Re: [RFC] exec: add a flag for "reasonable" execveat() comm

Tycho Andersen
2024-10-09T14:41:37Z

On Wed, Oct 02, 2024 at 02:34:43PM +0000, Zbigniew Jędrzejewski-Szmek wrote:
> On Tue, Sep 24, 2024 at 12:39:35PM -0500, Eric W. Biederman wrote:
> > Tycho Andersen <tycho@tycho.pizza> writes:
> >
> > > From: Tycho Andersen <tandersen@netflix.com>
> > >
> > > Zbigniew mentioned at Linux Plumber's that systemd is interested in
> > > switching to execveat() for service execution, but can't, because the
> > > contents of /proc/pid/comm are the file descriptor which was used,
> > > instead of the path to the binary. This makes the output of tools like
> > > top and ps useless, especially in a world where most fds are opened
> > > CLOEXEC so the number is truly meaningless.
> > >
> > > This patch adds an AT_ flag to fix up /proc/pid/comm to instead be the
> > > contents of argv[0], instead of the fdno.
>
> I tried this version (with a local modification to drop the flag and
> enable the new codepath if get_user_arg_ptr(argv, 0) returns nonnull
> as suggested later in the thread), and it seems to work as expected.
> In particular, 'pgrep' finds for the original name in case of
> symlinks.

Here is a version that only affects /proc/pid/comm, without a flag. We
still have to do the dance of keeping the user argv0 before actually
doing __set_task_comm(), since we want to surface the resulting fault
if people pass a bad argv0. Thoughts?

Tycho

diff --git a/fs/exec.c b/fs/exec.c
index dad402d55681..61de8a71f316 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1416,7 +1416,16 @@ int begin_new_exec(struct linux_binprm * bprm)
set_dumpable(current->mm, SUID_DUMP_USER);

perf_event_exec();
- __set_task_comm(me, kbasename(bprm->filename), true);
+
+ /*
+ * If fdpath was set, execveat() made up a path that will
+ * probably not be useful to admins running ps or similar.
+ * Let's fix it up to be something reasonable.
+ */
+ if (bprm->argv0)
+ __set_task_comm(me, kbasename(bprm->argv0), true);
+ else
+ __set_task_comm(me, kbasename(bprm->filename), true);

/* An exec changes our domain. We are no longer part of the thread
group */
@@ -1566,9 +1575,30 @@ static void free_bprm(struct linux_binprm *bprm)
if (bprm->interp != bprm->filename)
kfree(bprm->interp);
kfree(bprm->fdpath);
+ kfree(bprm->argv0);
kfree(bprm);
}

+static int bprm_add_fixup_comm(struct linux_binprm *bprm, struct user_arg_ptr argv)
+{
+ const char __user *p = get_user_arg_ptr(argv, 0);
+
+ /*
+ * In keeping with the logic in do_execveat_common(), we say p == NULL
+ * => "" for comm.
+ */
+ if (!p) {
+ bprm->argv0 = kstrdup("", GFP_KERNEL);
+ return 0;
+ }
+
+ bprm->argv0 = strndup_user(p, MAX_ARG_STRLEN);
+ if (bprm->argv0)
+ return 0;
+
+ return -EFAULT;
+}
+
static struct linux_binprm *alloc_bprm(int fd, struct filename *filename, int flags)
{
struct linux_binprm *bprm;
@@ -1975,6 +2005,10 @@ static int do_execveat_common(int fd, struct filename *filename,
goto out_ret;
}

+ retval = bprm_add_fixup_comm(bprm, argv);
+ if (retval != 0)
+ goto out_free;
+
retval = count(argv, MAX_ARG_STRINGS);
if (retval == 0)
pr_warn_once("process '%s' launched '%s' with NULL argv: empty string added\n",
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index e6c00e860951..0cd1f2d0e8c6 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -55,6 +55,7 @@ struct linux_binprm {
of the time same as filename, but could be
different for binfmt_{misc,script} */
const char *fdpath; /* generated filename for execveat */
+ const char *argv0; /* argv0 from execveat */
unsigned interp_flags;
int execfd; /* File descriptor of the executable */
unsigned long loader, exec;