting.org) with Svace.

Fixes: a74d5307caba ("IB/hfi1: Rework fault injection machinery")
Signed-off-by: Vitaliy Shevtsov <v.shevtsov@maxima.ru>
---
 drivers/infiniband/hw/hfi1/fault.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/infiniband/hw/hfi1/fault.c b/drivers/infiniband/hw/hfi1/fault.c
index ec9ee59fcf0c..2d87f9c8b89d 100644
--- a/drivers/infiniband/hw/hfi1/fault.c
+++ b/drivers/infiniband/hw/hfi1/fault.c
@@ -190,7 +190,8 @@ static ssize_t fault_opcodes_read(struct file *file, char __user *buf,
 		bit = find_next_bit(fault->opcodes, bitsize, zero);
 	}
 	debugfs_file_put(file->f_path.dentry);
-	data[size - 1] = '\n';
+	if (size)
+		data[size - 1] = '\n';
 	data[size] = '\0';
 	ret = simple_read_from_buffer(buf, len, pos, data, size);
 free_data:
-- 
2.47.1