Re: [Qemu-devel] [PATCH] Avoid crash in epoll_ctl with EPOLL_CTL_DEL

[Giuseppe Musacchio <thatlemon@gmail.com>]

Yes, that's much better if compatibility with such an old kernel version 
is wanted.

I suppose there's no need for me to re-send the patch.

On 30/05/19 18:12, Laurent Vivier wrote:
> Le 30/05/2019 à 18:00, Laurent Vivier a écrit :
>> Le 30/05/2019 à 17:25, Giuseppe Musacchio a écrit :
>>> The `event` parameter is ignored by the kernel if `op` is EPOLL_CTL_DEL,
>>> do the same and avoid returning EFAULT if garbage is passed instead of a
>>> valid pointer.
>>>
>>> Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
>>> ---
>>>   linux-user/syscall.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
>>> index 5e29e675e9..32d463d58d 100644
>>> --- a/linux-user/syscall.c
>>> +++ b/linux-user/syscall.c
>>> @@ -11329,7 +11329,7 @@ static abi_long do_syscall1(void *cpu_env, int
>>> num, abi_long arg1,
>>>       {
>>>           struct epoll_event ep;
>>>           struct epoll_event *epp = 0;
>>> -        if (arg4) {
>>> +        if (arg2 != EPOLL_CTL_DEL && arg4) {
>>>               struct target_epoll_event *target_ep;
>>>               if (!lock_user_struct(VERIFY_READ, target_ep, arg4, 1)) {
>>>                   return -TARGET_EFAULT;
>>
>> Reviewed-by: Laurent Vivier <laurent@vivier.eu>
>>
> 
> in fact, the BUGS section of epoll_ctl(2) says:
> 
> "In kernel versions before 2.6.9, the EPOLL_CTL_DEL operation required a
>   non-null pointer in event, even though this argument is ignored.  Since
>   Linux 2.6.9, event can be specified as NULL when  using  EPOLL_CTL_DEL.
>   Applications  that  need  to be portable to kernels before 2.6.9 should
>   specify a non-null pointer in event."
> 
> So something like this would be more portable:
> 
> @@ -11329,6 +11329,7 @@ static abi_long do_syscall1(void *cpu_env, int
> num, abi_long arg1,
>           struct epoll_event ep;
>           struct epoll_event *epp = 0;
>           if (arg4) {
> +            if (arg2 != EPOLL_CTL_DEL) {
>                   struct target_epoll_event *target_ep;
>                   if (!lock_user_struct(VERIFY_READ, target_ep, arg4, 1)) {
>                       return -TARGET_EFAULT;
> @@ -11340,6 +11341,11 @@ static abi_long do_syscall1(void *cpu_env, int
> num, abi_long arg1,
>                    */
>                   ep.data.u64 = tswap64(target_ep->data.u64);
>                   unlock_user_struct(target_ep, arg4, 0);
> +            }
> +            /*
> +             * before kernel 2.6.9, EPOLL_CTL_DEL operation required a
> +             * non-null pointer, even though this argument is ignored.
> +             * */
>               epp = &ep;
>           }
> 
> Thanks,
> Laurent
>