On attempts to block or set SIGTRAP handler,
for example, using sigaction syscall, we generate
an additional SIGSTOP:
if ((long)sa.SA_HANDLER == (long)SIG_ERR)
tprints("{SIG_ERR, ");
else if ((long)sa.SA_HANDLER == (long)SIG_DFL)
tprints("{SIG_DFL, ");
else if ((long)sa.SA_HANDLER == (long)SIG_IGN) {
if (tcp->u_arg[0] == SIGTRAP) {
tcp->flags |= TCB_SIGTRAPPED;
kill(tcp->pid, SIGSTOP);
}
tprints("{SIG_IGN, ");
}
else {
if (tcp->u_arg[0] == SIGTRAP) {
tcp->flags |= TCB_SIGTRAPPED;
kill(tcp->pid, SIGSTOP);
}
tprintf("{%#lx, ", (long) sa.SA_HANDLER);
}
In strace, it looks this way:
Entering in sys_sigaction(SIGTRAP):
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == 133}], __WALL, NULL) = 5848
ptrace(PTRACE_GETREGS, 5848, 0, 0x808d3e0) = 0
ptrace(PTRACE_PEEKDATA, 5848, 0xbf996274, [0x804825b]) = 0
ptrace(PTRACE_PEEKDATA, 5848, 0xbf996278, [0]) = 0
ptrace(PTRACE_PEEKDATA, 5848, 0xbf99627c, [0x4000000]) = 0
ptrace(PTRACE_PEEKDATA, 5848, 0xbf996280, [0x8048253]) = 0
Aha, it's the 'else' branch (in the above C code), send SIGSTOP:
kill(5848, SIGSTOP) = 0
print the log:
write(3, "sigaction(SIGTRAP, {0x804825b, [], SA_RESTORER, 0x8048253}, ", 60) =
60
go go go:
ptrace(PTRACE_SYSCALL, 5848, 0x1, SIG_0) = 0
--- {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=5848, si_status=SIGTRAP,
si_utime=0, si_stime=0} (Child exited) ---
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == 133}], __WALL, NULL) = 5848
exiting sigaction - print result:
ptrace(PTRACE_GETREGS, 5848, 0, 0x808d3e0) = 0
write(3, "NULL, 0x8048253) = 0\n", 21) = 21
continue:
ptrace(PTRACE_SYSCALL, 5848, 0x1, SIG_0) = 0
oops, got our STOP:
--- {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=5848, si_status=SIGSTOP,
si_utime=0, si_stime=0} (Child exited) ---
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSTOP}], __WALL, NULL) = 5848
ignore it:
ptrace(PTRACE_SYSCALL, 5848, 0x1, SIG_0) = 0
--- {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=5848, si_status=SIGTRAP,
si_utime=0, si_stime=0} (Child exited) ---
entering into next syscall, getpid:
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == 133}], __WALL, NULL) = 5848
ptrace(PTRACE_GETREGS, 5848, 0, 0x808d3e0) = 0
write(3, "getpid(", 7) = 7
...
Attached patch gets rid of this SIGSTOP sending/ignoring.
It appears to work just fine:
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP}], __WALL, NULL) = 5967
ptrace(PTRACE_GETREGS, 5967, 0, 0x808c2c0) = 0
ptrace(PTRACE_PEEKDATA, 5967, 0xbfe764b4, [0x804825b]) = 0
ptrace(PTRACE_PEEKDATA, 5967, 0xbfe764b8, [0]) = 0
ptrace(PTRACE_PEEKDATA, 5967, 0xbfe764bc, [0x4000000]) = 0
ptrace(PTRACE_PEEKDATA, 5967, 0xbfe764c0, [0x8048253]) = 0
write(3, "sigaction(SIGTRAP, {0x804825b, [], SA_RESTORER, 0x8048253}, ", 60) =
60
ptrace(PTRACE_SYSCALL, 5967, 0x1, SIG_0) = 0
--- {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=5967, si_status=SIGTRAP,
si_utime=0, si_stime=0} (Child exited) ---
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP}], __WALL, NULL) = 5967
ptrace(PTRACE_GETREGS, 5967, 0, 0x808c2c0) = 0
write(3, "NULL, 0x8048253) = 0\n", 21) = 21
ptrace(PTRACE_SYSCALL, 5967, 0x1, SIG_0) = 0
--- {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=5967, si_status=SIGTRAP,
si_utime=0, si_stime=0} (Child exited) ---
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP}], __WALL, NULL) = 5967
ptrace(PTRACE_GETREGS, 5967, 0, 0x808c2c0) = 0
write(3, "getpid(", 7) = 7
...
