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Date: Tue, 16 Oct 2012 23:47:52 +0200
From: boris brezillon <b.brezillon.musl@...il.com>
To: musl@...ts.openwall.com
Subject: Re: TLS (thread-local storage) support

2012/10/16 boris brezillon <b.brezillon.musl@...il.com>:
> Hi,
>
> First I'd like to thank Rich for adding TLS support (I started to work
> on it a few weeks ago but never had time to finish it).
>
> 2012/10/6 Daniel Cegiełka <daniel.cegielka@...il.com>:
>> 2012/10/5 Rich Felker <dalias@...ifal.cx>:
>>> On Thu, Oct 04, 2012 at 11:29:11PM +0200, Daniel Cegiełka wrote:
>>>> great news! Finally able to compile Go (lang)...
>>>
>>> Did Go fail with gcc's emulated TLS in libgcc?
>>
>> I tested Go with sabotage (with fresh musl). I'll try to do it again...
>> gcc in sabotage was compiled without support for TLS, so I didn't
>> expect that it will be successful:
>>
>> https://github.com/rofl0r/sabotage/blob/master/pkg/gcc4
>>
> There's at least one thing (maybe more) missing for go support with
> musl : gcc 'split-stack' support (see http://blog.nella.org/?p=849 and
> http://gcc.gnu.org/wiki/SplitStacks).
>
> I'm also interested in split stack support in musl but for other
> reasons (thread and coroutine stack automatic expansion).
>
> For x86/x86_64 split stack is implemented using a field inside the
> pthread struct which is accessed via %fs (or %gs for x86_64) and an
> offset.
>
> Currently this offset is defined at 0x30 (0x70 for x86_64) by the
> TARGET_THREAD_SPLIT_STACK_OFFSET but only if TARGET_LIBC_PROVIDES_SSP
> is defined (see gcc/config/i386/gnu-user.h or
> gcc/config/i386/gnu-user64.h).
>
> As far as I know musl does not support stack protection, but we could
> at least patch gcc to define TARGET_THREAD_SPLIT_STACK_OFFSET when
> using musl.
>
> We also need to reserve a field in the musl pthread struct. There are
> currently two fields named 'unused1' and 'unused2' but I'm not sure
> they're really unused in every supported arch.
>
>
> BTW, I'd like to work on a more integrated support of split stack in MUSL :
>
> 1) support in dynamic linker (see the last point of
> http://gcc.gnu.org/wiki/SplitStacks) : check split stack notes in
> shared libs (and program ?)
>
> 2) support in thread implementation : currently when a thread is
> created the stack limit is set afterward (see
> https://github.com/mirrors/gcc/blob/master/libgcc/generic-morestack-thread.c
> and https://github.com/mirrors/gcc/blob/master/libgcc/config/i386/morestack.S)
> and the stack size is supposed to be 16K (which is the minimum stack
> size). This means we may reallocate a new stack chunk even if the
> previous one (the first one) is not fully used.
> If stack limit is set by thread implementation, this can be set
> appropriately according to the stack size defined by the thread
> creator.
>
> 3) more optimizations I haven't thought about yet...
>
4) Compile musl with '-fsplit-stack' and add no_split_stack attribute
to appropriate functions (at least all functions called before
pthread_self_init because %gs or %fs register is unusable before this
call).

5) set main thread stack limit to 0 (pthread_self_init) : the main
thread stack grow is handled by the kernel.

6) add no-split-stack note to every asm file.

7) make split stack support optional (either by checking the
-fsplit-stack option in CFLAGS or with a specific option :
--enable-split-stack) : split stack adds overhead to every functions
(except for those with the 'no_split_stack' attribute).

> Do you have any concern about adding those features in musl ?
>
> Let me know if you see other issues I haven't noticed.
>
>
> Regards,
>
> Boris

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