[dualbus]

I don't think changing argv[0] in the current process will have any effect in the /proc file system.

And to do what you describe, there's `exec -a NAME' already:

  $ (exec -a NOT-BASH bash -c 'echo $0; ps -p $BASHPID -f')
  NOT-BASH
  UID        PID  PPID  C STIME TTY          TIME CMD
  dualbus  18210  2549  0 19:30 pts/1    00:00:00 NOT-BASH -c echo $0; ps -p $BASHPID -f

[LukeShu]

> I don't think changing argv[0] in the current process will have any effect in the /proc file system.

Yes it does. This is a standard trick for changing the process name at runtime, several daemons do this to change the process name of child processes created by fork() that aren't separate executable. For instance, OpenSSH's sshd sets the child-process for a session to "sshd: USERNAME [priv]".

`exec -a` lets you set argv[0] through an execve() call, but many times you want to set it without exec'ing a new program.

[chrisseaton]

> I don't think changing argv[0] in the current process will have any effect in the /proc file system.

Yes it does - that’s the whole point of changing it.

[dualbus]

I would like to understand how this would work.

argv is a buffer in Bash's process memory space. This is AFAIK, not shared in any way with the kernel.

How would the kernel know that a process wrote to the memory location of argv[0] and then reflect that in /proc?

This is what I tried:

  dualbus@system76-pc:~/src/gnu/bash$ ./bash -c 'echo $BASH_VERSION; ps -p $BASHPID -f; BASH_ARGV0=NOT-BASH; echo $0; ps -p $BASHPID -f; (ps -p $BASHPID -f && : do not optimize fork)'
  5.0.0(1)-rc1
  UID        PID  PPID  C STIME TTY          TIME CMD
  dualbus  27918 20628  0 20:16 pts/5    00:00:00 ./bash -c echo $BASH_VERSION; ps -p $BASHPID -f; BASH_ARGV0=NOT-BASH; echo $0; ps -p $BASHPID -f; (ps -p $BASHPID -f && : do not optimize fork)
  NOT-BASH 
  UID        PID  PPID  C STIME TTY          TIME CMD
  dualbus  27918 20628  0 20:16 pts/5    00:00:00 ./bash -c echo $BASH_VERSION; ps -p $BASHPID -f; BASH_ARGV0=NOT-BASH; echo $0; ps -p $BASHPID -f; (ps -p $BASHPID -f && : do not optimize fork)
  UID        PID  PPID  C STIME TTY          TIME CMD
  dualbus  27921 27918  0 20:16 pts/5    00:00:00 ./bash -c echo $BASH_VERSION; ps -p $BASHPID -f; BASH_ARGV0=NOT-BASH; echo $0; ps -p $BASHPID -f; (ps -p $BASHPID -f && : do not optimize fork)

[willglynn]

On Linux, reading /proc/<pid>/cmdline literally asks the kernel to reach into the target process's address space and fish out its argv[0]. This, erm... has some corner cases:

https://github.com/torvalds/linux/blob/v4.20/fs/proc/base.c#...

https://github.com/torvalds/linux/blob/v4.20/fs/proc/base.c#...

Changing the `ps` output in a cross platform way requires a number of platform-dependent strategies, e.g. how PostgreSQL does it:

https://github.com/postgres/postgres/blob/REL_11_1/src/backe...

[dualbus]

Awesome, thank you! This is super useful. I stand corrected.

[LukeShu]

The argv array (and the envp array) are in a page the kernel set up when it created the process, and the kernel holds on a reference to it, and remembers the addres of those arrays in the page. The kernel doesn't need to "watch" that memory, when you read from `/proc/$pid/cmdline` or `/proc/$pid/environ`, procfs literally reads directly from $pid's memory space (remember that the kernel controls the page table, it can look in to the memory space of any process it wants to). The kernel doesn't "know" that the value changed, it just reads the current value from process' memory.

[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.