Hi: the SWI is used for system APIs such as open, read, write. user mode applications call system APIs via SWI, which will change ARM mode from USER to SVC. so when vector_swi is called, Linux will do as below: 1. store r0~r12, these registers are universal for USR mode SVC mode. 2. store r13 and r14 of USER mode. Note, SWI is triggered from USER mode, so here Linux store USER mode's r13 and r14, rather than SVC's. for your two questions: 1. the ^ means to get USER mode registers, rather than current mode. 2. no matter how you arrange registers in opcode {}, stmdb will always push lr first, then sp. so after line 348, the stack view is as below:lr_usrsp_usrr12...r0 > Date: Thu, 16 Feb 2012 19:35:17 -0700
Subject: Re: arm assembly doubt From: subingangadharan@gmail.com To: suren@gatech.edu CC: kernelnewbies@kernelnewbies.org
Thanks for the answer. Actually this is what I am trying to understand.
ENTRY(vector_swi) 345 sub sp, sp, #S_FRAME_SIZE 346 stmia sp, {r0 - r12} @ Calling r0 - r12 347 ARM( add r8, sp, #S_PC ) 348 ARM( stmdb r8, {sp, lr}^ ) @ Calling sp, lr 349 THUMB( mov r8, sp ) 350 THUMB( store_user_sp_lr r8, r10, S_SP ) @ calling sp, lr 351 mrs r8, spsr @ called from non-FIQ mode, so ok. 352 str lr, [sp, #S_PC] @ Save calling PC 353 str r8, [sp, #S_PSR] @ Save CPSR 354 str r0, [sp, #S_OLD_R0]
In this case after the line number 348(if its in arm mode),will the kernel stack have the contents r0-r12,sp,lr in this order or r0-r12,lr,sp this one. Beccause I believe stmdb r8, {sp, lr}^ will push the sp first then lr. In that case sp and lr will be interchanged in struct pt_regs.
Please correct me if I am wrong.
On Wed, Feb 15, 2012 at 9:34 PM, Surenkumar Nihalani <suren@gatech.edu> wrote:
Hi, On Feb 15, 2012, at 11:30 PM, subin gangadharan wrote:
Hi ,
I am trying to understand how system call is implmented in linux for arm.And I am not that familiar with arm assembly.
Could any body please help me to understand what exactly this ^ does in this instruction stmdb r8,{sp,lr}^
-- With Regards Subin Gangadharan
I am not afraid and I am also not afraid of being afraid.
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Example: LDFMD sp!, {r0-r12, pc}^ - The ^ qualifier specifies that the CPSR is restored from the SPSR. It must be used only from a privileged mode.
-- With Regards Subin Gangadharan
I am not afraid and I am also not afraid of being afraid.
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