[RFC][PATCH v4 69/69] lookup_open(): don't bother with fallbacks to lookup+create

Al Viro
2020-03-13T23:54:50Z

We fall back to lookup+create (instead of atomic_open) in several cases:
1) we don't have write access to filesystem and O_TRUNC is
present in the flags. It's not something we want ->atomic_open() to
see - it just might go ahead and truncate the file. However, we can
pass it the flags sans O_TRUNC - eventually do_open() will call
handle_truncate() anyway.
2) we have O_CREAT | O_EXCL and we can't write to parent.
That's going to be an error, of course, but we want to know _which_
error should that be - might be EEXIST (if file exists), might be
EACCES or EROFS. Simply stripping O_CREAT (and checking if we see
ENOENT) would suffice, if not for O_EXCL. However, we used to have
->atomic_open() fully responsible for rejecting O_CREAT | O_EXCL
on existing file and just stripping O_CREAT would've disarmed
those checks. With nothing downstream to catch the problem -
FMODE_OPENED used to be "don't bother with EEXIST checks,
->atomic_open() has done those". Now EEXIST checks downstream
are skipped only if FMODE_CREATED is set - FMODE_OPENED alone
is not enough. That has eliminated the need to fall back onto
lookup+create path in this case.
3) O_WRONLY or O_RDWR when we have no write access to
filesystem, with nothing else objectionable. Fallback is
(and had always been) pointless.

IOW, we don't really need that fallback; all we need in such
cases is to trim O_TRUNC and O_CREAT properly.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/namei.c | 34 +++++++++-------------------------
1 file changed, 9 insertions(+), 25 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 36b15f5b09bd..f4ac81b60a95 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2938,9 +2938,6 @@ static struct dentry *atomic_open(struct nameidata *nd, struct dentry *dentry,
struct inode *dir = nd->path.dentry->d_inode;
int error;

- if (!(~open_flag & (O_EXCL | O_CREAT))) /* both O_EXCL and O_CREAT */
- open_flag &= ~O_TRUNC;
-
if (nd->flags & LOOKUP_DIRECTORY)
open_flag |= O_DIRECTORY;

@@ -3037,32 +3034,20 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
* Another problem is returing the "right" error value (e.g. for an
* O_EXCL open we want to return EEXIST not EROFS).
*/
+ if (unlikely(!got_write))
+ open_flag &= ~O_TRUNC;
if (open_flag & O_CREAT) {
+ if (open_flag & O_EXCL)
+ open_flag &= ~O_TRUNC;
if (!IS_POSIXACL(dir->d_inode))
mode &= ~current_umask();
- if (unlikely(!got_write)) {
- create_error = -EROFS;
- open_flag &= ~O_CREAT;
- if (open_flag & (O_EXCL | O_TRUNC))
- goto no_open;
- /* No side effects, safe to clear O_CREAT */
- } else {
+ if (likely(got_write))
create_error = may_o_create(&nd->path, dentry, mode);
- if (create_error) {
- open_flag &= ~O_CREAT;
- if (open_flag & O_EXCL)
- goto no_open;
- }
- }
- } else if ((open_flag & (O_TRUNC|O_WRONLY|O_RDWR)) &&
- unlikely(!got_write)) {
- /*
- * No O_CREATE -> atomicity not a requirement -> fall
- * back to lookup + open
- */
- goto no_open;
+ else
+ create_error = -EROFS;
}
-
+ if (create_error)
+ open_flag &= ~O_CREAT;
if (dir_inode->i_op->atomic_open) {
dentry = atomic_open(nd, dentry, file, open_flag, mode);
if (unlikely(create_error) && dentry == ERR_PTR(-ENOENT))
@@ -3070,7 +3055,6 @@ static struct dentry *lookup_open(struct nameidata *nd, struct file *file,
return dentry;
}

-no_open:
if (d_in_lookup(dentry)) {
struct dentry *res = dir_inode->i_op->lookup(dir_inode, dentry,
nd->flags);
--
2.11.0