It also works if I force strace to not use PTRACE_O_TRACESYSGOOD,
which means strace stops will be marked with SIGTRAP,
not (SIGTRAP | 0x80) - I wondered maybe that's when it's needed:
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP}], __WALL, NULL) = 6002
ptrace(PTRACE_GETREGS, 6002, 0, 0x808c2a0) = 0
ptrace(PTRACE_PEEKDATA, 6002, 0xbfa63254, [0x804825b]) = 0
ptrace(PTRACE_PEEKDATA, 6002, 0xbfa63258, [0]) = 0
ptrace(PTRACE_PEEKDATA, 6002, 0xbfa6325c, [0x4000000]) = 0
ptrace(PTRACE_PEEKDATA, 6002, 0xbfa63260, [0x8048253]) = 0
write(3, "sigaction(SIGTRAP, {0x804825b, [], SA_RESTORER, 0x8048253}, ", 60) =
60
ptrace(PTRACE_SYSCALL, 6002, 0x1, SIG_0) = 0
--- {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=6002, si_status=SIGTRAP,
si_utime=0, si_stime=0} (Child exited) ---
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP}], __WALL, NULL) = 6002
ptrace(PTRACE_GETREGS, 6002, 0, 0x808c2a0) = 0
write(3, "NULL, 0x8048253) = 0\n", 21) = 21
ptrace(PTRACE_SYSCALL, 6002, 0x1, SIG_0) = 0
--- {si_signo=SIGCHLD, si_code=CLD_TRAPPED, si_pid=6002, si_status=SIGTRAP,
si_utime=0, si_stime=0} (Child exited) ---
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGTRAP}], __WALL, NULL) = 6002
ptrace(PTRACE_GETREGS, 6002, 0, 0x808c2a0) = 0
write(3, "getpid(", 7) = 7
...
As you see, the same thing. No hangs alluded to by
/*
* Trapped attempt to block SIGTRAP
* Hope we are back in control now.
*/
comment which used to be there.
So, why we even have TCB_SIGTRAPPED? No one knows. It predates
version control: this code was present in the initial commit,
in 1999.
Moreover, TCB_SIGTRAPPED is not used in sys_rt_sigaction,
only in obsolete sys_signal, sys_sigaction, sys_sigsetmask,
and in some dead non-Linux code.
I think whatever bug it was fixing is gone long ago -
at least as long as sys_rt_sigaction is used by glibc.
Again, since glibc (and uclibc) uses sys_rt_sigaction
and sys_sigprocmask, modified code paths are not used
by most programs anyway.
