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After a bit of reading, it would seem that proper exception handling *is* possible - even if a bit clunky. The document I found basically hinted that you'd need a thread set up for handling async exceptions/signals properly... but at least sync exceptions/signals do get delivered to the thread that caused them.
I still think errno is bloody ugly though, even with the per-thread hack that violates "errno is an int"
|03 Oct 2006, 11:08||
Unfortunatelly I haven't found SYSCALL_SIGNAL, SYSCALL_SIGACTION in x64 Linux (but there is rt_sigaction, I'm going to try it).
Multithread idea looks well.
Skeleton for multiprocess handling for 64-bits (easy portable to 32-bit world) - may help for trying in multithread handling
start: mov eax,sys_fork syscall ; make second copy of program ; child has return value=0, parent return value = child's PID ; see: man fork or rax,rax js exit1 ; something went wrong... jnz parent_proc child_proc: ; see: ; man ptrace ; look for PTRACE_TRACEME ; Any signal delivered to this process will cause it to stop and its parent to ; be notified via wait. xor r10,r10 xor edx,edx xor esi,esi mov edi,PTRACE_TRACEME mov eax,sys_ptrace syscall ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; put the core of your program here ; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; parent_proc: mov [child_PID],rax parent_signal_wait: xor r10,r10 mov edx,WUNTRACED lea rsi,[child_status] ; or rdi,-1 ; use -1 for wait to child's children too, but we want to wait for child only mov rdi,qword [child_PID] mov eax,sys_wait4 syscall or rax,rax jns wait_L1 msg_err_wait_exit: lea rax,[msg_err_wait] jmp exit_msg wait_L1: mov eax,dword [child_status] WIFSTOPPED = 7Fh test al,WIFSTOPPED jnz wait_L2 ; Child exited ? Then we do the same... ; WEXITSTATUS eax and eax,0000FF00h sar eax,8 ; al=exitstatus ; display the exit status ;... jmp exit0 wait_L2: ; WIFSIGNALED eax and eax,7Fh inc eax sar al,1 jle wait_L3 ; display hexa content of AL register: ; ... jmp exit0 wait_L3: mov eax,dword [child_status] cmp al,WIFSTOPPED jnz wait_L7 ; WSTOPSIG eax and eax,0000FF00h sar eax,8 ; al holds signal now ; 0. display message with the signal number ; 1. then handle the exception and at the end choose only one of 1.A. or 1.B. ; 1.A. resume program: ; mov r10d,SIGCONT ; xor edx,edx ; mov rsi,qword [child_PID] ; mov edi,PTRACE_CONT ; mov eax,sys_ptrace ; syscall ; 1.B. or kill the program: ; xor r10,r10 ; xor edx,edx ; mov rsi,qword [child_PID] ; mov edi,PTRACE_KILL ; mov eax,sys_ptrace ; syscall wait_L7: jmp parent_signal_wait exit0: xor edi,edi exit: mov eax,sys_exit syscall exit1: mov edi,1 jmp exit
The above skeleton is from fdbg. I still don't know what to do when the child forks - something in the above code is missing. The above handling idea is the same as every debugger's job (it is an auto-debugger, or self-debugger).
|12 Jan 2007, 09:32||
if you want new thread instead of new process, then use
sys_clone with CLONE_THREAD
you need to mmap space for new thread (new stack, ...) before sys_clone
how to skip instruction causing singal ?
0. read registers sys_ptrace with PTRACE_GETREGS and get RIP (EIP) register
(offset of instruction pointer differs in x64 and x86 platform)
1. determine size of instruction:
1A read 16 bytes (2 qwords in x64, 4 dwords in x86) sys_ptrace PTRACE_PEEKTEXT
1B determine the size of the instruction by disassembling it, the most difficult task
you can use disasm engine for x64 or simplify it not to disassemble
the whole instruction but to determine instruction size only
I don't know whether there is any disasm engine for x86 written in asm
2. add instruction size to user.user_regs.rip (or ...eip)
3. write registers sys_ptrace PTRACE_SETREGS
4. continue run sys_ptrace PTRACE_CONT
After doing this you can only hope that skipping instruction doesn't alter program run too much...
|18 Jan 2007, 06:45||
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