[PATCH 03/14] d_path: regularize handling of root dentry in __dentry_path()

Al Viro
2021-05-19T00:51:14Z

All path-forming primitives boil down to sequence of prepend_name()
on dentries encountered along the way toward root. Each time we prepend
/ + dentry name to the buffer. Normally that does exactly what we want,
but there's a corner case when we don't call prepend_name() at all (in case
of __dentry_path() that happens if we are given root dentry). We obviously
want to end up with "/", rather than "", so this corner case needs to be
handled.

__dentry_path() used to manually put '/' in the end of buffer before
doing anything else, to be overwritten by the first call of prepend_name()
if one happens and to be left in place if we don't call prepend_name() at
all. That required manually checking that we had space in the buffer
(prepend_name() and prepend() take care of such checks themselves) and lead
to clumsy keeping track of return value.

A better approach is to check if the main loop has added anything
into the buffer and prepend "/" if it hasn't. A side benefit of using prepend()
is that it does the right thing if we'd already run out of buffer, making
the overflow-handling logics simpler.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/d_path.c | 21 +++++----------------
1 file changed, 5 insertions(+), 16 deletions(-)

diff --git a/fs/d_path.c b/fs/d_path.c
index 1a1cf05e7780..b3324ae7cfe2 100644
--- a/fs/d_path.c
+++ b/fs/d_path.c
@@ -329,31 +329,22 @@ char *simple_dname(struct dentry *dentry, char *buffer, int buflen)
static char *__dentry_path(const struct dentry *d, char *p, int buflen)
{
const struct dentry *dentry;
- char *end, *retval;
+ char *end;
int len, seq = 0;
- int error = 0;
-
- if (buflen < 1)
- goto Elong;

rcu_read_lock();
restart:
dentry = d;
end = p;
len = buflen;
- /* Get '/' right */
- retval = end-1;
- *retval = '/';
read_seqbegin_or_lock(&rename_lock, &seq);
while (!IS_ROOT(dentry)) {
const struct dentry *parent = dentry->d_parent;

prefetch(parent);
- error = prepend_name(&end, &len, &dentry->d_name);
- if (error)
+ if (unlikely(prepend_name(&end, &len, &dentry->d_name) < 0))
break;

- retval = end;
dentry = parent;
}
if (!(seq & 1))
@@ -363,11 +354,9 @@ static char *__dentry_path(const struct dentry *d, char *p, int buflen)
goto restart;
}
done_seqretry(&rename_lock, seq);
- if (error)
- goto Elong;
- return retval;
-Elong:
- return ERR_PTR(-ENAMETOOLONG);
+ if (len == buflen)
+ prepend(&end, &len, "/", 1);
+ return len >= 0 ? end : ERR_PTR(-ENAMETOOLONG);
}

char *dentry_path_raw(const struct dentry *dentry, char *buf, int buflen)
--
2.11.0