Test program I used for generating logs shown above
is below. Use:
strace -oLOG0 -s99 ./strace -o LOG -s99 ./test
--
vda
#include <ctype.h>
#include <dirent.h>
#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <netdb.h>
#include <setjmp.h>
#include <signal.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>
#include <stddef.h>
#include <string.h>
#include <syscall.h>
#include <sys/poll.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <termios.h>
#include <time.h>
#include <sys/param.h>
/* This is the sigaction structure from the Linux 2.1.20 kernel. */
struct old_kernel_sigaction {
__sighandler_t k_sa_handler;
unsigned long sa_mask;
unsigned long sa_flags;
void (*sa_restorer)(void);
};
#define SA_RESTORER 0x04000000
extern void restoreZ(void) __asm__ ("__restoreZ");
int sigactionZ(int sig, const struct sigaction *act, struct sigaction *oact)
{
int result;
struct old_kernel_sigaction kact, koact;
if (act) {
kact.k_sa_handler = act->sa_handler;
kact.sa_mask = act->sa_mask.__val[0];
kact.sa_flags = act->sa_flags | SA_RESTORER;
kact.sa_restorer = &restoreZ;
}
__asm__ __volatile__ (
" pushl %%ebx\n"
" movl %3, %%ebx\n"
" int $0x80\n"
" popl %%ebx\n"
: "=a" (result), "=m" (koact)
: "0" (__NR_sigaction), "r" (sig), "m" (kact),
"c" (act ? &kact : NULL),
"d" (oact ? &koact : NULL));
if (result < 0) {
__set_errno(-result);
return -1;
}
if (oact) {
oact->sa_handler = koact.k_sa_handler;
oact->sa_mask.__val[0] = koact.sa_mask;
oact->sa_flags = koact.sa_flags;
oact->sa_restorer = koact.sa_restorer;
}
return result;
}
#define RESTORE(name, syscall) RESTORE2(name, syscall)
#define RESTORE2(name, syscall) \
__asm__ ( \
".text\n" \
"__" #name ":\n" \
" popl %eax\n" \
" movl $" #syscall ", %eax\n" \
" int $0x80\n" \
);
RESTORE(restoreZ, __NR_sigreturn)
static void handler(int sig)
{
printf("Got sig:%d\n", sig);
}
int main(int argc, char **argv)
{
struct sigaction sa;
memset(&sa, 0, sizeof(sa));
sa.sa_handler = handler;
///signal(SIGTRAP, handler);
sigactionZ(SIGTRAP, &sa, NULL);
raise(SIGTRAP); // 1
sigset_t set;
sigemptyset(&set);
sigaddset(&set, SIGTRAP);
sigprocmask(SIG_BLOCK, &set, NULL);
raise(SIGTRAP);
// trap should not happen
sigprocmask(SIG_UNBLOCK, &set, NULL);
// 2 trap should happen here
///signal(SIGTRAP, SIG_IGN);
sa.sa_handler = SIG_IGN;
sigactionZ(SIGTRAP, &sa, NULL);
raise(SIGTRAP);
// trap should not happen
///signal(SIGTRAP, handler);
sa.sa_handler = handler;
sigactionZ(SIGTRAP, &sa, NULL);
raise(SIGTRAP); // 3
return 0;
}
diff -d -urpN strace.7/defs.h strace.8/defs.h
--- strace.7/defs.h 2012-01-18 15:18:36.384963666 +0100
+++ strace.8/defs.h 2012-01-16 13:09:14.313265676 +0100
@@ -477,7 +477,6 @@ struct tcb {
#define TCB_INSYSCALL 00010
#define TCB_ATTACHED 00020 /* Process is not our own child */
#define TCB_BPTSET 00100 /* "Breakpoint" set after fork(2) */
-#define TCB_SIGTRAPPED 00200 /* Process wanted to block SIGTRAP */
#define TCB_REPRINT 01000 /* We should reprint this syscall on exit */
#define TCB_FILTERED 02000 /* This system call has been filtered out */
