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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 1/8/19 7:49 PM, Peter Maydell wrote:
> 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>
> ---
> I chose to fix this by setting p to NULL and falling through
> to the normal-case codepath rather than having a call to
> pread/pwrite in the special-case if like 58cfa6c2e6eb5,
> because here the normal-case is a bit more complicated as
> it has the target_offset64() call in it.
> 38d840e6790c29f59 has "just return 0" for the NULL buffer
> case, but we can't do that here as that would not get the
> "negative offset should return -EINVAL" case write.
> ---
>  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 280137da8c2..b13a170e52e 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -9677,8 +9677,15 @@ 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) {

And this if() is now more readable, thanks.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

> +                return -TARGET_EFAULT;
> +            }
> +        }
>          ret = get_errno(pread64(arg1, p, arg3, target_offset64(arg4, arg5)));
>          unlock_user(p, arg2, ret);
>          return ret;
> @@ -9687,8 +9694,15 @@ 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;
>