On Sat, 21 Jan 2023 18:50:29 +1300
Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@xxxxxxxxx> 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@xxxxxxxxx>
> ---
> 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@xxxxxxxxxxxxxxx/
> ---
> 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);
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