To Do

Discuss d2431f633e6139a62e1575ec18830f7e81160cf0 with Samuel.
Validate our implementation against https://sourceware.org/glibc/wiki/TLSandSignals.

Documentation

IRC, freenode, #hurd, 2011-11-26

<tschwinge> In glibc multiarch support (strcasecmp for i686 SSE3, etc.)
  there is access to memory via gs: -- this will need to be changed for
  us, right?
<youpi> depends on the access
<tschwinge>       * `optimized strcasecmp and strncasecmp for x86-32`
  (multiarch),
<tschwinge>         76e3966e9efc3808a9e7ad09121c5dfc1211c20b +
<tschwinge>         6abf346582ba678f4850a88b4a5950593841df1d +
<tschwinge>         5583a0862cf94f71cbcde91c4043a20af65facca.  `gs`
  access.
<youpi> +       movl    __libc_tsd_LOCALE@GOTNTPOFF(%ebx), %eax
<youpi> that's handled by the linker fine
<youpi> it's only the things held in the tcb_t structure which can pose
  problem
<tschwinge> tcbhead_t?
<tschwinge> I'm looking at this.
<tschwinge> So, at gs:0, there is the TCB.
<tschwinge> And we have the same layout as NPTL/Linux, just that we
  don't have as much data there as they have.
<tschwinge> We're missing multiple_threads, sysinfo, sttack_guard,
  pointer_guard, gscope_flag, private_futex, __private_tm[5].
<tschwinge> So, if one of these is referenced (be it my name or by
  numeric offset), this is invalid for us.
<tschwinge> Anything else should work equivalently.
<youpi> yes
<youpi> usually the only numeric offset being used is 0
<youpi> so it would simply not build
<tschwinge> And the other offsers are generated via tcb-offsets.sym.
<tschwinge> glibc's elf/stackguard-macros.h is wrong for us (but not
  used anywhere apart from elf/tst-stackguard1.c, I think).

After commit a9538892adfbb9f092e0bb14ff3a1703973968af, it's sysdeps/i386/stackguard-macros.h; problem remains.

    <tschwinge> __thread __locale_t __libc_tsd_LOCALE = &_nl_global_locale;
      -- this means that a __libc_tsd_LOCALE values will be in the TLS
      segment, and this is what is being accessed from the assembler code
      with %gs:__libc_tsd_LOCALE@NTPOFF, and the linker will resolve this.
    <youpi> yes
    <youpi> see in the nm output, the libc_tsd symbols
    <youpi> these provide the offsets
    <tschwinge> youpi: Thank you, I'm now understanding this part of TLS
      much better.
    <youpi> have you had a look at the tls.pdf from Uli ?
    <youpi> all the gory details are there :)

Commit c61b4d41c9647a54a329aa021341c0eb032b793e, sourceware [BZ #15754], adds sysdeps/i386/stackguard-macros.h:POINTER_CHK_GUARD, which is not correct for us (at the moment), but it also shouldn't cause any harm, as this file is only used in elf/tst-ptrguard1.c and elf/tst-stackguard1.c, which now will fail to build for us, as we don't have a pointer_guard member in sysdeps/mach/hurd/tls.h:tcbhead_t.

We don't define THREAD_SET_POINTER_GUARD.