linux-kernel - [PATCH] iov_iter: Fix 9p virtio breakage

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Date:   Fri,  2 Nov 2018 17:16:51 +0000
From:   Marc Zyngier <marc.zyngier@....com>
To:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, Will Deacon <will.deacon@....com>
Subject: [PATCH] iov_iter: Fix 9p virtio breakage

When switching to the new iovec accessors, a negation got subtly
dropped, leading to 9p being remarkably broken (here with kvmtool):

[    7.430941] VFS: Mounted root (9p filesystem) on device 0:15.
[    7.432080] devtmpfs: mounted
[    7.432717] Freeing unused kernel memory: 1344K
[    7.433658] Run /virt/init as init process
  Warning: unable to translate guest address 0x7e00902ff000 to host
  Warning: unable to translate guest address 0x7e00902fefc0 to host
  Warning: unable to translate guest address 0x7e00902ff000 to host
  Warning: unable to translate guest address 0x7e008febef80 to host
  Warning: unable to translate guest address 0x7e008febf000 to host
  Warning: unable to translate guest address 0x7e008febef00 to host
  Warning: unable to translate guest address 0x7e008febf000 to host
[    7.436376] Kernel panic - not syncing: Requested init /virt/init failed (error -8).
[    7.437554] CPU: 29 PID: 1 Comm: swapper/0 Not tainted 4.19.0-rc8-02267-g00e23707442a #291
[    7.439006] Hardware name: linux,dummy-virt (DT)
[    7.439902] Call trace:
[    7.440387]  dump_backtrace+0x0/0x148
[    7.441104]  show_stack+0x14/0x20
[    7.441768]  dump_stack+0x90/0xb4
[    7.442425]  panic+0x120/0x27c
[    7.443036]  kernel_init+0xa4/0x100
[    7.443725]  ret_from_fork+0x10/0x18
[    7.444444] SMP: stopping secondary CPUs
[    7.445391] Kernel Offset: disabled
[    7.446169] CPU features: 0x0,23000438
[    7.446974] Memory Limit: none
[    7.447645] ---[ end Kernel panic - not syncing: Requested init /virt/init failed (error -8). ]---

Restoring the missing "!" brings the guest back to life.

Fixes: 00e23707442a ("iov_iter: Use accessor function")
Reported-by: Will Deacon <will.deacon@....com>
Signed-off-by: Marc Zyngier <marc.zyngier@....com>
---
 net/9p/trans_virtio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
index 91981970f542..b1d39cabf125 100644
--- a/net/9p/trans_virtio.c
+++ b/net/9p/trans_virtio.c
@@ -329,7 +329,7 @@ static int p9_get_mapped_pages(struct virtio_chan *chan,
 	if (!iov_iter_count(data))
 		return 0;
 
-	if (iov_iter_is_kvec(data)) {
+	if (!iov_iter_is_kvec(data)) {
 		int n;
 		/*
 		 * We allow only p9_max_pages pinned. We wait for the
-- 
2.19.1