Re: [PATCH] nptl: Start new threads with all signals blocked [BZ #25098]

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 14/10/2019 09:33, Florian Weimer wrote:
> New threads inherit the signal mask from the current thread.  This
> means that signal handlers can run on the newly created thread
> immediately after the kernel has created the userspace thread, even
> before glibc has initialized the TCB.  Consequently, new threads can
> observe uninitialized ctype data, among other things.
> 
> To address this, block all signals before starting the thread, and
> pass the original signal mask to the start routine wrapper.  On the
> new thread, first perform all thread initialization, and then unblock
> signals.
> 
> The cost of doing this is two rt_sigprocmask system calls on the old
> thread, and one rt_sigprocmask system call on the new thread.  (If
> there was a way to clone a new thread with a signals disabled, this
> could be brought down to one system call each.)  The thread descriptor
> increases in size, too, and sigset_t is fairly large.  This increase
> could be brought down by reusing space the in the descriptor which is
> not needed before running user code, or by switching to an internal
> sigset_t definition which only covers the signals supported by the
> kernel definition.  (Part of the thread descriptor size increase is
> already offset by reduced stack usage in the thread start wrapper
> routine after this commit.)

I think this change worth parametrizing it on Linux to save some space
on the pthread_t structure, since it save about 120 bytes per thread
and it is unlikely Linux will eventually increase the signal size.

> 
> Tested on aarch64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
> s390x-linux-gnu, x86_64-linux-gnu.
> 
> -----
>  nptl/descr.h          | 10 +++++++---
>  nptl/pthread_create.c | 50 ++++++++++++++++++++++++++------------------------
>  2 files changed, 33 insertions(+), 27 deletions(-)
> 
> diff --git a/nptl/descr.h b/nptl/descr.h
> index d3f863aa18..70d76bc63b 100644
> --- a/nptl/descr.h
> +++ b/nptl/descr.h
> @@ -332,9 +332,8 @@ struct pthread
>    /* True if thread must stop at startup time.  */
>    bool stopped_start;
>  
> -  /* The parent's cancel handling at the time of the pthread_create
> -     call.  This might be needed to undo the effects of a cancellation.  */
> -  int parent_cancelhandling;
> +  /* Formerly used for dealing with cancellation.  */
> +  int parent_cancelhandling_unsed;

I think the idea of keeping the fields was that tools that abuse 
the ABI and access such metadata directly could work across glibc
versions. However, the C11 thread state already changed the internal
layout so I don't see much gain on keep this idea.  I would say to 
just remove the field altogether.

Based on this I send a patch to remove the pid_ununsed field
as well.

>  
>    /* Lock to synchronize access to the descriptor.  */
>    int lock;
> @@ -391,6 +390,11 @@ struct pthread
>    /* Resolver state.  */
>    struct __res_state res;
>  
> +  /* Signal mask for the new thread.  Used during thread startup to
> +     restore the signal mask.  (Threads are launched with all signals
> +     masked.)  */
> +  sigset_t sigmask;
> +
>    /* Indicates whether is a C11 thread created by thrd_creat.  */
>    bool c11;
>  
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index 130937c3c4..940e5bdd4f 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -369,7 +369,6 @@ __free_tcb (struct pthread *pd)
>      }
>  }
>  
> -
>  /* Local function to start thread and handle cleanup.
>     createthread.c defines the macro START_THREAD_DEFN to the
>     declaration that its create_thread function will refer to, and
> @@ -385,10 +384,6 @@ START_THREAD_DEFN
>    /* Initialize pointers to locale data.  */
>    __ctype_init ();
>  
> -  /* Allow setxid from now onwards.  */
> -  if (__glibc_unlikely (atomic_exchange_acq (&pd->setxid_futex, 0) == -2))
> -    futex_wake (&pd->setxid_futex, 1, FUTEX_PRIVATE);
> -
>  #ifdef __NR_set_robust_list
>  # ifndef __ASSUME_SET_ROBUST_LIST
>    if (__set_robust_list_avail >= 0)

Ok.

