Re: [Qemu-stable] [Qemu-devel] [PATCH] linux-user: write(fd, NULL, 0) parity with linux's treatment of same

[Philippe Mathieu-Daudé]

On Sat, Sep 8, 2018 at 6:04 PM Tony Garnock-Jones
<address@hidden> wrote:
>
> Bring linux-user write(2) handling into line with linux for the case
> of a 0-byte write with a NULL buffer. Based on a patch originally
> written by Zhuowei Zhang.
>
> Addresses https://bugs.launchpad.net/qemu/+bug/1716292.
>
> From Zhuowei Zhang's patch 
> (https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg08073.html):
>
>     Linux returns success for the special case of calling write with a
>     zero-length NULL buffer: compiling and running
>
>     int main() {
>        ssize_t ret = write(STDOUT_FILENO, NULL, 0);
>        fprintf(stderr, "write returned %ld\n", ret);
>        return 0;
>     }
>
>     gives "write returned 0" when run directly, but "write returned
>     -1" in QEMU.
>
>     This commit checks for this situation and returns success if
>     found.
>
> Subsequent discussion raised the following questions (and my answers):
>
>  - Q. Should TARGET_NR_read pass through to safe_read in this
>       situation too?
>    A. I'm wary of changing unrelated code to the specific problem I'm
>       addressing. TARGET_NR_read is already consistent with Linux for
>       this case.
>
>  - Q. Do pread64/pwrite64 need to be changed similarly?
>    A. Experiment suggests not: both linux and linux-user yield -1 for
>       NULL 0-length reads/writes.
>
> Signed-off-by: Tony Garnock-Jones <address@hidden>

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

> ---
>  linux-user/syscall.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 850b72a0c7..8f46540534 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -8168,6 +8168,9 @@ static abi_long do_syscall1(void *cpu_env, int num, 
> abi_long arg1,
>          }
>          return ret;
>      case TARGET_NR_write:
> +        if (arg2 == 0 && arg3 == 0) {
> +            return get_errno(safe_write(arg1, 0, 0));
> +        }
>          if (!(p = lock_user(VERIFY_READ, arg2, arg3, 1)))
>              return -TARGET_EFAULT;
>          if (fd_trans_target_to_host_data(arg1)) {
> --
> 2.18.0
>
>
> [University of Glasgow: The Times Scottish University of the Year 2018]
>