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Subject[PATCH 1/4] Add path resolution flag LOOKUP_NEVER_FOLLOW
Date
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
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