linux-user: fix emulation of accept4/getpeername/getsockname/recvfrom syscalls

System calls that return a socket address do so by writing the (possibly
truncated) address into the provided buffer space, but setting the
addrlen parameter to the actual size of the address.  To determine how
much to copy back to the target memory the emulation needs to remember
the incoming value of the addrlen parameter, so that it doesn't write
past the buffer limits.

Signed-off-by: Andreas Schwab <schwab@suse.de>
---
 linux-user/syscall.c | 41 +++++++++++++++++++++++++----------------
 1 file changed, 25 insertions(+), 16 deletions(-)

On 14/02/2019 12:43, Andreas Schwab wrote:
> System calls that return a socket address do so by writing the (possibly
> truncated) address into the provided buffer space, but setting the
> addrlen parameter to the actual size of the address.  To determine how
> much to copy back to the target memory the emulation needs to remember
> the incoming value of the addrlen parameter, so that it doesn't write
> past the buffer limits.
> 
> Signed-off-by: Andreas Schwab <schwab@suse.de>
> ---
>  linux-user/syscall.c | 41 +++++++++++++++++++++++++----------------
>  1 file changed, 25 insertions(+), 16 deletions(-)
> 
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 4a320850bb..f24743a2b2 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -2885,7 +2885,7 @@ static abi_long do_sendrecvmmsg(int fd, abi_ulong target_msgvec,
>  static abi_long do_accept4(int fd, abi_ulong target_addr,
>                             abi_ulong target_addrlen_addr, int flags)
>  {
> -    socklen_t addrlen;
> +    socklen_t addrlen, ret_addrlen;
>      void *addr;
>      abi_long ret;
>      int host_flags;
> @@ -2909,11 +2909,13 @@ static abi_long do_accept4(int fd, abi_ulong target_addr,
>  
>      addr = alloca(addrlen);
>  
> -    ret = get_errno(safe_accept4(fd, addr, &addrlen, host_flags));
> +    ret_addrlen = addrlen;
> +    ret = get_errno(safe_accept4(fd, addr, &ret_addrlen, host_flags));
>      if (!is_error(ret)) {
> -        host_to_target_sockaddr(target_addr, addr, addrlen);
> -        if (put_user_u32(addrlen, target_addrlen_addr))
> +        host_to_target_sockaddr(target_addr, addr, MIN(addrlen, ret_addrlen));
> +        if (put_user_u32(ret_addrlen, target_addrlen_addr)) {
>              ret = -TARGET_EFAULT;
> +        }
>      }
>      return ret;
>  }
> @@ -2922,7 +2924,7 @@ static abi_long do_accept4(int fd, abi_ulong target_addr,
>  static abi_long do_getpeername(int fd, abi_ulong target_addr,
>                                 abi_ulong target_addrlen_addr)
>  {
> -    socklen_t addrlen;
> +    socklen_t addrlen, ret_addrlen;
>      void *addr;
>      abi_long ret;
>  
> @@ -2938,11 +2940,13 @@ static abi_long do_getpeername(int fd, abi_ulong target_addr,
>  
>      addr = alloca(addrlen);
>  
> -    ret = get_errno(getpeername(fd, addr, &addrlen));
> +    ret_addrlen = addrlen;
> +    ret = get_errno(getpeername(fd, addr, &ret_addrlen));
>      if (!is_error(ret)) {
> -        host_to_target_sockaddr(target_addr, addr, addrlen);
> -        if (put_user_u32(addrlen, target_addrlen_addr))
> +        host_to_target_sockaddr(target_addr, addr, MIN(addrlen, ret_addrlen));
> +        if (put_user_u32(ret_addrlen, target_addrlen_addr)) {
>              ret = -TARGET_EFAULT;
> +        }
>      }
>      return ret;
>  }
> @@ -2951,7 +2955,7 @@ static abi_long do_getpeername(int fd, abi_ulong target_addr,
>  static abi_long do_getsockname(int fd, abi_ulong target_addr,
>                                 abi_ulong target_addrlen_addr)
>  {
> -    socklen_t addrlen;
> +    socklen_t addrlen, ret_addrlen;
>      void *addr;
>      abi_long ret;
>  
> @@ -2967,11 +2971,13 @@ static abi_long do_getsockname(int fd, abi_ulong target_addr,
>  
>      addr = alloca(addrlen);
>  
> -    ret = get_errno(getsockname(fd, addr, &addrlen));
> +    ret_addrlen = addrlen;
> +    ret = get_errno(getsockname(fd, addr, &ret_addrlen));
>      if (!is_error(ret)) {
> -        host_to_target_sockaddr(target_addr, addr, addrlen);
> -        if (put_user_u32(addrlen, target_addrlen_addr))
> +        host_to_target_sockaddr(target_addr, addr, MIN(addrlen, ret_addrlen));
> +        if (put_user_u32(ret_addrlen, target_addrlen_addr)) {
>              ret = -TARGET_EFAULT;
> +        }
>      }
>      return ret;
>  }
> @@ -3043,7 +3049,7 @@ static abi_long do_recvfrom(int fd, abi_ulong msg, size_t len, int flags,
>                              abi_ulong target_addr,
>                              abi_ulong target_addrlen)
>  {
> -    socklen_t addrlen;
> +    socklen_t addrlen, ret_addrlen;
>      void *addr;
>      void *host_msg;
>      abi_long ret;
> @@ -3061,10 +3067,12 @@ static abi_long do_recvfrom(int fd, abi_ulong msg, size_t len, int flags,
>              goto fail;
>          }
>          addr = alloca(addrlen);
> +        ret_addrlen = addrlen;
>          ret = get_errno(safe_recvfrom(fd, host_msg, len, flags,
> -                                      addr, &addrlen));
> +                                      addr, &ret_addrlen));
>      } else {
>          addr = NULL; /* To keep compiler quiet.  */
> +        addrlen = 0; /* To keep compiler quiet.  */
>          ret = get_errno(safe_recvfrom(fd, host_msg, len, flags, NULL, 0));
>      }
>      if (!is_error(ret)) {
> @@ -3077,8 +3085,9 @@ static abi_long do_recvfrom(int fd, abi_ulong msg, size_t len, int flags,
>              }
>          }
>          if (target_addr) {
> -            host_to_target_sockaddr(target_addr, addr, addrlen);
> -            if (put_user_u32(addrlen, target_addrlen)) {
> +            host_to_target_sockaddr(target_addr, addr,
> +                                    MIN(addrlen, ret_addrlen));
> +            if (put_user_u32(ret_addrlen, target_addrlen)) {
>                  ret = -TARGET_EFAULT;
>                  goto fail;
>              }
> 

Applied to my linux-user branch.

Thanks,
Laurent