[PATCH RESEND 1/3] mm/mlock: make a zero length request a no-op whatever the alignment

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

A zero length mlock() or munlock() at an unaligned address is rounded up
to a page, so mlock(addr + 5, 0) locks a page that was never asked for,
and munlock(addr + 5, 0) unlocks one the caller may still rely on.  An
aligned zero length is already a no-op.

Return early for a zero length, before the rounding and before
can_do_mlock().  This is a uapi change: with RLIMIT_MEMLOCK=0 and no
CAP_IPC_LOCK, mlock(addr, 0) returns 0 instead of -EPERM, which is what
munlock(addr, 0) already returns.  man 2 mlock does not say what a zero
length should do, and -EINVAL would break callers that pass one today and
get 0.

Reported-by: Park Tae-sun <ts930@dgu.ac.kr>
Link: https://lore.kernel.org/all/179066531783.50175.13377521828379196177@dgu.ac.kr/
Signed-off-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>
---
 mm/mlock.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/mm/mlock.c b/mm/mlock.c
index 1c70a7829394..673a8433a0a9 100644
--- a/mm/mlock.c
+++ b/mm/mlock.c
@@ -612,6 +612,9 @@ static __must_check int do_mlock(unsigned long start, size_t len,
 
 	start = untagged_addr(start);
 
+	if (!len)
+		return 0;
+
 	if (!can_do_mlock())
 		return -EPERM;
 
@@ -678,6 +681,9 @@ SYSCALL_DEFINE2(munlock, unsigned long, start, size_t, len)
 
 	start = untagged_addr(start);
 
+	if (!len)
+		return 0;
+
 	len = PAGE_ALIGN(len + (offset_in_page(start)));
 	start &= PAGE_MASK;
 
-- 
2.56.0