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Subjectint 0x40
Date
Hi,
I relatively new to the kernel and am trying to understand how the linux
kernel handles interrupts. For this I attempted to
create an int 0x40 by adding a set_system_gate(64, &system_call) in traps.c.
I verfied by giving out print statements within set_system_gate that 64 is
being set during initialization (though it isnt a surprise that it is being
set). But when i give an int 0x40 in a user level assembly program I get
segmentation fault, (a SIGSEGV signal is sent to the process). I have tried
adding another function in entry.S called my_system_call and reproducing the
code in system_call with a jmp ret_from_sys_call at the end. Also tried
giving an empty C function for my_system_call all with the same result.

My assembly prints out hello world, from the linux assembly how to,
reproduced here
If i replace int 0x80 with my int 0x40 I end up with a segmentation fault.
Is there any thing other than set_system_gate and writing the my_system_call
handler, that i need to do to have a successful int 0x40? I had tried
a) commenting out just the deference of the system handler function
within system_call (call *sys_call_table(0, %eax, 4) )
b) using &system_call itself in set_system_gate
but still the same situation.

Any suggestions will be appreciated.

Thanks
Raman


.data # section declaration

msg:
.string "Hello, world!\n" # our dear string
len = . - msg # length of our dear string

.text # section declaration

# we must export the entry point to the ELF linker or
.global _start # loader. They conventionally recognize _start as their
# entry point. Use ld -e foo to override the default.

_start:

# write our string to stdout

movl $len,%edx # third argument: message length
movl $msg,%ecx # second argument: pointer to message to write
movl $1,%ebx # first argument: file handle (stdout)
movl $4,%eax # system call number (sys_write)
int $0x80 # call kernel

# and exit

movl $0,%ebx # first argument: exit code
movl $1,%eax # system call number (sys_exit)
int $0x80 # call kernel




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