[UNIKRAFT PATCH 5/7] lib/vfscore: Fix double refcounting bug

[Cristian Banu <cristb@xxxxxxxxx>]

This patch removes the `dref()` line in `sys_open` which would result
in an unbounded growth of the refcount. open() would take two
references: one via dentry_lookup() in namei(), and the other via
dref(); close() would release only one reference via drele().

As to the comment also deleted in this patch, it is true that OSv uses
intrusive_ptr, however it is created with the second argument
add_ref = false, hence the "std::move()" comment below in the original
OSv source code.

Now close() correctly releases the dentry if there are no open files
left.

Signed-off-by: Cristian Banu <cristb@xxxxxxxxx>
---
 lib/vfscore/syscalls.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/vfscore/syscalls.c b/lib/vfscore/syscalls.c
index 9a132b7..ce51274 100644
--- a/lib/vfscore/syscalls.c
+++ b/lib/vfscore/syscalls.c
@@ -205,8 +205,10 @@ sys_open(char *path, int flags, mode_t mode, struct 
vfscore_file **fpp)
        fhold(fp);
        fp->f_flags = flags;
 
-       // OSv was using a intrusive_ptr which was increasing the refcount
-       dref(dp);
+       /*
+        * Don't need to increase refcount here, we already hold a reference
+        * to dp from namei().
+        */
        // change to std::move once dp is a dentry_ref
        fp->f_dentry = dp;
        dp = NULL;
-- 
2.26.2