[lxc-devel] [PATCH v2] use PR_SET_MM_MAP instead of PR_SET_MM

[Serge Hallyn]

Quoting Tycho Andersen (tycho.andersen at canonical.com):
> PR_SET_MM_MAP can be called as non-root, which we are in the unprivileged
> (or nested) case.
> 
> Also, let's not do the strcpy() for the new cmdline until after we're sure
> the prctl succeeded. This means that even if it does fail, we won't
> mutilate the command line like we did before, it just won't be as pretty.
> 
> v2: remember to chop off bits of the string that are too long
> 
> Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>

Acked-by: Serge E. Hallyn <serge.hallyn at ubuntu.com>

> ---
>  src/lxc/utils.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++----------
>  1 file changed, 58 insertions(+), 12 deletions(-)
> 
> diff --git a/src/lxc/utils.c b/src/lxc/utils.c
> index 01774c0..d592243 100644
> --- a/src/lxc/utils.c
> +++ b/src/lxc/utils.c
> @@ -1347,7 +1347,15 @@ int setproctitle(char *title)
>  	char buf[2048], *tmp;
>  	FILE *f;
>  	int i, len, ret = 0;
> -	unsigned long arg_start, arg_end, env_start, env_end;
> +
> +	/* We don't really need to know all of this stuff, but unfortunately
> +	 * PR_SET_MM_MAP requires us to set it all at once, so we have to
> +	 * figure it out anyway.
> +	 */
> +	unsigned long start_data, end_data, start_brk, start_code, end_code,
> +			start_stack, arg_start, arg_end, env_start, env_end,
> +			brk_val;
> +	struct prctl_mm_map prctl_map;
>  
>  	f = fopen_cloexec("/proc/self/stat", "r");
>  	if (!f) {
> @@ -1360,23 +1368,42 @@ int setproctitle(char *title)
>  		return -1;
>  	}
>  
> -	/* Skip the first 47 fields, column 48-51 are ARG_START and
> -	 * ARG_END. */
> +	/* Skip the first 25 fields, column 26-28 are start_code, end_code,
> +	 * and start_stack */
>  	tmp = strchr(buf, ' ');
> -	for (i = 0; i < 46; i++) {
> +	for (i = 0; i < 24; i++) {
>  		if (!tmp)
>  			return -1;
>  		tmp = strchr(tmp+1, ' ');
>  	}
> -
>  	if (!tmp)
>  		return -1;
>  
> -	i = sscanf(tmp, "%lu %lu %lu %lu", &arg_start, &arg_end, &env_start, &env_end);
> -	if (i != 4) {
> +	i = sscanf(tmp, "%lu %lu %lu", &start_code, &end_code, &start_stack);
> +	if (i != 3)
>  		return -1;
> +
> +	/* Skip the next 19 fields, column 45-51 are start_data to arg_end */
> +	for (i = 0; i < 19; i++) {
> +		if (!tmp)
> +			return -1;
> +		tmp = strchr(tmp+1, ' ');
>  	}
>  
> +	if (!tmp)
> +		return -1;
> +
> +	i = sscanf(tmp, "%lu %lu %lu %lu %lu %lu %lu",
> +		&start_data,
> +		&end_data,
> +		&start_brk,
> +		&arg_start,
> +		&arg_end,
> +		&env_start,
> +		&env_end);
> +	if (i != 7)
> +		return -1;
> +
>  	/* Include the null byte here, because in the calculations below we
>  	 * want to have room for it. */
>  	len = strlen(title) + 1;
> @@ -1386,6 +1413,7 @@ int setproctitle(char *title)
>  	if (len > env_end - arg_start) {
>  		arg_end = env_end;
>  		len = env_end - arg_start;
> +		title[len-1] = '\0';
>  	} else {
>  		/* Only truncate the environment if we're actually going to
>  		 * overwrite part of it. */
> @@ -1402,12 +1430,30 @@ int setproctitle(char *title)
>  
>  	}
>  
> -	strcpy((char*)arg_start, title);
> +	brk_val = syscall(__NR_brk, 0);
>  
> -	ret |= prctl(PR_SET_MM, PR_SET_MM_ARG_START,   arg_start, 0, 0);
> -	ret |= prctl(PR_SET_MM, PR_SET_MM_ARG_END,     arg_end, 0, 0);
> -	ret |= prctl(PR_SET_MM, PR_SET_MM_ENV_START,   env_start, 0, 0);
> -	ret |= prctl(PR_SET_MM, PR_SET_MM_ENV_END,     env_end, 0, 0);
> +	prctl_map = (struct prctl_mm_map) {
> +		.start_code = start_code,
> +		.end_code = end_code,
> +		.start_stack = start_stack,
> +		.start_data = start_data,
> +		.end_data = end_data,
> +		.start_brk = start_brk,
> +		.brk = brk_val,
> +		.arg_start = arg_start,
> +		.arg_end = arg_end,
> +		.env_start = env_start,
> +		.env_end = env_end,
> +		.auxv = NULL,
> +		.auxv_size = 0,
> +		.exe_fd = -1,
> +	};
> +
> +	ret = prctl(PR_SET_MM, PR_SET_MM_MAP, (long) &prctl_map, sizeof(prctl_map), 0);
> +	if (ret == 0)
> +		strcpy((char*)arg_start, title);
> +	else
> +		SYSERROR("setting cmdline failed");
>  
>  	return ret;
>  }
> -- 
> 2.5.0
> 
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