Re: dump, restore

On Fri, 9 Aug 2024, I wrote:

> 
> On Sat, 3 Aug 2024, Stan Johnson wrote:
> 
> > 
> > Using "-a" appears to be a better option than just specifying a really 
> > long tape size. Unfortunately, it also doesn't work. The problem seems 
> > to affect only m68k; ppc-32, ppc-64, x86-32 and x86-64 all work as 
> > expected...
> 
> I reproduced the problem in QEMU and found it went away when I ran dump 
> under Linux v5.6. So I went through a lot of "git bisect" steps and the 
> culprit appears to be commit ef2c41cf38a7 ("clone3: allow spawning 
> processes into cgroups"). That seems plausible, since we are seeing an 
> error from fork_clone_io() below...
> 
> #ifdef __linux__
> #if defined(SYS_clone) && defined(CLONE_IO)
> pid_t
> fork_clone_io(void)
> {
>         return syscall(SYS_clone, CLONE_ARGS);
> }
> #endif
> #endif
> 
> That code bypasses the C library so I suppose it's not too surprising 
> that different architectures give different results...
> 
> Anyway, if I run dump under strace I see no CLONE_INTO_CGROUP flag:
> 
> clone(child_stack=NULL, flags=CLONE_IO|SIGCHLD) = -1 EBADF (Bad file 
> descriptor)
> 
> The -EBADF result was introduced into cgroup_css_set_fork() by the 
> commit above. That should not happen unless CLONE_INTO_CGROUP was set, 
> but strace says its not. So I don't know what's going on here.
> 

Here's what gdb says, FWIW...

# gdb
GNU gdb (Debian 13.1-3) 13.1
...
(gdb) file /usr/sbin/dump
Reading symbols from /usr/sbin/dump...
Reading symbols from /usr/lib/debug/.build-id/24/071a827207bee9c025d364137514447279302b.debug...
(gdb) run -0f /dev/null /dev/sda
Starting program: /usr/sbin/dump -0f /dev/null /dev/sda
  DUMP: Date of this level 0 dump: Fri Aug  9 23:37:15 2024
  DUMP: Dumping /dev/sda (an unlisted file system) to /dev/null
  DUMP: Label: none
  DUMP: Writing 10 Kilobyte records
  DUMP: mapping (Pass I) [regular files]
  DUMP: mapping (Pass II) [directories]
  DUMP: estimated 3595695 blocks.
  DUMP: Context save fork fails in parent 671
[Inferior 1 (process 671) exited with code 03]
(gdb) b fork_clone_io
Breakpoint 1 at 0x80009dbc: file tape.c, line 740.
(gdb) run -0f /dev/null /dev/sda
Starting program: /usr/sbin/dump -0f /dev/null /dev/sda
  DUMP: Date of this level 0 dump: Fri Aug  9 23:38:17 2024
  DUMP: Dumping /dev/sda (an unlisted file system) to /dev/null
  DUMP: Label: none
  DUMP: Writing 10 Kilobyte records
  DUMP: mapping (Pass I) [regular files]
  DUMP: mapping (Pass II) [directories]
  DUMP: estimated 3595695 blocks.

Program received signal SIGSEGV, Segmentation fault.
0x00000001 in ?? ()
(gdb) l fork_clone_io
warning: Source file is more recent than executable.
735
736     #ifdef __linux__
737     #if defined(SYS_clone) && defined(CLONE_IO)
738     pid_t
739     fork_clone_io(void)
740     {
741        pid_t res,parent;
742        parent=getppid();            /* az hackety hack... */
743
744        res=syscall(SYS_clone, CLONE_ARGS);
745        getppid();
746        /* as per clone call manpage: caching! */
747        getpid();
748     #ifdef __alpha__
749        syscall(SYS_getxpid);
750     #else
751         syscall(SYS_getpid);
752     #endif
753
754        /* az: clone manpage doesn't say jack about what the
(gdb) disas fork_clone_io 
Dump of assembler code for function fork_clone_io:
   0x80009dbc <+0>:     movel %d3,%sp@-
   0x80009dbe <+2>:     movel %d2,%sp@-
   0x80009dc0 <+4>:     bsrl 0x80004200 <getppid@plt>
   0x80009dc6 <+10>:    movel %d0,%d3
   0x80009dc8 <+12>:    clrl %sp@-
   0x80009dca <+14>:    clrl %sp@-
   0x80009dcc <+16>:    clrl %sp@-
   0x80009dce <+18>:    movel #-2147483631,%sp@-
   0x80009dd4 <+24>:    pea 0x78
   0x80009dd8 <+28>:    bsrl 0x80003fd0 <syscall@plt>
   0x80009dde <+34>:    movel %d0,%d2
   0x80009de0 <+36>:    bsrl 0x80004200 <getppid@plt>
   0x80009de6 <+42>:    bsrl 0x80003c9c <getpid@plt>
   0x80009dec <+48>:    pea 0x14
   0x80009df0 <+52>:    bsrl 0x80003fd0 <syscall@plt>
   0x80009df6 <+58>:    bsrl 0x80004200 <getppid@plt>
   0x80009dfc <+64>:    lea %sp@(24),%sp
   0x80009e00 <+68>:    cmpl %d0,%d3
   0x80009e02 <+70>:    beqs 0x80009e06 <fork_clone_io+74>
   0x80009e04 <+72>:    clrl %d2
   0x80009e06 <+74>:    movel %d2,%d0
   0x80009e08 <+76>:    movel %sp@+,%d2
   0x80009e0a <+78>:    movel %sp@+,%d3
   0x80009e0c <+80>:    rts
End of assembler dump.
(gdb) 

Is this clone syscall (0x78) really executing sys_clone3()? Also,
-2147483631 == CLONE_IO|SIGCHLD like strace said.

And why does it crash when I set a break point?