User mode emulation returns wrong value for write(fd, NULL, 0)

[Zhuowei Zhang]

QEMU version: latest master (fcea73709b966a7ded9efa7b106ea50c7fe9025c)
OS version: Ubuntu 14.04.5
Configured with: ../configure --target-list=x86_64-linux-user

QEMU Linux usermode emulation does not handle write() syscalls with zero length and a null pointer correctly: on Linux this returns 0, but in emulation this returns -1.

I ran into this while using an aarch64 abuild-tar from Alpine Linux in user-mode emulation; here's the minimized reproduction test case:

zhuowei@zhuowei-tablet:/tmp$ cat writezerobytes.c
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int main() {
 ssize_t ret = write(STDOUT_FILENO, NULL, 0);
 fprintf(stderr, "write returned %ld\n", ret);
 return 0;
}
zhuowei@zhuowei-tablet:/tmp$ gcc -o writezerobytes writezerobytes.c
zhuowei@zhuowei-tablet:/tmp$ uname -a
Linux zhuowei-tablet 3.13.0-129-generic #178-Ubuntu SMP Fri Aug 11 12:48:20 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
zhuowei@zhuowei-tablet:/tmp$ ./writezerobytes
write returned 0
zhuowei@zhuowei-tablet:/tmp$ /media/zhuowei/redhd/docs/repos/qemu/build4/x86_64-linux-user/qemu-x86_64 ./writezerobytes
write returned -1
zhuowei@zhuowei-tablet:/tmp$ /media/zhuowei/redhd/docs/repos/qemu/build4/x86_64-linux-user/qemu-x86_64 --version
qemu-x86_64 version 2.10.50 (v2.10.0-471-gfcea737-dirty)
Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers

[Tony Garnock-Jones (tonyg)]

This happens for me also, with qemu version 2.12.0 (Debian 1:2.12+dfsg-3).

An initial patch was proposed here: https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg08073.html

Discussion pointed out some problems, and the patch languished and was not accepted.

Here is a summary of the changes needed for it to be more likely for the patch to be accepted: https://lists.gnu.org/archive/html/qemu-devel/2018-02/msg03964.html

 - change from "ret = 0" to something like "ret = get_errno(safe_write(arg1, NULL, 0))"
 - change TARGET_NR_read to do the same, instead of its current short-circuit behaviour for count==0
 - check pread64/pwrite64 to see if they need a similar change as well

[Eric Blake (eblake)]

On 09/07/2018 06:51 AM, Tony Garnock-Jones wrote:
> ** Patch added: "0001-Bring-linux-user-write-2-handling-into-line-with-lin.patch"
> https://bugs.launchpad.net/qemu/+bug/1716292/+attachment/5186008/+files/0001-Bring-linux-user-write-2-handling-into-line-with-lin.patch

While a developer can chase a URL, our CI tools can't. Can you please
also send that patch directly to <email address hidden>, so that it gets
the same level of review as other patches?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc. [phone]
Virtualization: qemu.org | libvirt.org

[Thomas Huth (th-huth)]

Fix has been committed here:
https://git.qemu.org/?p=qemu.git;a=commitdiff;h=58cfa6c2e6eb51b23cc98