#ifdef LINUX
diff -d -urpN strace.7/signal.c strace.8/signal.c
--- strace.7/signal.c 2012-01-18 15:19:26.334601560 +0100
+++ strace.8/signal.c 2012-01-18 15:16:33.872902944 +0100
@@ -842,24 +842,12 @@ sys_sigvec(struct tcb *tcp)
tprints("{SIG_DFL}");
break;
case (int) SIG_IGN:
- if (tcp->u_arg[0] == SIGTRAP) {
- tcp->flags |= TCB_SIGTRAPPED;
- kill(tcp->pid, SIGSTOP);
- }
tprints("{SIG_IGN}");
break;
case (int) SIG_HOLD:
- if (tcp->u_arg[0] == SIGTRAP) {
- tcp->flags |= TCB_SIGTRAPPED;
- kill(tcp->pid, SIGSTOP);
- }
- tprints("SIG_HOLD");
+ tprints("{SIG_HOLD}");
break;
default:
- if (tcp->u_arg[0] == SIGTRAP) {
- tcp->flags |= TCB_SIGTRAPPED;
- kill(tcp->pid, SIGSTOP);
- }
tprintf("{%#lx, ", (unsigned long) sv.sv_handler);
printsigmask(&sv.sv_mask, 0);
tprints(", ");
@@ -923,14 +911,6 @@ sys_sigsetmask(struct tcb *tcp)
sigset_t sigm;
long_to_sigset(tcp->u_arg[0], &sigm);
printsigmask(&sigm, 0);
-#ifndef USE_PROCFS
- if ((tcp->u_arg[0] & sigmask(SIGTRAP))) {
- /* Mark attempt to block SIGTRAP */
- tcp->flags |= TCB_SIGTRAPPED;
- /* Send unblockable signal */
- kill(tcp->pid, SIGSTOP);
- }
-#endif /* !USE_PROCFS */
}
else if (!syserror(tcp)) {
sigset_t sigm;
@@ -1005,24 +985,10 @@ sys_sigaction(struct tcb *tcp)
tprints("{SIG_ERR, ");
else if ((long)sa.SA_HANDLER == (long)SIG_DFL)
tprints("{SIG_DFL, ");
- else if ((long)sa.SA_HANDLER == (long)SIG_IGN) {
-#ifndef USE_PROCFS
- if (tcp->u_arg[0] == SIGTRAP) {
- tcp->flags |= TCB_SIGTRAPPED;
- kill(tcp->pid, SIGSTOP);
- }
-#endif /* !USE_PROCFS */
+ else if ((long)sa.SA_HANDLER == (long)SIG_IGN)
tprints("{SIG_IGN, ");
- }
- else {
-#ifndef USE_PROCFS
- if (tcp->u_arg[0] == SIGTRAP) {
- tcp->flags |= TCB_SIGTRAPPED;
- kill(tcp->pid, SIGSTOP);
- }
-#endif /* !USE_PROCFS */
+ else
tprintf("{%#lx, ", (long) sa.SA_HANDLER);
- }
#ifndef LINUX
printsigmask(&sa.sa_mask, 0);
#else
@@ -1060,21 +1026,9 @@ sys_signal(struct tcb *tcp)
tprints("SIG_DFL");
break;
case (long) SIG_IGN:
-#ifndef USE_PROCFS
- if (tcp->u_arg[0] == SIGTRAP) {
- tcp->flags |= TCB_SIGTRAPPED;
- kill(tcp->pid, SIGSTOP);
- }
-#endif /* !USE_PROCFS */
tprints("SIG_IGN");
break;
default:
-#ifndef USE_PROCFS
- if (tcp->u_arg[0] == SIGTRAP) {
- tcp->flags |= TCB_SIGTRAPPED;
- kill(tcp->pid, SIGSTOP);
- }
-#endif /* !USE_PROCFS */
tprintf("%#lx", tcp->u_arg[1]);
}
return 0;
diff -d -urpN strace.7/strace.c strace.8/strace.c
--- strace.7/strace.c 2012-01-18 15:18:36.386963635 +0100
+++ strace.8/strace.c 2012-01-16 13:09:04.314285771 +0100
@@ -2540,15 +2540,6 @@ trace()
}
if (sig != syscall_trap_sig) {
- if (sig == SIGSTOP &&
- (tcp->flags & TCB_SIGTRAPPED)) {
- /*
- * Trapped attempt to block SIGTRAP
- * Hope we are back in control now.
- */
- tcp->flags &= ~(TCB_INSYSCALL | TCB_SIGTRAPPED);
- goto restart_tracee_with_sig_0;
- }
if (cflag != CFLAG_ONLY_STATS
&& (qual_flags[sig] & QUAL_SIGNAL)) {
siginfo_t si;
------------------------------------------------------------------------------
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