Re: [RFC PATCH 1/1] idr: do not create idr if new id would be outside given range

[Matthew Wilcox <willy@infradead.org>]

On Thu, Nov 27, 2025 at 02:11:02PM +0000, Matthew Wilcox wrote:
> Hm.  That's not what it does for me.  It gives me id == 1, which isn't
> correct!  I'll look into that, but it'd be helpful to know what
> combination of inputs gives us 2.

Oh, never mind, I see what's happening.

int idr_alloc(struct idr *idr, void *ptr, int start, int end, gfp_t gfp)

        ret = idr_alloc_u32(idr, ptr, &id, end > 0 ? end - 1 : INT_MAX, gfp);
so it's passing 0 as 'max' to idr_alloc_u32() which does:

        slot = idr_get_free(&idr->idr_rt, &iter, gfp, max - base);

and max - base becomes -1 or rather ULONG_MAX, and so we'll literally
allocate any number.  If the first slot is full, we'll get back 1
and then add 'base' to it, giving 2.

Here's the new test-case:

+void idr_alloc2_test(void)
+{
+       int id;
+       struct idr idr = IDR_INIT_BASE(idr, 1);
+
+       id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL);
+       assert(id == -ENOSPC);
+
+       id = idr_alloc(&idr, idr_alloc2_test, 1, 2, GFP_KERNEL);
+       assert(id == 1);
+
+       id = idr_alloc(&idr, idr_alloc2_test, 0, 1, GFP_KERNEL);
+       assert(id == -ENOSPC);
+
+       id = idr_alloc(&idr, idr_alloc2_test, 0, 2, GFP_KERNEL);
+       assert(id == -ENOSPC);
+
+       idr_destroy(&idr);
+}

and with this patch, it passes:

+++ b/lib/idr.c
@@ -40,6 +40,8 @@ int idr_alloc_u32(struct idr *idr, void *ptr, u32 *nextid,

        if (WARN_ON_ONCE(!(idr->idr_rt.xa_flags & ROOT_IS_IDR)))
                idr->idr_rt.xa_flags |= IDR_RT_MARKER;
+       if (max < base)
+               return -ENOSPC;

        id = (id < base) ? 0 : id - base;
        radix_tree_iter_init(&iter, id);