-----Original Message-----
From: Justin He
Sent: Friday, June 25, 2021 5:18 PM
To: Al Viro <viro@zeniv.linux.org.uk>; Linus Torvalds <torvalds@linux-
foundation.org>
Cc: Petr Mladek <pmladek@suse.com>; Steven Rostedt <rostedt@goodmis.org>;
Sergey Senozhatsky [off-list ref]; Andy Shevchenko
[off-list ref]; Rasmus Villemoes
[off-list ref]; Jonathan Corbet [off-list ref]; Heiko
Carstens [off-list ref]; Vasily Gorbik [off-list ref]; Christian
Borntraeger [off-list ref]; Eric W . Biederman
[off-list ref]; Darrick J. Wong [off-list ref]; Peter
Zijlstra (Intel) [off-list ref]; Ira Weiny [off-list ref];
Eric Biggers [off-list ref]; Ahmed S. Darwish
[off-list ref]; open list:DOCUMENTATION <linux-
doc@vger.kernel.org>; Linux Kernel Mailing List <linux-
kernel@vger.kernel.org>; linux-s390 [off-list ref]; linux-
fsdevel [off-list ref]
Subject: RE: [PATCH 07/14] d_path: lift -ENAMETOOLONG handling into callers
of prepend_path()

Hi Al

quoted

-----Original Message-----
From: Al Viro <redacted> On Behalf Of Al Viro
Sent: Wednesday, May 19, 2021 8:49 AM
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Justin He <redacted>; Petr Mladek <pmladek@suse.com>; Steven
Rostedt [off-list ref]; Sergey Senozhatsky
[off-list ref]; Andy Shevchenko
[off-list ref]; Rasmus Villemoes
[off-list ref]; Jonathan Corbet [off-list ref]; Heiko
Carstens [off-list ref]; Vasily Gorbik [off-list ref];

Christian

quoted

Borntraeger [off-list ref]; Eric W . Biederman
[off-list ref]; Darrick J. Wong [off-list ref]; Peter
Zijlstra (Intel) [off-list ref]; Ira Weiny [off-list ref];
Eric Biggers [off-list ref]; Ahmed S. Darwish
[off-list ref]; open list:DOCUMENTATION <linux-
doc@vger.kernel.org>; Linux Kernel Mailing List <linux-
kernel@vger.kernel.org>; linux-s390 [off-list ref]; linux-
fsdevel [off-list ref]
Subject: [PATCH 07/14] d_path: lift -ENAMETOOLONG handling into callers

of

quoted

prepend_path()

The only negative value ever returned by prepend_path() is -ENAMETOOLONG
and callers can recognize that situation (overflow) by looking at the
sign of buflen.  Lift that into the callers; we already have the
same logics (buf if buflen is non-negative, ERR_PTR(-ENAMETOOLONG)
otherwise)
in several places and that'll become a new primitive several commits down
the road.

Make prepend_path() return 0 instead of -ENAMETOOLONG.  That makes for
saner calling conventions (0/1/2/3/-ENAMETOOLONG is obnoxious) and
callers actually get simpler, especially once the aforementioned
primitive gets added.

In prepend_path() itself we switch prepending the / (in case of
empty path) to use of prepend() - no need to open-code that, compiler
will do the right thing.  It's exactly the same logics as in
__dentry_path().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/d_path.c | 39 +++++++++++----------------------------
 1 file changed, 11 insertions(+), 28 deletions(-)

diff --git a/fs/d_path.c b/fs/d_path.c
index 72b8087aaf9c..327cc3744554 100644
--- a/fs/d_path.c
+++ b/fs/d_path.c

@@ -127,8 +127,7 @@ static int prepend_path(const struct path *path,
 		}
 		parent = dentry->d_parent;
 		prefetch(parent);
-		error = prepend_name(&bptr, &blen, &dentry->d_name);
-		if (error)
+		if (unlikely(prepend_name(&bptr, &blen, &dentry->d_name) < 0))
 			break;

 		dentry = parent;

@@ -149,12 +148,9 @@ static int prepend_path(const struct path *path,
 	}
 	done_seqretry(&mount_lock, m_seq);

-	if (error >= 0 && bptr == *buffer) {
-		if (--blen < 0)
-			error = -ENAMETOOLONG;
-		else
-			*--bptr = '/';
-	}
+	if (blen == *buflen)
+		prepend(&bptr, &blen, "/", 1);
+
 	*buffer = bptr;
 	*buflen = blen;
 	return error;

@@ -181,16 +177,11 @@ char *__d_path(const struct path *path,
 	       char *buf, int buflen)
 {
 	char *res = buf + buflen;
-	int error;

 	prepend(&res, &buflen, "", 1);
-	error = prepend_path(path, root, &res, &buflen);
-
-	if (error < 0)
-		return ERR_PTR(error);
-	if (error > 0)
+	if (prepend_path(path, root, &res, &buflen) > 0)
 		return NULL;
-	return res;
+	return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);
 }

 char *d_absolute_path(const struct path *path,

@@ -198,16 +189,11 @@ char *d_absolute_path(const struct path *path,
 {
 	struct path root = {};
 	char *res = buf + buflen;
-	int error;

 	prepend(&res, &buflen, "", 1);
-	error = prepend_path(path, &root, &res, &buflen);
-
-	if (error > 1)
-		error = -EINVAL;
-	if (error < 0)
-		return ERR_PTR(error);
-	return res;
+	if (prepend_path(path, &root, &res, &buflen) > 1)
+		return ERR_PTR(-EINVAL);
+	return buflen >= 0 ? res : ERR_PTR(-ENAMETOOLONG);

This patch is *correct*.
But do you mind changing like:
if (buflen >= 0 || error == 1)
	return res;
else
	return ERR_PTR(-ENAMETOOLONG);

The reason why I comment here is that I will change the
prepend_name in __prepend_path to prepend_name_with_len.
The latter will go through all the dentries recursively instead
of returning false if p.len<0.
So (error == 1 && buflen < 0) is possible.

Please ignore it, this is not relevant to this patch itself, I can
draft a new patch if it is needed.

Hence:
Reviewed-by: Jia He <redacted>