Subject: Re: [PATCH v6 09/10] run-command: add close_fd_above_stderr option

From: Junio C Hamano <[email-masked]>
Date: Wed, 25 Feb 2026 13:41:59 -0800

"Paul Tarjan via GitGitGadget" <[email-masked]> writes:

> From: Paul Tarjan <[email-masked]>
>
> Add a new option to struct child_process that closes file descriptors
> 3 and above in the child after forking but before exec.  This prevents
> long-running child processes from inheriting pipe endpoints or other
> descriptors from the parent environment.
>
> The upper bound for the fd scan comes from sysconf(_SC_OPEN_MAX),
> capped at 4096 to avoid excessive iteration when the limit is set
> very high.
>
> Signed-off-by: Paul Tarjan <[email-masked]>
> ---
>  run-command.c | 11 +++++++++++
>  run-command.h |  9 +++++++++
>  2 files changed, 20 insertions(+)

All makes sense.

I somehow find it a bit surprising that we did not already have this
feature anywhere, as closing all except for the low file descriptors
connected to stdio by default is fairly a common thing to do.

> diff --git a/run-command.c b/run-command.c
> index e3e02475cc..cbadcf5ff8 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -832,6 +832,17 @@ fail_pipe:
>  			child_close(cmd->out);
>  		}
>  
> +		if (cmd->close_fd_above_stderr) {
> +			long max_fd = sysconf(_SC_OPEN_MAX);
> +			int fd;
> +			if (max_fd < 0 || max_fd > 4096)
> +				max_fd = 4096;
> +			for (fd = 3; fd < max_fd; fd++) {
> +				if (fd != child_notifier)
> +					close(fd);
> +			}
> +		}
> +
>  		if (cmd->dir && chdir(cmd->dir))
>  			child_die(CHILD_ERR_CHDIR);
>  
> diff --git a/run-command.h b/run-command.h
> index 0df25e445f..a1aa1b1069 100644
> --- a/run-command.h
> +++ b/run-command.h
> @@ -141,6 +141,15 @@ struct child_process {
>  	unsigned stdout_to_stderr:1;
>  	unsigned clean_on_exit:1;
>  	unsigned wait_after_clean:1;
> +
> +	/**
> +	 * Close file descriptors 3 and above in the child after forking
> +	 * but before exec.  This prevents the long-running child from
> +	 * inheriting pipe endpoints or other descriptors from the parent
> +	 * environment (e.g., the test harness).
> +	 */
> +	unsigned close_fd_above_stderr:1;
> +
>  	void (*clean_on_exit_handler)(struct child_process *process);
>  };