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Re: [Qemu-devel] [PULL v2 07/49] util: check the return value of fcntl i

From: Brad Smith
Subject: Re: [Qemu-devel] [PULL v2 07/49] util: check the return value of fcntl in qemu_set_{block, nonblock}
Date: Fri, 25 Jan 2019 14:04:15 -0500
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0

On 1/25/2019 1:53 PM, Philippe Mathieu-Daudé wrote:


On 1/15/19 9:04 PM, Michael S. Tsirkin wrote:
From: Li Qiang <address@hidden>

Assert that the return value is not an error. This is like commit
7e6478e7d4f for qemu_set_cloexec.

Signed-off-by: Li Qiang <address@hidden>
Reviewed-by: Thomas Huth <address@hidden>
Reviewed-by: Michael S. Tsirkin <address@hidden>
Signed-off-by: Michael S. Tsirkin <address@hidden>
  util/oslib-posix.c | 8 ++++++--
  1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/util/oslib-posix.c b/util/oslib-posix.c
index c1bee2a581..4ce1ba9ca4 100644
--- a/util/oslib-posix.c
+++ b/util/oslib-posix.c
@@ -233,14 +233,18 @@ void qemu_set_block(int fd)
      int f;
      f = fcntl(fd, F_GETFL);
-    fcntl(fd, F_SETFL, f & ~O_NONBLOCK);
+    assert(f != -1);
+    f = fcntl(fd, F_SETFL, f & ~O_NONBLOCK);
+    assert(f != -1);
void qemu_set_nonblock(int fd)
      int f;
      f = fcntl(fd, F_GETFL);
-    fcntl(fd, F_SETFL, f | O_NONBLOCK);
+    assert(f != -1);
+    f = fcntl(fd, F_SETFL, f | O_NONBLOCK);
+    assert(f != -1);
This commit breaks OpenBSD, when trying to start QEMU I get:
assertion "f != -1" failed: file "util/oslib-posix.c", line 247,
function "qemu_set_nonblock"

Having a quick look at gdb, the last device opened is /dev/null, and
when fcntl() fails we have errno = ENODEV.

     19 ENODEV Operation not supported by device.
     An attempt was made to apply an inappropriate function to a device,
     for example, trying to read a write-only device such as a printer.

Digging further I found a recent commit which could fix this problem:
"fcntl(F_SETFL) invokes the FIONBIO and FIOASYNC ioctls internally, so
the memory devices (/dev/null, /dev/zero, etc) need to permit them."

Brad: Do you think this might be the fix? If so, any idea what is the
first release to contain this fix? I don't know OpenBSD and can't figure
this out... Also, what would be the cleaner QEMU fix?

I don't know. But that commit was included with 6.3 or newer.

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