bpf: Fix bpf_throw warning on 32-bit arch

On 32-bit architectures, the pointer width is 32-bit, while we try to
cast from a u64 down to it, the compiler complains on mismatch in
integer size. Fix this by first casting to long which should match
the pointer width on targets supported by Linux.

Fixes: ec5290a ("bpf: Prevent KASAN false positive with bpf_throw")
Reported-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Tested-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230918155233.297024-3-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -2488,7 +2488,7 @@ __bpf_kfunc void bpf_throw(u64 cookie)
 	 * deeper stack depths than ctx.sp as we do not return from bpf_throw,
 	 * which skips compiler generated instrumentation to do the same.
 	 */
-	kasan_unpoison_task_stack_below((void *)ctx.sp);
+	kasan_unpoison_task_stack_below((void *)(long)ctx.sp);
 	ctx.aux->bpf_exception_cb(cookie, ctx.sp, ctx.bp);
 	WARN(1, "A call to BPF exception callback should never return\n");
 }