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