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

[Kees Cook <keescook@chromium.org>]

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...

-Kees

-- 
Kees Cook