io_uring: only test SQPOLL cpu after we've verified it

We currently call cpu_possible() even if we don't use the CPU. Move the
test under the SQ_AFF branch, which is the only place where we'll use
the value. Do the cpu_possible() test AFTER we've limited it to a max
of NR_CPUS. This avoids triggering the following warning:

WARNING: CPU: 1 PID: 7600 at include/linux/cpumask.h:121 cpu_max_bits_warn

if CONFIG_DEBUG_PER_CPU_MAPS is enabled.

While in there, also move the SQ thread idle period assignment inside
SETUP_SQPOLL, as we don't use it otherwise either.

Reported-by: syzbot+cd714a07c6de2bc34293@syzkaller.appspotmail.com
Fixes: 6c271ce2f1d5 ("io_uring: add submission polling")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 8 insertions, 4 deletions

diff --git a/fs/io_uring.c b/fs/io_uring.c
index e5008c1b82be..24355e0c47f0 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2241,10 +2241,6 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
         mmgrab(current->mm);
         ctx->sqo_mm = current->mm;
 
-        ctx->sq_thread_idle = msecs_to_jiffies(p->sq_thread_idle);
-        if (!ctx->sq_thread_idle)
-                ctx->sq_thread_idle = HZ;
-
         ret = -EINVAL;
         if (!cpu_possible(p->sq_thread_cpu))
                 goto err;
@@ -2254,10 +2250,18 @@ static int io_sq_offload_start(struct io_ring_ctx *ctx,
                 if (!capable(CAP_SYS_ADMIN))
                         goto err;
 
+                ctx->sq_thread_idle = msecs_to_jiffies(p->sq_thread_idle);
+                if (!ctx->sq_thread_idle)
+                        ctx->sq_thread_idle = HZ;
+
                 if (p->flags & IORING_SETUP_SQ_AFF) {
                         int cpu;
 
                         cpu = array_index_nospec(p->sq_thread_cpu, NR_CPUS);
+                        ret = -EINVAL;
+                        if (!cpu_possible(p->sq_thread_cpu))
+                                goto err;
+
                         ctx->sqo_thread = kthread_create_on_cpu(io_sq_thread,
                                                         ctx, cpu,
                                                         "io_uring-sq");