Re: [Qemu-devel] [PATCH 2/2] linux-user: Don't use alloca() for epoll_wait's epoll event array

[Peter Maydell]

Ping? It looks like patch 1/2 of this series got into the
recent linux-user pullreq, but this one (2/2) didn't. Do
you want a resend as a standalone patch?

thanks
-- PMM

On 18 July 2016 at 15:36, Peter Maydell <address@hidden> wrote:
> The epoll event array which epoll_wait() allocates has a size
> determined by the guest which could potentially be quite large.
> Use g_try_new() rather than alloca() so that we can fail more
> cleanly if the guest hands us an oversize value. (ENOMEM is
> not a documented return value for epoll_wait() but in practice
> some kernel configurations can return it -- see for instance
> sys_oabi_epoll_wait() on ARM.)
>
> This rearrangement includes fixing a bug where we were
> incorrectly passing a negative length to unlock_user() in
> the error-exit codepath.
>
> Signed-off-by: Peter Maydell <address@hidden>
> ---
>  linux-user/syscall.c | 17 +++++++++++++----
>  1 file changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 3552295..721d7b1 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -11035,7 +11035,12 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> arg1,
>              goto efault;
>          }
>
> -        ep = alloca(maxevents * sizeof(struct epoll_event));
> +        ep = g_try_new(struct epoll_event, maxevents);
> +        if (!ep) {
> +            unlock_user(target_ep, arg2, 0);
> +            ret = -TARGET_ENOMEM;
> +            break;
> +        }
>
>          switch (num) {
>  #if defined(TARGET_NR_epoll_pwait)
> @@ -11053,8 +11058,8 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> arg1,
>                  target_set = lock_user(VERIFY_READ, arg5,
>                                         sizeof(target_sigset_t), 1);
>                  if (!target_set) {
> -                    unlock_user(target_ep, arg2, 0);
> -                    goto efault;
> +                    ret = -TARGET_EFAULT;
> +                    break;
>                  }
>                  target_to_host_sigset(set, target_set);
>                  unlock_user(target_set, arg5, 0);
> @@ -11082,8 +11087,12 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
> arg1,
>                  target_ep[i].events = tswap32(ep[i].events);
>                  target_ep[i].data.u64 = tswap64(ep[i].data.u64);
>              }
> +            unlock_user(target_ep, arg2,
> +                        ret * sizeof(struct target_epoll_event));
> +        } else {
> +            unlock_user(target_ep, arg2, 0);
>          }
> -        unlock_user(target_ep, arg2, ret * sizeof(struct 
> target_epoll_event));
> +        g_free(ep);
>          break;
>      }
>  #endif
> --
> 1.9.1