Re: dump, restore

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Finn,

Am 10.08.2024 um 12:42 schrieb Finn Thain:
>
> 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:

strace may not be aware of the CLONE_INTO_CGROUP flag yet? How old is 
your strace binary?

>> 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,

Nope, syscall no. 120 calls __sys_clone() which in turn calls 
m68k_clone() which emulates sys_clone() (roundabout way due to different 
calling conventions on m68k).

clone3 is syscall 435 (calling __sys_clone3() -> m68k_clone3() -> 
sys_clone3()).

But as long as syscall() takes care of the calling convention, I see no 
reason why that way of calling sys_clone() would fail.

Cheers,

	Michael


> -2147483631 == CLONE_IO|SIGCHLD like strace said.
>
> And why does it crash when I set a break point?
>