strace -f cannot trace failing execve()

[Michael Holzheu]

Hi all,

I found a problem running strace 4.5.1 under s390(x) using the -f option
with a program that calls execve() which fails.

Here is the testprogram:
========================

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>

int main(void)
{
        pid_t child;
        if(child = fork()){
                int status;
                printf("parent\n");
                waitpid(child,&status,0);
        } else {
                printf("child\n");
                execve("blubber",NULL,NULL);
                printf("child\n");
        }
        return 0;
}


If I call "strace -f ./testme" under Linux 2.6.2 strace hangs:
==============================================================

clone(Process 2224 attached
child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD,
child_tidptr=0x4016d2e8) = 2224
[pid  2223] fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1),
...}) = 0
[pid  2223] mmap(NULL, 4096, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40018000
[pid  2223] write(1, "parent\n", 7parent
)     = 7
[pid  2223] wait4(2224, Process 2223 suspended
 <unfinished ...>
[pid  2224] fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1),
...}) = 0
[pid  2224] mmap(NULL, 4096, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x40018000
[pid  2224] write(1, "child\n", 6child
)      = 6
[pid  2224] execve("blubber", [0], [/* 0 vars */]) = -1 ENOENT (No such
file or directory)
child
[pid  2224] close(1)                    = 91
[pid  2224] setup()                     = 248
PANIC: attached pid 2224 exited

<<< Here hangs strace


If I call "strace -f ./testme" under Linux 2.4.21 strace prints wrong
systemcalls:
==================================================================================

fork(Process 597 attached (waiting for parent)
Process 597 resumed (parent 596 ready)
)                                  = 597
[pid   597] fstat64(1,  <unfinished ...>
[pid   596] fstat64(1,  <unfinished ...>
[pid   597] <... fstat64 resumed> {st_mode=S_IFCHR|0620,
st_rdev=makedev(136, 0), ...}) = 0
[pid   596] <... fstat64 resumed> {st_mode=S_IFCHR|0620,
st_rdev=makedev(136, 0), ...}) = 0

....

[pid   596] <... write resumed> )       = 7
[pid   597] execve("blubber", [0], [/* 0 vars */] <unfinished ...>
[pid   596] wait4(597, Process 596 suspended
 <unfinished ...>
[pid   597] <... execve resumed> )      = -1 ENOENT (No such file or
directory)
child
[pid   597] close(1)                    = -1 ENOSYS (Function not
implemented) >>>>> Wrong system call: Should be write
[pid   597] setup()                     = -1 ENOSYS (Function not
implemented) >>>>> Wrong system call: Should be munmap
[pid   597] exit_group(0)               = ?
Process 596 resumed
Process 597 detached
<... wait4 resumed> [WIFEXITED(s) && WEXITSTATUS(s) == 0], 0, NULL) = 597
--- SIGCHLD (Child exited) @ 0 (0) ---
munmap(0x40018000, 4096)                = 0
exit_group(0)                           = ?



  The problem occurs, if execve fails with an error code. E.g. if the file
  to excecute is not available/readable.

  If the execve systemcall is traced, strace sets TCB_WAITEXECVE in the
  flags an then "forgets" the next SIGTRAP. This works, if the execve was
  successfull, but does not work, if execve failed. Therefore I think that
  we have to check if execve failed and reset the flags in this case:

  I have a bugfix for the problem:

  syscall.c:

  --- syscall.c.orig      2004-02-20 15:55:32.576657126 +0100
  +++ syscall.c   2004-02-20 15:57:16.276657126 +0100
  @@ -2183,6 +2183,12 @@
                                  tprintf("= %#lx", tcp->u_rval);
                  }
                  else if (!(sys_res & RVAL_NONE) && u_error) {
  +#ifdef TCB_WAITEXECVE
  +                       if(tcp->scno == SYS_execve){
  +                               tcp->flags &= ~TCB_WAITEXECVE;
  +                       }
  +#endif
  +
                          switch (u_error) {
   #ifdef LINUX
                          case ERESTARTSYS:

  (See attached file: strace-4.5.1-execve.diff)


  What do you think?

Best Regards

       Michael

------------------------------------------------------------------------
Linux for E-Server Development
Phone: +49-7031-16-2360,  Bld 71032-03-U09
Email: holzheu at de.ibm.com
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