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

[Edouard Klein <edou@rdklein.fr>]

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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