Date: Mon, 04 Aug 2003 15:15:04 -0400 (EDT) From: John Baldwin <jhb@FreeBSD.org> To: Ryan Sommers <ryans@gamersimpact.com> Cc: freebsd-hackers@freebsd.org Subject: RE: Assembly Syscall Question Message-ID: <XFMail.20030804151504.jhb@FreeBSD.org> In-Reply-To: <20030731201227.28952.qmail@neuroflux.com>
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On 31-Jul-2003 Ryan Sommers wrote: > When making a system call to the kernel why is it necessary to push the > syscall value onto the stack when you don't call another function? > > Example: > > access.the.bsd.kernel: > int 80h > ret > > func: > mov eax, 4 ; Write > call access.the.bsd.kernel > ; End > > Works. However: > func: > mov eax, 4 ; Write > int 80h > ; End > > Doesn't. > > Now, if you change it to: > > func: > mov eax, 4 ; Write > push eax > int 80h > ; End > > It does work. I was able to find, "By default, the FreeBSD kernel uses the C > calling convention. Further, although the kernel is accessed using int 80h, > it is assumed the program will call a function that issues int 80h, rather > than issuing int 80h directly," in the developer's handbook. But I can't > figure out why the second example doesn't work. Is the call instruction > pushing the value onto the stack in addition to pushing the instruction > pointer on? > > Thank you in advance. > PS I'm not on the list. First off, why are you using asm for userland stuff? Secondly, the kernel assumes that all the other arguments besides the syscall to execute (i.e. %eax) are passed on the user stack. Thus, it has to have a set location relative to the user stack pointer to find the arguments. It allows for a return IP from a call instruction to be at the top of the stack. You can tell this by looking at syscall() in sys/i386/i386/trap.c: params = (caddr_t)frame.tf_esp + sizeof(int); code = frame.tf_eax; orig_tf_eflags = frame.tf_eflags; params is a userland pointer to the function arguments. Adding the sizeof(int) skips over the saved return address, or in your 3rd case, the dummy %eax value. -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve!" - http://www.FreeBSD.org/
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