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Calling a function in kernel initialization code

Hussain M
Note: This was originally posted on 3rd September 2012 at http://forums.arm.com

As part of a project I'm trying to modify the entry point to an ARM Linux kernel through the file entry-common.S. I'm not quite familiar with the ARM architecture but I'm learning. What I need to do is when a system call is about to happen, for some system call numbers, I do some pre-call by calling another system call and then continue calling the desired system call.

So, I had to modify entry-common.S as follows:




....
stmdb   sp!, {r4, r5}     @ push fifth and sixth args
tst ip, #_TIF_SYSCALL_TRACE  @ are we tracing syscalls?
bne __sys_trace

cmp scno, #NR_syscalls      @ check upper syscall limit

@ added code
mov ip, scno
stmdb   sp!, {r4}
mov  r4, #255
add  scno, r4, #115   @ preparing the system call number
pop  {r4}
adr lr,a2cont        @ getting the return address for the called function
ldrcc   pc, [tbl, scno, lsl #2]  @ my system call
a2cont:
mov  scno, ip   @ restoring the original desired system call

@ added code ends

adr lr, BSYM(ret_fast_syscall)  @ return address 
ldrcc   pc, [tbl, scno, lsl #2]  @ call sys_* routine
    ...


[font="Arial,"]

[size="2"]This code does not work. The issue is with the my first call:

adr lr,a2contldrcc   pc, [tbl, scno, lsl #2]
[/size]

[size="2"]It looks like the call does not return to the appropriate location. If I remove these two lines, the kernel proceeds with no issues. I'm probably missing something about the call stack in ARM. Any thoughts is highly appreciated.

[/size]
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