[Qemu-devel] [PATCH v2] linux-user: fix wait* syscall status returns

[Alexander Graf]

When calling wait4 or waitpid with a status pointer and WNOHANG, the
syscall can potentially not modify the status pointer input. Now if we
have guest code like:

  int status = 0;
  waitpid(pid, &status, WNOHANG);
  if (status)
     <breakage>

then we have to make sure that in case status did not change we actually
return the guest's initialized status variable instead of our own uninitialized.
We fail to do so today, as we proxy everything through an uninitialized status
variable which for me ended up always containing the last error code.

This patch fixes some test cases when building yast2-core in OBS for ARM.

Signed-off-by: Alexander Graf <address@hidden>

---

v1 -> v2:

  - take Peter's comment into account and just not write status back when
    wait*'s return value is 0
---
 linux-user/syscall.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 3e6f3bd..5810e2a 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -4835,7 +4835,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
         {
             int status;
             ret = get_errno(waitpid(arg1, &status, arg3));
-            if (!is_error(ret) && arg2
+            if (!is_error(ret) && arg2 && ret
                 && put_user_s32(host_to_target_waitstatus(status), arg2))
                 goto efault;
         }
@@ -6391,7 +6391,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
                 rusage_ptr = NULL;
             ret = get_errno(wait4(arg1, &status, arg3, rusage_ptr));
             if (!is_error(ret)) {
-                if (status_ptr) {
+                if (status_ptr && ret) {
                     status = host_to_target_waitstatus(status);
                     if (put_user_s32(status, status_ptr))
                         goto efault;
-- 
1.6.0.2