strace: Shows wrong system call

[Dmitry V. Levin]

On Wed, Jan 09, 2008 at 12:12:20AM +0200, Petri Latvala wrote:
> Package: strace
> Version: 4.5.14-2
> Severity: normal

This comes from quite an old Debian bug report
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=459820
which seems to be actual.

> -- BEGIN TESTCASE PROGRAM --
> 
> #include <stdio.h>
> #include <unistd.h>
> 
> int main ()
> {
>  printf("--------------------\n");
>  __asm__("movl $2, %eax; int $0x80");
>  printf("[i am %d]\n", getpid());
>  return 0;
> }
> 
> -- END TESTCASE PROGRAM --
> 
> The testcase code executes fork() by calling it directly. That is,
> putting 2 (SYS_fork on i386) into eax and doing int 0x80. Let's see how
> this runs on amd64:
> 
> --------------------
> [i am 10869]
> [i am 10868]
> 
> Yep, forks. Now let's see what strace tells about it:
> 
> <snip>
> write(1, "--------------------\n", 21--------------------
> )  = 21
> open(umovestr: Input/output error
> 0x1, O_WRONLY|0x14[i am 10873]
> )                = 10873
> --- SIGCHLD (Child exited) @ 0 (0) ---
> getpid()                                = 10872
> write(1, "[i am 10872]\n", 13[i am 10872]
> )          = 13
> exit_group(13)                          = ?
> Process 10872 detached
> 
> 
> The heading line is printed, then there's an open() printed instead of a
> fork, yet it did fork.
> 
> Possible cause: 2 is SYS_fork on i386, and this int 0x80 is i386 way of
> syscalling. amd64 way is SYSCALL orsomethingicantremember, and 2 is
> SYS_open on amd64. I don't know whether strace is confused, or whether
> ptrace() gives out something silly.

There is a commented out code in get_scno() that implements another method
of personality detection.  Could somebody more familiar with this subject
have a look?


-- 
ldv
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