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

[Laurent Vivier <laurent@vivier.eu>]

Le 08/09/2018 à 20:22, Tony Garnock-Jones a écrit :
> 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 <tonygarnockjones@gmail.com>
> ---
>  linux-user/syscall.c | 3 +++
>  1 file changed, 3 insertions(+)

Reviewed-by: Laurent Vivier <laurent@vivier.eu>