The r241025 fixed the case when a binary, executed from nullfs mount,

was still possible to open for write from the lower filesystem.  There
is a symmetric situation where the binary could already has file
descriptors opened for write, but it can be executed from the nullfs
overlay.

Handle the issue by passing one v_writecount reference to the lower
vnode if nullfs vnode has non-zero v_writecount.  Note that only one
write reference can be donated, since nullfs only keeps one use
reference on the lower vnode.  Always use the lower vnode v_writecount
for the checks.

Introduce the VOP_GET_WRITECOUNT to read v_writecount, which is
currently always bypassed to the lower vnode, and VOP_ADD_WRITECOUNT
to manipulate the v_writecount value, which manages a single bypass
reference to the lower vnode.  Caling the VOPs instead of directly
accessing v_writecount provide the fix described in the previous
paragraph.

Tested by:	pho
MFC after:	3 weeks

1 files changed, 5 insertions, 3 deletions

diff --git a/sys/compat/linux/linux_misc.c b/sys/compat/linux/linux_misc.c
index 6da3b19..7587272 100644
--- a/sys/compat/linux/linux_misc.c
+++ b/sys/compat/linux/linux_misc.c
@@ -246,8 +246,7 @@ linux_uselib(struct thread *td, struct linux_uselib_args *args)
 	unsigned long bss_size;
 	char *library;
 	ssize_t aresid;
-	int error;
-	int locked;
+	int error, locked, writecount;
 
 	LCONVPATHEXIST(td, args->library, &library);
 
@@ -277,7 +276,10 @@ linux_uselib(struct thread *td, struct linux_uselib_args *args)
 	locked = 1;
 
 	/* Writable? */
-	if (vp->v_writecount) {
+	error = VOP_GET_WRITECOUNT(vp, &writecount);
+	if (error != 0)
+		goto cleanup;
+	if (writecount != 0) {
 		error = ETXTBSY;
 		goto cleanup;
 	}