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such kernels cannot support threads, but the thread pointer is also
important for other purposes, most notably stack protector. without a
valid thread pointer, all code compiled with stack protector will
crash. the same applies to any use of thread-local storage by
applications or libraries.
the concept of this patch is to fall back to using the modify_ldt
syscall, which has been around since linux 1.0, to setup the gs
segment register. since the kernel does not have a way to
automatically assign ldt entries, use of slot zero is hard-coded. if
this fallback path is used, __set_thread_area returns a positive value
(rather than the usual zero for success, or negative for error)
indicating to the caller that the thread pointer was successfully set,
but only for the main thread, and that thread creation will not work
properly. the code in __init_tp has been changed accordingly to record
this result for later use by pthread_create.
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several things are changed. first, i have removed the old __uniclone
function signature and replaced it with the "standard" linux
__clone/clone signature. this was necessary to expose clone to
applications anyway, and it makes it easier to port __clone to new
archs, since it's now testable independently of pthread_create.
secondly, i have removed all references to the ugly ldt descriptor
structure (i386 only) from the c code and pthread structure. in places
where it is needed, it is now created on the stack just when it's
needed, in assembly code. thus, the i386 __clone function takes the
desired thread pointer as its argument, rather than an ldt descriptor
pointer, just like on all other sane archs. this should not affect
applications since there is really no way an application can use clone
with threads/tls in a way that doesn't horribly conflict with and
clobber the underlying implementation's use. applications are expected
to use clone only for creating actual processes, possibly with new
namespace features and whatnot.
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this seems to be necessary to make the linker accept the functions in
a shared library (perhaps to generate PLT entries?)
strictly speaking libc-internal asm should not need it. i might clean
that up later.
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these are useless and have caused problems for users trying to build
with non-gnu tools like tcc's assembler.
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