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

[Tycho Andersen]

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@xxxxxxxxxxx>
> > >
> > > 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;