[chrisseaton]

Any process can write into the argv they get from the kernel.

The kernel doesn’t need to monitor for reads - when proc reads it it’s read from the process.

It doesn’t need to be specially ‘shared’ with the kernel. The kernel can of course ready any memory it wants to from the process at any time.

I’ve implemented setting argv[0] in another language myself.

[dualbus]

Can you show an example of how this would work with BASH_ARGV0?

[chrisseaton]

Sorry I don’t know anything about how Bash is implemented, but when someone assigns to that variable Bash just needs to write that string to argv.

[dualbus]

I tried python, bash and even C, none of them update /proc/self/comm when argv[0] is updated:

  dualbus@system76-pc:~$ cat argv0.c
  #include <stdio.h>
  #include <string.h>
  int main(int argc, char **argv) {
      FILE *fp;
      char buf[256]; // XXX :-)
      strcpy(argv[0], "XYZ");
      //puts(argv[0]);
      fp = fopen("/proc/self/comm", "r");
      fread(&buf, 1, 256, fp);
      buf[255] = '\0';
      puts(buf);
  }
  dualbus@system76-pc:~$ gcc -o argv0 argv0.c  -Wall
  dualbus@system76-pc:~$ ./argv0
  argv0

  dualbus@system76-pc:~$ python -c 'import sys; sys.argv[0] = "XYZ"; print(open("/proc/self/comm").read())'
  python

  dualbus@system76-pc:~$ ~/src/gnu/bash/bash -c 'BASH_ARGV0="XYZ"; cat /proc/$BASHPID/comm'
  bash

Furthermore, https://github.com/torvalds/linux/blob/master/Documentation/... says:

  > 3.6   /proc/<pid>/comm  & /proc/<pid>/task/<tid>/comm
  > --------------------------------------------------------
  > These files provide a method to access a tasks comm value. It also allows for
  > a task to set its own or one of its thread siblings comm value. The comm value
  > is limited in size compared to the cmdline value, so writing anything longer
  > then the kernel's TASK_COMM_LEN (currently 16 chars) will result in a truncated
  > comm value.

Which works as advertised:

  dualbus@system76-pc:~$ ~/src/gnu/bash/bash -c 'echo -n XYZ > /proc/$BASHPID/comm; ps -p $BASHPID'
    PID TTY          TIME CMD
  28797 pts/6    00:00:00 XYZ

Can you show me an example, in any language, where updating argv[0] causes ps (or /proc/self/comm) to show the updated value?

EDIT: formatting.

EDIT2: I stand corrected, see willglynn's comment.

[LukeShu]

You're looking at the wrong file. Setting argv[0] doesn't update /proc/self/comm, it updates /proc/self/cmdline.

demo.c:

    #include <unistd.h>
    #include <string.h>
    
    int main(int argc, char *argv[]) {
    	strcpy(argv[0], "frob");
    	sleep(100);
    }

Terminal 1:

    $ make demo
    cc     demo.c   -o demo
    $ ./demo --greppable

Terminal 2:

    $ ps aux|grep greppable
    luke     25858  0.0  0.0   2164   752 pts/0    S+   00:32   0:00 frob o --greppable
    luke     25931  0.0  0.0   8192  2356 pts/5    S+   00:32   0:00 grep --color=auto greppable
    $ cat -v /proc/25858/cmdline
    frob^@o^@--greppable^@$

[chrisseaton]

    $ ruby -e "\$0 = 'im not lying to you'; sleep"
    $ ps