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.mayd...@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 <phi...@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;
>