> @@ -402,21 +397,6 @@ START_THREAD_DEFN
>      }
>  #endif
>  
> -#ifdef SIGCANCEL
> -  /* If the parent was running cancellation handlers while creating
> -     the thread the new thread inherited the signal mask.  Reset the
> -     cancellation signal mask.  */
> -  if (__glibc_unlikely (pd->parent_cancelhandling & CANCELING_BITMASK))
> -    {
> -      INTERNAL_SYSCALL_DECL (err);
> -      sigset_t mask;
> -      __sigemptyset (&mask);
> -      __sigaddset (&mask, SIGCANCEL);
> -      (void) INTERNAL_SYSCALL (rt_sigprocmask, err, 4, SIG_UNBLOCK, &mask,
> -			       NULL, _NSIG / 8);
> -    }
> -#endif
> -
>    /* This is where the try/finally block should be created.  For
>       compilers without that support we do use setjmp.  */
>    struct pthread_unwind_buf unwind_buf;

Ok.

> @@ -438,6 +418,12 @@ START_THREAD_DEFN
>    unwind_buf.priv.data.prev = NULL;
>    unwind_buf.priv.data.cleanup = NULL;
>  
> +  __libc_signal_restore_set (&pd->sigmask);
> +
> +  /* Allow setxid from now onwards.  */
> +  if (__glibc_unlikely (atomic_exchange_acq (&pd->setxid_futex, 0) == -2))
> +    futex_wake (&pd->setxid_futex, 1, FUTEX_PRIVATE);
> +
>    if (__glibc_likely (! not_first_call))
>      {
>        /* Store the new cleanup handler info.  */
> @@ -728,10 +714,6 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
>    CHECK_THREAD_SYSINFO (pd);
>  #endif
>  
> -  /* Inform start_thread (above) about cancellation state that might
> -     translate into inherited signal state.  */
> -  pd->parent_cancelhandling = THREAD_GETMEM (THREAD_SELF, cancelhandling);
> -
>    /* Determine scheduling parameters for the thread.  */
>    if (__builtin_expect ((iattr->flags & ATTR_FLAG_NOTINHERITSCHED) != 0, 0)
>        && (iattr->flags & (ATTR_FLAG_SCHED_SET | ATTR_FLAG_POLICY_SET)) != 0)
> @@ -777,6 +759,22 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
>       ownership of PD (see CONCURRENCY NOTES above).  */
>    bool stopped_start = false; bool thread_ran = false;
>  
> +  /* Block alll signals, so that the new thread starts out with
> +     signals disabled.  This avoids race conditions in the thread
> +     startup.  */

s/alll/all

> +  sigset_t original_sigmask;
> +  __libc_signal_block_all (&original_sigmask);
> +
> +  /* Conceptually, the new thread needs to inherit the signal mask of
> +     this thread.  Therefore, it needs to restore the saved signal
> +     mask of this thread, so save it in the startup information.  */
> +  pd->sigmask = original_sigmask;
> +#ifdef SIGCANCEL
> +  /* Reset the cancellation signal mask in case this thread is running
> +     cancellation.  */
> +  __sigdelset (&pd->sigmask, SIGCANCEL);
> +#endif
> +

Do we still have a nptl target that does not have SIGCANCEL support?

>    /* Start the thread.  */
>    if (__glibc_unlikely (report_thread_creation (pd)))
>      {
> @@ -819,6 +817,10 @@ __pthread_create_2_1 (pthread_t *newthread, const pthread_attr_t *attr,
>      retval = create_thread (pd, iattr, &stopped_start,
>  			    STACK_VARIABLES_ARGS, &thread_ran);
>  
> +  /* Return to the previous signal mask, after creating the new
> +     thread.  */
> +  __libc_signal_restore_set (&original_sigmask);
> +
>    if (__glibc_unlikely (retval != 0))
>      {
>        if (thread_ran)
> 

Ok.