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lookup_open(): don't bother with fallbacks to lookup+create

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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>
This commit is contained in:
Al Viro 2020-03-12 14:07:27 -04:00
parent d489cf9a3e
commit 99a4a90c8e
1 changed files with 9 additions and 25 deletions
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34
fs/namei.c
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@ -2939,9 +2939,6 @@ 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;
@ -3038,32 +3035,20 @@ 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))
@ -3071,7 +3056,6 @@ 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);