linux内核中的信号机制--信号处理

1. regs->regs->ARM_lr=retcode;//返回地址，当用户态信号处理函数结束时，就会把这个地址作为返回地址
2. regs->ARM_pc=handler;//信号处理函数
3. regs->ARM_cpsr=cpsr;
5. return0;
6. }

当setup_frame()返回后，一切准备就绪，因此可以从内核态返回了，这样就顺利过渡到用户态的信号处理函数。当这个函数处理结束后，会通过retcode再次进入内核态，现在我们看看retcode是如何处理的，下面的代码选自glibc(2.3.2)：

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1. #include
3. /*IfnoSA_RESTORERfunctionwasspecifiedbytheapplicationweuse
4. oneofthese.Thisavoidstheneedforthekerneltosynthesiseareturn
5. instructiononthestack,whichwouldinvolveexpensivecacheflushes.*/
7. ENTRY(__default_sa_restorer)
8. swiSYS_ify(sigreturn)
10. #ifdef__NR_rt_sigreturn
12. ENTRY(__default_rt_sa_restorer)
13. swiSYS_ify(rt_sigreturn)
15. #defineSYS_ify(syscall_name)(__NR_##syscall_name)

下面具体看看sys_sigreturn()的定义：

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1. asmlinkageintsys_sigreturn(structpt_regs*regs)
2. {
3. structsigframe__user*frame;
4. sigset_tset;
6. /*Alwaysmakeanypendingrestartedsystemcallsreturn-EINTR*/
7. current_thread_info()->restart_block.fn=do_no_restart_syscall;
9. /*
10. *Sincewestackedthesignalona64-bitboundary,
11. *thenspshouldbewordalignedhere.Ifits
12. *not,thentheuseristryingtomesswithus.
13. */
14. if(regs->ARM_sp&7)
15. gotobadframe;
17. frame=(structsigframe__user*)regs->ARM_sp;
19. if(!access_ok(VERIFY_READ,frame,sizeof(*frame)))
20. gotobadframe;
21. if(__get_user(set.sig[0],&frame->sc.oldmask)
22. ||(_NSIG_WORDS>1
23. &&__copy_from_user(&set.sig[1],&frame->extramask,
24. sizeof(frame->extramask))))
25. gotobadframe;
27. sigdelsetmask(&set,~_BLOCKABLE);
28. spin_lock_irq(¤t->sighand->siglock);
29. current->blocked=set;
30. recalc_sigpending();
31. spin_unlock_irq(¤tt->sighand->siglock);
33. if(restore_sigcontext(regs,&frame->sc,&frame->aux))
34. gotobadframe;
36. /*SendSIGTRAPifweresingle-stepping*/
37. if(current->ptrace&PT_SINGLESTEP){
38. ptrace_cancel_bpt(current);
39. send_sig(SIGTRAP,current,1);
40. }
42. returnregs->ARM_r0;
44. badframe:
45. force_sig(SIGSEGV,current);
46. return0;
47. }

这个函数主要调用restore_sigcontext()根据用户态态堆栈上的sigframe备份还原pt_regs。

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1. staticint
2. restore_sigcontext(structpt_regs*regs,structsigcontext__user*sc,
3. structaux_sigframe__user*aux)
4. {
5. interr=0;
7. __get_user_error(regs->ARM_r0,&sc->arm_r0,err);
8. __get_user_error(regs->ARM_r1,&sc->arm_r1,err);
9. __get_user_error(regs->ARM_r2,&sc->arm_r2,err);
10. __get_user_error(regs->ARM_r3,&sc->arm_r3,err);
11. __get_user_error(regs->ARM_r4,&sc->arm_r4,err);
12. __get_user_error(regs->ARM_r5,&sc->arm_r5,err);
13. __get_user_error(regs->ARM_r6,&sc->arm_r6,err);
14. __get_user_error(regs->ARM_r7,&sc->arm_r7,err);
15. __get_user_error(regs->ARM_r8,&sc->arm_r8,err);
16. __get_user_error(regs->ARM_r9,&sc->arm_r9,err);
17. __get_user_error(regs->ARM_r10,&sc->arm_r10,err);
18. __get_user_error(regs->ARM_fp,&sc->arm_fp,err);
19. __get_user_error(regs->ARM_ip,&sc->arm_ip,err);
20. __get_user_error(regs->ARM_sp,&sc->arm_sp,err);
21. __get_user_error(regs->ARM_lr,&sc->arm_lr,err);
22. __get_user_error(regs->ARM_pc,&sc->arm_pc,err);
23. __get_user_error(regs->ARM_cpsr,&sc->arm_cpsr,err);
25. err|=!valid_user_regs(regs);
27. #ifdefCONFIG_IWMMXT
28. if(err==0&&test_thread_flag(TIF_USING_IWMMXT))
29. err|=restore_iwmmxt_context(&aux->iwmmxt);
30. #endif
31. #ifdefCONFIG_VFP
32. //if(err==0)
33. //err|=vfp_restore_state(&aux->vfp);
34. #endif
36. returnerr;
37. }

restore_sigcontext()很简单，当它返回后，通过sys_sigreturn()在内核态堆栈建立的pt_regs已经变为调用信号处理函数之前的pt_regs。这样，sys_sigreturn()返回用户态时，就顺利地过渡到处理之前的状态了。