[PATCH 1/4] Add path resolution flag LOOKUP_NEVER_FOLLOW

[demiobenour@gmail.com]

From: "Demi M. Obenour" <demiobenour@gmail.com>

This adds the flag LOOKUP_NEVER_FOLLOW to path resolution, which tells
the code in fs/namei.c to never follow symlinks.  This flag overrides
LOOKUP_FOLLOW, since this makes internal APIs simpler: code can set the
flag without needing to also clear LOOKUP_FOLLOW, which is often set by
default.

This is a prerequisite to adding O_PATHSTATIC, but is also useful for
kernel-internal use.
---
 fs/namei.c            | 4 +++-
 include/linux/namei.h | 2 ++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 914178cdbe94..54fbd2c7ba82 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1720,7 +1720,9 @@ static int pick_link(struct nameidata *nd, struct path *link,
 {
 	int error;
 	struct saved *last;
-	if (unlikely(nd->total_link_count++ >= MAXSYMLINKS)) {
+	const int max_symlinks = (nd->flags & LOOKUP_NEVER_FOLLOW) ?
+		0 : MAXSYMLINKS;
+	if (unlikely(nd->total_link_count++ >= max_symlinks)) {
 		path_to_nameidata(link, nd);
 		return -ELOOP;
 	}
diff --git a/include/linux/namei.h b/include/linux/namei.h
index a78606e8e3df..f065502a653d 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -24,10 +24,12 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
  *  - internal "there are more path components" flag
  *  - dentry cache is untrusted; force a real lookup
  *  - suppress terminal automount
+ *  - never follow symbolic links, even internally
  */
 #define LOOKUP_FOLLOW		0x0001
 #define LOOKUP_DIRECTORY	0x0002
 #define LOOKUP_AUTOMOUNT	0x0004
+#define LOOKUP_NEVER_FOLLOW	0x0008
 
 #define LOOKUP_PARENT		0x0010
 #define LOOKUP_REVAL		0x0020
-- 
2.20.1