[Qemu-devel] [PATCH v2] syscall: fix special case of write(fd, NULL, 0)

[zhuoweizhang]

From: Zhuowei Zhang <address@hidden>

Linux returns success for the special case of calling write with a zero-length
NULL buffer: compiling and running

```
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

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 calls the real syscall with a NULL
buffer and zero length, which gives the correct return value.

Signed-off-by: Zhuowei Zhang <address@hidden>
---
 linux-user/syscall.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 9b6364a..60769c0 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -7783,6 +7783,11 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
arg1,
         }
         break;
     case TARGET_NR_write:
+        if (arg2 == 0 && arg3 == 0) {
+            /* special case: write(fd, NULL, 0) returns success. */
+            ret = get_errno(safe_write(arg1, NULL, 0));
+            break;
+        }
         if (!(p = lock_user(VERIFY_READ, arg2, arg3, 1)))
             goto efault;
         if (fd_trans_target_to_host_data(arg1)) {
-- 
1.9.1


.