[PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm

[Josh Triplett]

Currently, execve allocates an mm and parses argv and envp before
checking if the path exists. However, the common case of a $PATH search
may have several failed calls to exec before a single success. Do a
filename lookup for the purposes of returning ENOENT before doing more
expensive operations.

This does not create a TOCTTOU race, because this can only happen if the
file didn't exist at some point during the exec call, and that point is
permitted to be when we did our lookup.

To measure performance, I ran 2000 fork and execvpe calls with a
seven-element PATH in which the file was found in the seventh directory
(representative of the common case as /usr/bin is the seventh directory
on my $PATH), as well as 2000 fork and execve calls with an absolute
path to an existing binary. I recorded the minimum time for each, to
eliminate noise from context switches and similar.

Without fast-path:
fork/execvpe: 49876ns
fork/execve:  32773ns

With fast-path:
fork/execvpe: 36890ns
fork/execve:  32069ns

The cost of the additional lookup seems to be in the noise for a
successful exec, but it provides a 26% improvement for the path search
case by speeding up the six failed execs.

Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
---

Discussed this at Plumbers with Kees Cook; turned out to be even more of
a win than anticipated.

 fs/exec.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 9a5ca7b82bfc..fe786aeb2f1b 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1881,6 +1881,16 @@ static int do_execveat_common(int fd, struct filename *filename,
 	if (IS_ERR(filename))
 		return PTR_ERR(filename);
 
+	/* Fast-path ENOENT for $PATH search failures, before we alloc an mm or
+	 * parse arguments. */
+	if (fd == AT_FDCWD && flags == 0 && filename->name[0] == '/') {
+		struct path path;
+		retval = filename_lookup(AT_FDCWD, filename, 0, &path, NULL);
+		if (retval == -ENOENT)
+			goto out_ret;
+		path_put(&path);
+	}
+
 	/*
 	 * We move the actual failure in case of RLIMIT_NPROC excess from
 	 * set*uid() to execve() because too many poorly written programs
-- 
2.37.2