[Qemu-devel] [PULL 2/2] linux-user: make pwrite64/pread64(fd, NULL, 0, offset) return 0

[Laurent Vivier]

From: Peter Maydell <address@hidden>

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 <address@hidden>
Reviewed-by: Laurent Vivier <address@hidden>
Reviewed-by: Philippe Mathieu-Daudé <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Laurent Vivier <address@hidden>
---
 linux-user/syscall.c | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/linux-user/syscall.c b/linux-user/syscall.c
index 29bac79b61..d6605b8b50 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;
-- 
2.20.1