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

[Josh Triplett <josh@joshtriplett.org>]

On Fri, Sep 16, 2022 at 07:38:37AM -0700, Kees Cook wrote:
> On Fri, Sep 16, 2022 at 02:41:30PM +0100, Josh Triplett wrote:
> > 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.
> 
> At first I didn't understand how you were seeing this, since I'm so used
> to watching shell scripts under tracing, which correctly use stat():
> 
> $ strace bash -c foo
> stat("/home/keescook/bin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory)
> stat("/usr/local/sbin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory)
> stat("/usr/local/bin/foo", 0x7ffe1f9ddea0) = -1 ENOENT (No such file or directory)
> stat("/usr/sbin/foo", 0x7ffe1f9ddea0)   = -1 ENOENT (No such file or directory)
> stat("/usr/bin/foo", 0x7ffe1f9ddea0)    = -1 ENOENT (No such file or directory)
> stat("/sbin/foo", 0x7ffe1f9ddea0)       = -1 ENOENT (No such file or directory)
> stat("/bin/foo", 0x7ffe1f9ddea0)        = -1 ENOENT (No such file or directory)
> 
> But I see, yes, glibc tries to actually call execve(), which, as you
> say, is extremely heavy:
> 
> $ strace ./execvpe
> ...
> execve("/home/kees/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory)
> execve("/usr/local/sbin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory)
> execve("/usr/local/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory)
> execve("/usr/sbin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory)
> execve("/usr/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory)
> execve("/sbin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory)
> execve("/bin/foo", ["./execvpe"], 0x7ffc542bff38 /* 33 vars */) = -1 ENOENT (No such file or directory)
> 
> This really seems much more like a glibc bug. The shell does it correctly...

musl does the same thing, as do python and perl (likely via execvp or
posix_spawnp). As does gcc when it executes `as`. And I've seen more
than a few programs hand-implement a PATH search the same way. Seems
worth optimizing for.

And with io_uring_spawn, it'll be the substantially easier approach,
since it'll just require one pass (series of execs) rather than two
(stats then exec).

- Josh Triplett