Re: [PATCH] i386: Add _startup_sbrk and _startup_fatal [BZ #21913]

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Aug 7, 2017 at 6:26 AM, Zack Weinberg <zackw@panix.com> wrote:
> On Mon, Aug 7, 2017 at 9:21 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Aug 7, 2017 at 6:11 AM, Zack Weinberg <zackw@panix.com> wrote:
>>> On Mon, Aug 7, 2017 at 8:38 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Mon, Aug 7, 2017 at 4:47 AM, Zack Weinberg <zackw@panix.com> wrote:
>>>>
>>>> To initialize TLS, we need a single syscall, sbrk.
>>>
>>> I presume this is to allocate memory for the TLS block.  But why can't
>>> the TLS block - remember, this is for the main thread, in a statically
>>> linked binary - be statically allocated data?
>>
>> I can look into it.

The code is

#if TLS_TCB_AT_TP
  /* Align the TCB offset to the maximum alignment, as
     _dl_allocate_tls_storage (in elf/dl-tls.c) does using __libc_memalign
     and dl_tls_static_align.  */
  tcb_offset = roundup (memsz + GL(dl_tls_static_size), max_align);
  tlsblock = __sbrk (tcb_offset + TLS_INIT_TCB_SIZE + max_align);
#elif TLS_DTV_AT_TP
  tcb_offset = roundup (TLS_INIT_TCB_SIZE, align ?: 1);
  tlsblock = __sbrk (tcb_offset + memsz + max_align
                     + TLS_PRE_TCB_SIZE + GL(dl_tls_static_size));
  tlsblock += TLS_PRE_TCB_SIZE;

Are you suggesting that we use a static data with a fixed size and
check the size at run-time?

>> But tunables_strdup also calls sbrk before TLS is initialized.  We
>> still need it even if TLS initialization doesn't.
>
> Again, does this _need_ to happen that early, or could it be moved later?
>

We have

 __tunables_init (__environ);

  ARCH_INIT_CPU_FEATURES ();

  /* Perform IREL{,A} relocations.  */
  ARCH_SETUP_IREL ();

  /* The stack guard goes into the TCB, so initialize it early.  */
  __libc_setup_tls ();

__libc_setup_tls  should be called after ARCH_INIT_CPU_FEATURES and
ARCH_SETUP_IREL, which must be called after  __tunables_init since
__tunables_init can change CPU features.

Given this, we need a working sbrk before TLS is initialized.

-- 
H.J.