| Message ID | 21902.1433166736@warthog.procyon.org.uk (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon, Jun 1, 2015 at 3:52 PM, David Howells <dhowells@redhat.com> wrote: > In ovl_dentry_open(), ovl_drop_write() is called after vfs_open() - but is > this actually necessary? Can't we just drop it post-copyup? After all, > that's all we wanted the write lock for, right? Hmm, that could result in a race where remount r/o of upper fs comes in between copy-up and vfs_open() so copy-up succeeds but the actual open fails. It's harmless, though, and not very likely. So I guess your patch is OK. Thanks, Miklos > > David > --- > --- a/fs/overlayfs/inode.c > +++ b/fs/overlayfs/inode.c > @@ -356,16 +356,14 @@ static int ovl_dentry_open(struct dentry *dentry, struct inode *inode, > err = ovl_copy_up_last(dentry, NULL, true); > else > err = ovl_copy_up(dentry); > + ovl_drop_write(dentry); > if (err) > - goto out_drop_write; > + goto out; > > ovl_path_upper(dentry, &realpath); > } > > err = vfs_open(&realpath, d_backing_inode(realpath.dentry), file, cred); > -out_drop_write: > - if (want_write) > - ovl_drop_write(dentry); > out: > return err; > } -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Miklos Szeredi <miklos@szeredi.hu> wrote: > > In ovl_dentry_open(), ovl_drop_write() is called after vfs_open() - but is > > this actually necessary? Can't we just drop it post-copyup? After all, > > that's all we wanted the write lock for, right? > > Hmm, that could result in a race where remount r/o of upper fs comes > in between copy-up and vfs_open() so copy-up succeeds but the actual > open fails. It's harmless, though, and not very likely. So I guess > your patch is OK. That race is there anyway if there's no copy up, right? David -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, Jun 1, 2015 at 5:45 PM, David Howells <dhowells@redhat.com> wrote: > Miklos Szeredi <miklos@szeredi.hu> wrote: > >> > In ovl_dentry_open(), ovl_drop_write() is called after vfs_open() - but is >> > this actually necessary? Can't we just drop it post-copyup? After all, >> > that's all we wanted the write lock for, right? >> >> Hmm, that could result in a race where remount r/o of upper fs comes >> in between copy-up and vfs_open() so copy-up succeeds but the actual >> open fails. It's harmless, though, and not very likely. So I guess >> your patch is OK. > > That race is there anyway if there's no copy up, right? No. The race I'm talking about is that with your patch it's possible that the file will be copied up, but open will return -EROFS. Without your patch, that is not possible since holding write counter for the mnt over both the copy-up and the open ensures that the filesystem cannot become read-only in the middle. So your patch changes behavior, but the new behavior is acceptable, because there's no major change in semantics (it should only be detectable by the increased disk usage in the rare case of the failed open). Thanks, Miklos -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Miklos Szeredi <miklos@szeredi.hu> wrote: > >> Hmm, that could result in a race where remount r/o of upper fs comes > >> in between copy-up and vfs_open() so copy-up succeeds but the actual > >> open fails. It's harmless, though, and not very likely. So I guess > >> your patch is OK. > > > > That race is there anyway if there's no copy up, right? > > No. The race I'm talking about is that with your patch it's possible > that the file will be copied up, but open will return -EROFS. Ah, I see what you're getting at. > Without your patch, that is not possible since holding write counter > for the mnt over both the copy-up and the open ensures that the > filesystem cannot become read-only in the middle. > > So your patch changes behavior, but the new behavior is acceptable, > because there's no major change in semantics (it should only be > detectable by the increased disk usage in the rare case of the failed > open). Okay. David -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
--- a/fs/overlayfs/inode.c +++ b/fs/overlayfs/inode.c @@ -356,16 +356,14 @@ static int ovl_dentry_open(struct dentry *dentry, struct inode *inode, err = ovl_copy_up_last(dentry, NULL, true); else err = ovl_copy_up(dentry); + ovl_drop_write(dentry); if (err) - goto out_drop_write; + goto out; ovl_path_upper(dentry, &realpath); } err = vfs_open(&realpath, d_backing_inode(realpath.dentry), file, cred); -out_drop_write: - if (want_write) - ovl_drop_write(dentry); out: return err; }