Re: Stepping through entry.S

[Randy.Dunlap]

On Wed, 3 Apr 2002, Randy.Dunlap wrote:

| On Wed, 3 Apr 2002, Sridhar N wrote:
|
| | Hello (recycled)
|
| | 	I was trying to trace the handling of the system calls, and to step through
| | the entry.S on i386 machine.  I basically used this:
| |
| | 	srin_entryS_debug_mesg: 		#My addition
| | 		.asciz "some relevant message\n"
| | 		ALIGN
| | 	tracesys:			#haven't changed anything here..
| | 		.
| | 		.
| | 		.
| | 		jae tracesys_exit
| | 		pushl $srin_entryS_debug_mesg		#just this
| | 		call SYMBOL_NAME(printk)	# and this
| | 		call *SYMBOL_NAME(sys_call_table)(,%eax,4)
| | 		movl %eax,EAX(%esp)		# save the return value
| | 	tracesys_exit:
| |
| | Shouldn't this call printk everytime a system call is made or atleast crash
| | the kernel if something is dead wrong ? ( It isn't .. everything seems normal
| | as though the printk isn't there )  Also,  how can  i know the values in the
| | specific registers in that file ? Specifically, whenever a system call is
| | made, what registers store what values ? I'm using kernel 2.4.7 on a K6-2.
|
| The tracesys: label (code) is only used if ptrace is enabled for
| the task.  Is it enabled?  If not, you aren't executing this code
| at all.
|
| 	testb $0x02,tsk_ptrace(%ebx)	# PT_TRACESYS
| 	jne tracesys
|
| For the register interface, AFAIK, see the gcc docs, such as
| Extensions to the C Language Family:
|   http://gcc.gnu.org/onlinedocs/gcc-2.95.3/gcc_4.html
| and search for /regparm/ .
| <quote>
| regparm (number)
|  On the Intel 386, the regparm attribute causes the compiler to pass up
|  to number integer arguments in registers EAX, EDX, and ECX instead of
|  on the stack. Functions that take a variable number of arguments will
|  continue to be passed all of their arguments on the stack.
| </quote>
| Someone please correct or add to this.  :)

actually for x86, linux/include/asm-i386/unistd.h defines
_syscall0() ... thru _syscall6(), which load N registers as appropriate
and then execute int $0x80.

-- 
~Randy