linux-user: make pwrite64/pread64(fd, NULL, 0, offset) return 0

Linux returns success if pwrite64() or pread64() are called with a
zero length NULL buffer, but QEMU was returning -TARGET_EFAULT.

This is the same bug that we fixed in commit 58cfa6c2e6eb51b23cc9
for the write syscall, and long before that in 38d840e6790c29f59
for the read syscall.

Fixes: https://bugs.launchpad.net/qemu/+bug/1810433

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190108184900.9654-1-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

1 files changed, 18 insertions, 4 deletions

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 29bac79..d6605b8 100644
--- a/linux-user/syscall.c
+++ b/linux-user/syscall.c
@@ -9716,8 +9716,15 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
             arg4 = arg5;
             arg5 = arg6;
         }
-        if (!(p = lock_user(VERIFY_WRITE, arg2, arg3, 0)))
-            return -TARGET_EFAULT;
+        if (arg2 == 0 && arg3 == 0) {
+            /* Special-case NULL buffer and zero length, which should succeed */
+            p = 0;
+        } else {
+            p = lock_user(VERIFY_WRITE, arg2, arg3, 0);
+            if (!p) {
+                return -TARGET_EFAULT;
+            }
+        }
         ret = get_errno(pread64(arg1, p, arg3, target_offset64(arg4, arg5)));
         unlock_user(p, arg2, ret);
         return ret;
@@ -9726,8 +9733,15 @@ static abi_long do_syscall1(void *cpu_env, int num, abi_long arg1,
             arg4 = arg5;
             arg5 = arg6;
         }
-        if (!(p = lock_user(VERIFY_READ, arg2, arg3, 1)))
-            return -TARGET_EFAULT;
+        if (arg2 == 0 && arg3 == 0) {
+            /* Special-case NULL buffer and zero length, which should succeed */
+            p = 0;
+        } else {
+            p = lock_user(VERIFY_READ, arg2, arg3, 1);
+            if (!p) {
+                return -TARGET_EFAULT;
+            }
+        }
         ret = get_errno(pwrite64(arg1, p, arg3, target_offset64(arg4, arg5)));
         unlock_user(p, arg2, 0);
         return ret;