[SOLVED] TLS callback in which Calling sprintf

TLS callback in which Calling sprintf_s leads to crash

Here is my code:

#pragma comment (linker, "/INCLUDE:_tls_used")
#pragma comment (linker, "/INCLUDE:p_tls_callback1")
#pragma const_seg(push)
#pragma const_seg(".CRT$XLAAA")
EXTERN_C const PIMAGE_TLS_CALLBACK p_tls_callback1 = tls_start_protect;
#pragma const_seg(pop)

and following piece of code is directly called by tls_start_protect.

    char buf[10];
    sprintf_s(buf, 10, "hello\n");

and it crashes.

0:000> k
 # Child-SP          RetAddr           Call Site
00 0000007c`e676eb58 00007ff6`e3b04829 AGTtest!__crtFlsGetValue+0x10 [f:\dd\vctools\crt\crtw32\misc\winapisupp.c @ 422] 
01 0000007c`e676eb60 00007ff6`e3b047f3 AGTtest!_getptd_noexit+0x1d [f:\dd\vctools\crt\crtw32\startup\tidtable.c @ 277] 
02 0000007c`e676eb90 00007ff6`e3b03737 AGTtest!_getptd+0xb [f:\dd\vctools\crt\crtw32\startup\tidtable.c @ 337] 
03 0000007c`e676ebc0 00007ff6`e3b06030 AGTtest!_LocaleUpdate::_LocaleUpdate+0x1b [f:\dd\vctools\crt\crtw32\h\setlocal.h @ 248] 
04 0000007c`e676ebf0 00007ff6`e3b02816 AGTtest!_output_s_l+0x6c [f:\dd\vctools\crt\crtw32\stdio\output.c @ 1028] 
05 0000007c`e676ef10 00007ff6`e3b028a8 AGTtest!_vsnprintf_helper+0x92 [f:\dd\vctools\crt\crtw32\stdio\vsprintf.c @ 140] 
06 0000007c`e676ef80 00007ff6`e3b025a3 AGTtest!_vsprintf_s_l+0x3c [f:\dd\vctools\crt\crtw32\stdio\vsprintf.c @ 237] 
07 0000007c`e676efc0 00007ff6`e3b0112f AGTtest!sprintf_s+0x1f [f:\dd\vctools\crt\crtw32\stdio\sprintf.c @ 216] 
08 0000007c`e676f000 00007ffb`bd6a52c8 AGTtest!tls_start_protect+0x1f [d:\repos\antidebug\agt\tls_callback.c @ 83] 
09 0000007c`e676f040 00007ffb`bd6a1577 ntdll!LdrpCallInitRoutine+0x4c
0a 0000007c`e676f0a0 00007ffb`bd7201cd ntdll!LdrpCallTlsInitializers+0x93
0b 0000007c`e676f120 00007ffb`bd75166d ntdll!LdrpInitializeProcess+0x1c99
0c 0000007c`e676f510 00007ffb`bd706d5e ntdll!_LdrpInitialize+0x4a8b9
0d 0000007c`e676f590 00000000`00000000 ntdll!LdrInitializeThunk+0xe

Similarly family of _vscprintf(format, args) also crash in __crtFlsGetValue.

I wonder it's too early to call family of printfs, before which, initialization haven't been done. What I know is that TLS callback(Only DLL_PROCESS_ATTACH) is executed after 'ntdll!Ldr*' load all dependent modules and before 'EOP'.

Question:

Any detail about the initialization of _vscprintf, is it done by "CRT" in some CPP constructors' code???
Any other restriction of TLS callback???
If I do need to call _vscprintf in TLS Callback, how? (somehow, I just wanna print before main)

Solution

you use static linked CRT - this is visible from your stack trace. static linked CRT in exe initialized after exe entry point is called. but tls callback on DLL_PROCESS_ATACH called before exe entry point. in this case your static CRT yet not initialized and any call to this CRT code can crash. solution - use dynamic linked CRT in separate DLL - in this case it already will be initialized before TLS callback