[PATCH 3/7] maccess: Skip setup for zero-sized kernel nofault copies

[Muhammad Usama Anjum]

A zero-sized kernel nofault copy does not enter an access loop, but it
still disables and re-enables page faults.

Zero sizes are valid. BPF probe-read helpers accept them, and KGDB memory
packets may carry a zero length.

Return before changing page-fault state when there is nothing to copy.
For reads, keep architecture-specific address validation before the fast
path so its behavior is unchanged.

Signed-off-by: Muhammad Usama Anjum <usama.anjum at arm.com>
---
 mm/maccess.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/mm/maccess.c b/mm/maccess.c
index 486559d688583..c59a0e092d24a 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -35,6 +35,8 @@ long copy_from_kernel_nofault(void *dst, const void *src, size_t size)
 
 	if (!copy_from_kernel_nofault_allowed(src, size))
 		return -ERANGE;
+	if (!size)
+		return 0;
 
 	pagefault_disable();
 	if (!(align & 7))
@@ -65,6 +67,9 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
 {
 	unsigned long align = 0;
 
+	if (!size)
+		return 0;
+
 	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS))
 		align = (unsigned long)dst | (unsigned long)src;
 
-- 
2.47.3