Start printf 6_takarakasai
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This Presentation for start_printf#6 at 2014/03/29. It describes some kernel walk throw guide of system call request from userland with source code of NetBSD6.0.1.
Start printf 6_takarakasai
- 3. int命令どこ？ src/lib/libc/arch/i386/SYS.h 75～ #define OSYSTRAP(x) ¥ movl _SYSNAM(x),%eax ;¥ int $0x80 int と 0x80 でgrepするとこれが出てくるけど…
- 4. libc の Makefile.inc から追う lib/libc/sys/Makefile.inc 167～ ASMDEPS= ${.CURDIR}/sys/Makefile.inc ${ARCHDIR}/SYS.h ¥ ${DESTDIR}/usr/include/sys/syscall.h … ${WEAKASM}: ${ASMDEPS} ${_MKTARGET_CREATE} printf '#include "SYS.h"¥nWSYSCALL(${.PREFIX},_sys_${.PREFIX})¥nWEAK_ALIAS(_${.PREFIX},_sys_${.PREFIX})¥n' >${.TARGET} … WEAKASM= accept.S __aio_suspend50.S close.S connect.S execve.S ¥ … __wait450.S write.S writev.S write.S は./sys/Makefile.inc, i386/SYS.h, $DEST/usr/include/sys/syscall.h から生成される. printf で${.TARGET} = write.S が作成される.
- 5. 展開してみる lib/libc/sys/Makefile.inc printf '#include "SYS.h"¥nWSYSCALL(${.PREFIX},_sys_${.PREFIX})¥nWEAK_ALIAS(_${.PREFIX},_sys_${.PREFIX})¥n' >${.TARGET} WSYSCALL, WEAK_ALIAS ？ #include “SYS.h” WSYSCALL(${.PREFIX},_sys_${.PREFIX}) WEAK_ALIAS(_${.PREFIX},_sys_${.PREFIX}) ${.TARGET} #include “SYS.h” WSYSCALL(write,_sys_write) WEAK_ALIAS(_write,_sys_write) write.S
- 6. WSYSCALL, WEAK_ALIAS sys/arch/i386/include/asm.h 195- #define WEAK_ALIAS(alias,sym) ¥ .weak alias; ¥ alias = sym #endif #include “SYS.h” .weak write alias = _sys_write PSEDO(_sys_write, write) .weak _write alias = _sys_write write.S sys/arch/i386/include/asm.h 195- #define WSYSCALL(weak,strong) ¥ WEAK_ALIAS(weak,strong); ¥ PSEUDO(strong,weak)
- 7. さらに展開 #include “SYS.h” .weak write alias = _sys_write ENTRY(_sys_write); SYSTRAP(wirte) >> #define SYSTRAP(x) OSYSTRAP(x) jc 2f; ret; 2: _SYSCALL_ERR .weak _write alias = _sys_write write.S
- 8. さらにさらに展開 #include “SYS.h” .weak write alias = _sys_write ENTRY(_sys_write); movl $(SYS_write),%eax ; int $0x80 jc 2f; ret; 2: _SYSCALL_ERR .weak _write alias = _sys_write write.S #define SYS_write 4 sys/sys/syscall.h:28 eax に システムコール番号入れて int 0x80 よんでる！
- 9. さらにさらにさらに展開 #include “SYS.h” .weak write alias = _sys_write .text: _ALIGN_TEXT; .globl _sys_write; .type _sys_write; @function: _sys_write: movl $(SYS_write),%eax ; int $0x80 jc 2f; ret; 2: jmp __cerror // cの関数 .weak _write alias = _sys_write write.S weak シンボル( write, _write) global / text シンボル( _sys_write) Undefined シンボル (__cerror) がnetbsd の libc.a にある？
- 10. ほんとにあってる？ >> nm libc.a … write.o: U __cerror 00000000 T _sys_write 00000000 W _write 00000000 W write … write.S あってるようです