Messages in this thread | | | Date | Fri, 9 Apr 2004 10:34:04 +0200 | From | Peter Seiderer <> | Subject | Question about PTRACE_DETACH |
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Hello, shouldn't ptrace(PTRACE_DETACH, pid, NULL, SIGSTOP) stop the child?
The Valgrind [1] sequence call to stop a child, change the registers and attach a debugger to the child is as following (or take a look at [2] function start_debugger(), or the attached test program):
if ((pid = fork()) == 0) { ptrace(PTRACE_TRACEME, ...); kill(getpid(), SIGSTOP); // should never be reached } else { waitpid(pid, ...); ptrace(PTRACE_SETREGS, ...) ptrace(PTRACE_DETACH, pid, NULL, SIGSTOP) // attach gdb to child }
But the child seems to resume execution after the PTRACE_DETACH call (in contradiction to what the ptrace man page [3] tells).
Example programm gdb_attach.c attached to the e-mail.
gcc -Wall -g -o g db_attach gdb_attach.c ./gdb_attach parent child parent: cmdl: gdb gdb_attach 1509 child: should never be reached <gdb output snipped> 0x400d4411 in nanosleep () from /lib/libc.so.6 (gdb) bt #0 0x400d4411 in nanosleep () from /lib/libc.so.6 #1 0x400d43a8 in sleep () from /lib/libc.so.6 #2 0x08048704 in start_debugger () at gdb_attach.c:20 #3 0x08048889 in main (argc=1, argv=0xbffff444) at gdb_attach.c:46 #4 0x400499ed in __libc_start_main () from /lib/libc.so.6
One 'workaround' is to send an additional SIGSTOP from the parent: gcc -Wall -g -DADDITIONAL_SIGSTOP -o gdb_attach gdb_attach.c ./gdb_attach parent child parent: cmdl: gdb gdb_attach 1519 <gdb output> 0x4005aab1 in kill () from /lib/libc.so.6 (gdb) bt #0 0x4005aab1 in kill () from /lib/libc.so.6 #1 0x080486ec in start_debugger () at gdb_attach.c:17 #2 0x080488c5 in main (argc=1, argv=0xbffff444) at gdb_attach.c:46 #3 0x400499ed in __libc_start_main () from /lib/libc.so.6 (gdb)
It this a PTRACE_DETACH bug (kernel used 2.4.25)? If not, is the additional SIGSTOP a valid 'workaround'? If not, is there a better workaround/solution?
Thanks in advance for your help, Peter Seiderer
[1] http://valgrind.kde.org/
[2] http://webcvs.kde.org/cgi-bin/cvsweb.cgi/~checkout~/valgrind/coregrind/vg_main.c?rev=1.147&content-type=text/plain
[3] http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi?coll=linux&db=man&fname=/usr/share/catman/man2/ptrace.2.html PTRACE_DETACH: ... Restarts the stopped child as for PTRACE_CONT ... PTRACE_CONT: ... Restarts the stopped child process. If data is non-zero and not SIGSTOP, it is interpreted as a signal to be delivered to the child ...
-- ------------------------------------------------------------------------ Peter Seiderer E-Mail: seiderer123@ciselant.de #include <stdio.h> #include <sys/ptrace.h> #include <sys/types.h> #include <unistd.h> #include <signal.h> #include <sys/wait.h> #include <errno.h> #include <string.h> #include <stdlib.h> #include <assert.h>
void start_debugger() { int pid; if ((pid = fork()) == 0) { printf("child\n"); ptrace(PTRACE_TRACEME, 0, NULL, NULL); kill(getpid(), SIGSTOP); while(1){ printf("child: should never be reached\n"); sleep(1); } } else if (pid > 0) { int status; printf("parent\n"); assert(waitpid(pid, &status, 0) == pid); assert(WIFSTOPPED(status)); assert(WSTOPSIG(status) == SIGSTOP); #ifdef ADDITIONAL_SIGSTOP // send additional SIGSTOP to work around possible PTRACE_DETACH bug assert(kill(pid, SIGSTOP) == 0); #endif assert(ptrace(PTRACE_DETACH, pid, NULL, SIGSTOP) == 0); char buf[1024]; snprintf(buf, 1024, "gdb gdb_attach %d", pid); printf("parent: cmdl: %s\n", buf); system(buf); } else { printf("parent: fork failed\n"); } }
int main(int argc, char *argv[]) { start_debugger(); return 0; }
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