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Re: [Qemu-trivial] [Qemu-devel] [PATCH] linux-user: write(fd, NULL, 0) p
From: |
Peter Maydell |
Subject: |
Re: [Qemu-trivial] [Qemu-devel] [PATCH] linux-user: write(fd, NULL, 0) parity with linux's treatment of same |
Date: |
Thu, 3 Jan 2019 18:31:13 +0000 |
On Sat, 8 Sep 2018 at 22:04, 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.
Hi; following up on this, we've just had
https://bugs.launchpad.net/qemu/+bug/1810433 which is
a report of the same NULL/0 bug for pwrite64. Looking at the
kernel code I see that both the write and pwrite64 syscalls
go through the same vfs_write() common function, so their
behaviour for NULL/0 should be identical. Experimentally,
stracing the 1810433 test program gives
pwrite64(3, NULL, 0, 0) = 0
so we do indeed need to special case NULL/0 there as well
as in write().
The extra fix should be straightforward -- does anybody
feel like writing up a patch for it?
thanks
-- PMM
- Re: [Qemu-trivial] [Qemu-devel] [PATCH] linux-user: write(fd, NULL, 0) parity with linux's treatment of same,
Peter Maydell <=