Re: [PATCH v3] trace-cmd: Open code execvp routine to avoid multiple execve syscalls

[Steven Rostedt <rostedt@goodmis.org>]

On Sat, 21 Jan 2023 18:50:29 +1300
Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com> wrote:

> In tracecmd/trace-record.c:<run_cmd>, trace-cmd record -F <executable>
> is launched via the libc's execvp() routine. The way that execvp() routine
> works is by invoking execve syscall for every entry on the $PATH if
> command specified is neither absolute nor relative which can come across
> as a bit cryptic to untrained eyes.
> 
> - absolute path example:
> 
>         # trace-cmd record -p function_graph \
>                 -g __x64_sys_execve -O nofuncgraph-irqs \
>                 -n __cond_resched --max-graph-depth 1  \
>                 -F /usr/bin/echo "ftrace" > /dev/null
> 
>         # trace-cmd report
>         echo-172994 [000] 185539.798539: funcgraph_entry:      ! 803.376 us |  __x64_sys_execve();
> 
> - PATH-dependent path example:
> 
>         # trace-cmd record -p function_graph \
>                 -g __x64_sys_execve -O nofuncgraph-irqs \
>                 -n __cond_resched --max-graph-depth 1  \
>                 -F echo "ftrace" > /dev/null
> 
>         # trace-cmd report
>         echo-172656 [002] 185009.671586: funcgraph_entry:      ! 288.732 us |  __x64_sys_execve();
>         echo-172656 [002] 185009.671879: funcgraph_entry:      ! 158.337 us |  __x64_sys_execve();
>         echo-172656 [002] 185009.672042: funcgraph_entry:      ! 161.843 us |  __x64_sys_execve();
>         echo-172656 [002] 185009.672207: funcgraph_entry:      ! 157.656 us |  __x64_sys_execve();
>         echo-172656 [002] 185009.672369: funcgraph_entry:      ! 156.343 us |  __x64_sys_execve();
>         echo-172656 [002] 185009.672529: funcgraph_entry:      ! 863.629 us |  __x64_sys_execve();
> 
> Open code the libc's execvp routine into trace-cmd so ftrace will only
> start recording once the command is found when it needs to be found in
> PATH.
> 
> Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
> ---
> Changelog:
> 
> - v3: simplify path traversal code via strtok_r. (Req. Steven Rostedt)
> - v2: open code execvp routine into trace-cmd. (Req. Steven Rostedt)
> - v1: https://lore.kernel.org/linux-trace-devel/Y7dUo6woh9Y31cdl@mail.google.com/
> ---
>  tracecmd/trace-record.c | 48 +++++++++++++++++++++++++++++++++++------
>  1 file changed, 41 insertions(+), 7 deletions(-)
> 
> diff --git a/tracecmd/trace-record.c b/tracecmd/trace-record.c
> index 7f0cebe..c111ef6 100644
> --- a/tracecmd/trace-record.c
> +++ b/tracecmd/trace-record.c
> @@ -1683,6 +1683,46 @@ static int change_user(const char *user)
>  	return 0;
>  }
>  
> +static void execute_program(int argc, char **argv)
> +{
> +	char buf[PATH_MAX + NAME_MAX + 1];
> +	char *path;
> +	char *entry;
> +	char *saveptr;
> +
> +	/*
> +	 * if command specified by user is neither absolute nor
> +	 * relative than we search for it in $PATH.
> +	 */
> +	if (*argv[0] != '/' && *argv[0] != '.') {

All you really need to do is check if '/' exists in the name, as the path
lookup in bash will not add PATH in such cases:

 # mkdir /usr/bin/direct
 # cp /usr/bin/grep /usr/bin/direct
 # direct/grep
-bash: direct/grep: No such file or directory

So, the above really has to be just "if (strchr(argv[0], '/') == NULL)"

No need to look for '.'. As you could an application that starts with "."
and it should not think it's a path name in such cases.

In fact, if you have a '/' in the name. It will use relative paths.

 # mv /usr/bin/direct .
 # direct/grep -h
Usage: grep [OPTION]... PATTERNS [FILE]...
Try 'grep --help' for more information.

So appending PATH in such cases is incorrect, and will produce unexpected
results.

-- Steve

> +		path = getenv("PATH");
> +
> +		if (!path)
> +			die("can't search for '%s' if $PATH is NULL", argv[0]);
> +
> +		for (entry = strtok_r(path, ":", &saveptr);
> +		     entry; entry = strtok_r(NULL, ":", &saveptr)) {
> +
> +			snprintf(buf, sizeof(buf), "%s/%s", entry, argv[0]);
> +
> +			/* does it exist and can we execute it? */
> +			if (access(buf, X_OK) == 0)
> +				break;
> +
> +		}
> +	} else {
> +		strncpy(buf, argv[0], sizeof(buf));
> +	}
> +
> +	tracecmd_enable_tracing();
> +	if (execve(buf, argv, environ)) {
> +		fprintf(stderr, "\n********************\n");
> +		fprintf(stderr, " Unable to exec %s\n", argv[0]);
> +		fprintf(stderr, "********************\n");
> +		die("Failed to exec %s", argv[0]);
> +	}
> +}
> +
>  static void run_cmd(enum trace_type type, const char *user, int argc, char **argv)
>  {
>  	int status;
> @@ -1693,7 +1733,6 @@ static void run_cmd(enum trace_type type, const char *user, int argc, char **arg
>  	if (!pid) {
>  		/* child */
>  		update_task_filter();
> -		tracecmd_enable_tracing();
>  		if (!fork_process)
>  			enable_ptrace();
>  		/*
> @@ -1709,12 +1748,7 @@ static void run_cmd(enum trace_type type, const char *user, int argc, char **arg
>  		if (change_user(user) < 0)
>  			die("Failed to change user to %s", user);
>  
> -		if (execvp(argv[0], argv)) {
> -			fprintf(stderr, "\n********************\n");
> -			fprintf(stderr, " Unable to exec %s\n", argv[0]);
> -			fprintf(stderr, "********************\n");
> -			die("Failed to exec %s", argv[0]);
> -		}
> +		execute_program(argc, argv);
>  	}
>  	if (fork_process)
>  		exit(0);