objpool: fix choosing allocation for percpu slots

objpool intends to use vmalloc for default (non-atomic) allocations of
percpu slots and objects. However, the condition checking if GFP flags
set any bit of GFP_ATOMIC is wrong b/c GFP_ATOMIC is a combination of bits
(__GFP_HIGH|__GFP_KSWAPD_RECLAIM) and so `pool->gfp & GFP_ATOMIC` will
be true if either bit is set. Since GFP_ATOMIC and GFP_KERNEL share the
___GFP_KSWAPD_RECLAIM bit, kmalloc will be used in cases when GFP_KERNEL
is specified, i.e. in all current usages of objpool.

This may lead to unexpected OOM errors since kmalloc cannot allocate
large amounts of memory.

For instance, objpool is used by fprobe rethook which in turn is used by
BPF kretprobe.multi and kprobe.session probe types. Trying to attach
these to all kernel functions with libbpf using

    SEC("kprobe.session/*")
    int kprobe(struct pt_regs *ctx)
    {
        [...]
    }

fails on objpool slot allocation with ENOMEM.

Fix the condition to truly use vmalloc by default.

Link: https://lore.kernel.org/all/[email protected]/

Fixes: b4edb8d ("lib: objpool added: ring-array based lockless MPMC")
Signed-off-by: Viktor Malik <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Reviewed-by: Matt Wu <[email protected]>
Signed-off-by: Masami Hiramatsu (Google) <[email protected]>

lib/objpool.c

@@ -76,7 +76,7 @@ objpool_init_percpu_slots(struct objpool_head *pool, int nr_objs,
 		 * mimimal size of vmalloc is one page since vmalloc would
 		 * always align the requested size to page size
 		 */
-		if (pool->gfp & GFP_ATOMIC)
+		if ((pool->gfp & GFP_ATOMIC) == GFP_ATOMIC)
 			slot = kmalloc_node(size, pool->gfp, cpu_to_node(i));
 		else
 			slot = __vmalloc_node(size, sizeof(void *), pool->gfp,