Re: [9fans] Inferno: starting a process in a stopped state

[Edouard Klein <edou@rdklein.fr>]

OK, my echo stop <>/prog/33/dbgctl was wrong.

It opens /prog/33/dbgctl for writing and reading, but only reads from
it, output still goes to the standard output.

Here what I came up with next:

echo stop <>[5]/prog/4/dbgctl >[1=5]

This opens /prog/4/dbgctl RW on FD 5, and then redirects the standard
output to FD5.

I tried it on a text file, it works in redirecting the output.

However, the process is not stopped when I write to its dbgctl file.

It continues happily eating CPU cycles jumping to itself, its
/prog/N/status still shows "ready", and reading /prog/N/stack yields

cat: error reading /prog/4/stack: thread must be stopped

I'll try again tomorrow but would appreciate any help or hints.

Edouard Klein <edou@rdklein.fr> writes:

> For the sake of people stumbling on this post a long time from now:
>
> I solved my immediate problem by running:
>
> echo stop <>/prog/33/dbgctl
>
> The <> redirection opens the file for reading and writing, not just for
> writing, and solves the "permission denied" issue.
>
> However, Inferno's sh(1) man pages mentions
>
>        A file descriptor may be redirected to an already  open  descriptor  by
>        writing  >[fd0=fd1] or <[fd0=fd1].  Fd1 is a previously opened file de‐
>        scriptor and fd0 becomes a new copy (in the sense of sys-dup(2)) of it.
>
> But I see no hint on how to actually open a file and get a file
> descriptor.
>
> It seems that there is a way to call limbo from sh, so maybe the open
> system call can be called from sh, but I haven't discovered the syntax
> yet.
>
> Anyway, if anybody knows, I'll be glad to know too, in the meantime, my
> current hurdle is cleared so off I go.
>
> Cheers,
>
> Edouard.
>
> Edouard Klein <edou@rdklein.fr> writes:
>
>> Thanks Ron :)
>>
>> In DIS assembly, this is written as jmp $0, which can be changed to jmp
>> $1 when I want to disable the loop and spare myself from decreasing all
>> PC offsets in the code.
>>
>>
>> Now, I have another problem: /prog/N/dbgctl can't be written to :/
>>
>>
>> Here is how to reproduce:
>>
>> Save the file below as hello.s
>> asm hello.s  # Assemble it
>> hello&  # Run it
>> bind '#p' /prog  # Mount /prog
>> ps  # Get the pid, assume e.g. 33
>> echo stop > /prog/33/dbgctl
>>
>> This yields
>> sh: cannot open /prog/33/dbgctl: permission denied
>>
>> instead of stopping the program.
>>
>> But I can still kill it with
>> echo kill > /prog/33/ctl
>> sh: 33 "Command":killed
>>
>> I have no idea what's wrong. The dbgctl has mode --rw-r--r-- so I should
>> be able to write to it.
>>
>> devprog.c has this snippet
>>
>>
>>      case Qdbgctl:
>>              if(SECURE || p->group->flags&Pprivatemem || omode != ORDWR)
>>                      error(Eperm);
>>
>> Which seems to indicate that the simple > redirection may fail because
>> it would only by an open in write mode.
>>
>> Is there a way, with Inferno's sh, to open a file in RW and get a file
>> descriptor (kinda like bash's exec N<> fname) ?
>>
>> Is the problem stemming from something else ?
>>
>> Any ideas, no matter how far fetched or off the cuff, would be welcome
>> as I'm banging my head against the wall, and the wall is starting to
>> suffer.
>>
>> Thanks in advance,
>>
>> Cheers,
>>
>> Edouard
>>
>>
>>
>> ------------hello.s--------------
>> #0
>>     jmp     $1
>>      load    0(mp),$0,12(mp)  # Loads "$Sys" into @mp+12, loading the link descriptors at $0
>>                               # ???: why 12 in @mp+12
>>                               # ???: where is $0 ?
>>      frame   $1,44(fp)        # Stores in @fp+44 (44 bytes after the current frame pointer)
>>                               # a pointer to a new stack frame to local-call to function of type $1
>>                               # ???: Why +44 ?
>>      movp    4(mp),32(44(fp)) # 32 after said new frame, store a pointer to @mp+4, the string to print
>>                               # ???: Why 32 ?
>>      lea     40(fp),16(44(fp))    # Stores the address of @fp+40 in 16 after said new frame
>>                               # ???: Why 40 ? Maybe because type $2 is 40 bytes long
>>                               # ???: Why 16 ?
>>      mcall   44(fp),$0,12(mp) # Call, in module @mp+12 ($Sys), the first function in linkage record array $0, in frame @fp+44
>>      ret                          # Return
>>      entry   0, 2             # Entrypoint is at the beginning of the instruction stream (offset 0)
>>                                   # The function type of this entry point is $2 (offset 2)
>>      desc    $0,16,"f0"       # Type $0 is 16 bytes ==  4 words long, all pointers
>>      desc    $1,40,"0080"     # Type $1 is 40 bytes == 10 words long, the ninth is a pointer
>>      desc    $2,48,"00c0"     # Type $2 is 48 bytes == 12 words long, the ninth and tenth are pointers
>>      var     @mp,16               # Define the module pointer, of length 16
>>      string  @mp+0,"$Sys"     # The name of the module to load
>>      string  @mp+4,"hello world and more\n"  # The string to print
>>      module  Command          # The module name
>>      link    2,0,0x4244b354,"init" # The doc is wrong, its desc, pointer, ... not pointer, desc...
>>                               # it is the list of exported symbols for the module.
>>      ldts    @ldt,1           # This look like the imported symbols from $Sys
>>      word    @ldt+0,1
>>      ext     @ldt+4,0xac849033,"print"
>>      source  "/tmp/quatorze/hello.b"
>>
>>
>>
>> ron minnich <rminnich@gmail.com> writes:
>>
>>> I put a
>>> 1:jump 1
>>> At the start, when I need to do this and have no other way.
>>> 
>>> On Mon, Jul 22, 2024 at 01:28 Edouard Klein <edou@rdklein.fr> wrote:
>>> 
>>> Hi !
>>> 
>>> I'm writing dis assembly, and I would like to debug my program right
>>> from the entrypoint.
>>> 
>>> I intend to use the /prog filesystem for that, thanks to which one can
>>> stop a program by echoing "stop" to dbgctl file.
>>> 
>>> However, what I need to do is stop the program at the very start.
>>> 
>>> I have multiple ideas, such as introducing a sleep or a read that would
>>> let me enough time to stop the program, but they have the bad property
>>> of modifying the program.
>>> 
>>> I could create a loader program that would exec the target program and
>>> stop the loader right before the exec, and I suspect (but I'm not sure)
>>> that would let me stop the target program.
>>> 
>>> But these all sound very rube goldbergy to me.
>>> 
>>> Is there a way to start a program in a stopped state that I have
>>> overlooked ?
>>> 
>>> Thanks in advance,
>>> 
>>> Cheers,
>>> 
>>> Edouard.
>>> 
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