76 lines
3 KiB
Text
76 lines
3 KiB
Text
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Contributed by Max Kaehn
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All cygwin threads have separate context in an object of class _cygtls. The
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storage for this object is kept on the stack in the bottom CYGTLS_PADSIZE
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bytes. Each thread references the storage via the Thread Environment Block
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(aka Thread Information Block), which Windows maintains for each user thread
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in the system, with the address in the FS segment register. The memory
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is laid out as in the NT_TIB structure from <w32api/winnt.h>:
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typedef struct _NT_TIB {
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struct _EXCEPTION_REGISTRATION_RECORD *ExceptionList;
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PVOID StackBase;
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PVOID StackLimit;
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PVOID SubSystemTib;
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_ANONYMOUS_UNION union {
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PVOID FiberData;
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DWORD Version;
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} DUMMYUNIONNAME;
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PVOID ArbitraryUserPointer;
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struct _NT_TIB *Self;
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} NT_TIB,*PNT_TIB;
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Cygwin sees it like this:
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extern exception_list *_except_list asm ("%fs:0"); // exceptions.cc
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extern char *_tlsbase __asm__ ("%fs:4"); // cygtls.h
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extern char *_tlstop __asm__ ("%fs:8"); // cygtls.h
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And accesses cygtls like this:
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#define _my_tls (((_cygtls *) _tlsbase)[-1]) // cygtls.h
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Initialization always goes through _cygtls::init_thread(). It works
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in the following ways:
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* In the main thread, _dll_crt0() provides CYGTLS_PADSIZE bytes on the stack
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and passes them to initialize_main_tls(), which calls _cygtls::init_thread().
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It then calls dll_crt0_1(), which terminates with cygwin_exit() rather than
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by returning, so the storage never goes out of scope.
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If you load cygwin1.dll dynamically from a non-cygwin application, it is
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vital that the bottom CYGTLS_PADSIZE bytes of the stack are not in use
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before you call cygwin_dll_init(). See winsup/testsuite/cygload for
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more information.
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* Threads other than the main thread receive DLL_THREAD_ATTACH messages
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to dll_entry() (in init.cc).
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- dll_entry() calls munge_threadfunc(), which grabs the function pointer
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for the thread from the stack frame and substitutes threadfunc_fe(),
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- which then passes the original function pointer to _cygtls::call(),
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- which then allocates CYGTLS_PADSIZE bytes on the stack and hands them
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to call2(),
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- which allocates an exception_list object on the stack and hands it to
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init_exceptions() (in exceptions.cc), which attaches it to the end of
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the list of exception handlers, changing _except_list (aka
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tib->ExceptionList), then passes the cygtls storage to init_thread().
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call2() calls ExitThread() instead of returning, so the storage never
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goes out of scope.
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Note that the padding isn't necessarily going to be just where the _cygtls
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structure lives; it just makes sure there's enough room on the stack when the
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CYGTLS_PADSIZE bytes down from there are overwritten.
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Debugging
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You can examine the segment registers in gdb via "info w32 selector $fs"
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(which is using GetThreadSelectorEntry()) to get results like this:
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Selector $fs
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0x03b: base=0x7ffdd000 limit=0x00000fff 32-bit Data (Read/Write, Exp-up)
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Priviledge level = 3. Byte granular.
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"x/3x 0x7ffdd000" will give you _except_list, _tlsbase, and _tlstop.
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