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

[Edouard Klein <edou@rdklein.fr>]

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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