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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