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[Qemu-devel] [PULL v2 4/5] linux-user: write(fd, NULL, 0) parity with li

From: Laurent Vivier
Subject: [Qemu-devel] [PULL v2 4/5] linux-user: write(fd, NULL, 0) parity with linux's treatment of same
Date: Tue, 25 Sep 2018 22:51:28 +0200

From: Tony Garnock-Jones <address@hidden>

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 

    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

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>
Reviewed-by: Laurent Vivier <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Laurent Vivier <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 58fb967499..019af632df 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -6772,6 +6772,9 @@ 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)) {

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