> On Jul 7, 2016, at 12:52, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote: > > > On Jul 7, 2016, at 12:16 PM, Trond Myklebust wrote: > >> >>> On Jul 7, 2016, at 01:53, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote: >>> >>> It's great when we can shave an extra RPC, but not at the expense >>> of correctness. >>> We should not return EPERM (from vfs_create/mknod/mkdir) if the >>> name already exists, even if we have no write access in parent. >>> >>> Since the check in nfs_permission is clearly not enough to stave >>> off this, just throw in the extra READ access to actually >>> go through. >>> >>> Signed-off-by: Oleg Drokin <green@xxxxxxxxxxxxxx> >>> --- >>> fs/nfs/dir.c | 4 +++- >>> 1 file changed, 3 insertions(+), 1 deletion(-) >>> >>> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c >>> index d8015a0..8c7835b 100644 >>> --- a/fs/nfs/dir.c >>> +++ b/fs/nfs/dir.c >>> @@ -1383,8 +1383,10 @@ struct dentry *nfs_lookup(struct inode *dir, struct dentry * dentry, unsigned in >>> /* >>> * If we're doing an exclusive create, optimize away the lookup >>> * but don't hash the dentry. >>> + * This optimization only works if we can write in the parent. >>> */ >>> - if (nfs_is_exclusive_create(dir, flags)) >>> + if (nfs_is_exclusive_create(dir, flags) && >>> + (inode_permission(dir, MAY_WRITE | MAY_READ | MAY_EXEC) == 0)) >>> return NULL; >>> >> >> NACK. The only write permission we should care about on the client side is whether or not the filesystem is mounted read-only. All other permissions are checked by the server. > > Right. This was mostly a discussion piece. > The problem here is nfs_permission() returns 0 if you check for > inode_permission(dir, MAY_WRITE | MAY_EXEC) (as in may_create), but then > some other checks in the kernel still catch on to the fact that the directory > is not writeable, so we have a premature failure with EPERM and server never sees > this request which breaks things. Are these VFS level checks? Which ones? > > (the read-only mount is not handled as well at the moment of course and my patch > does not address this issue either, but it's easier to address in the VFS, like > in filename_create() or something). > > I see that two major consumers of this nfs_permission MAY_WRITE|!MAY_READ check > are creates and deletes and with deletes we had a lookup already, so it already > looked up the child and revalidated the parent. > For creates, a revalidation still might be needed, I guess and that was the main driver > behind this check? And that only when you do current dir creates, because otherwise > the parent would have been revalidated in lookup? > Is this the major case why that check is actually there? > > Just trying to see how to approach this better without breaking the applications. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
