[Libguestfs] [libnbd PATCH v4 0/2] lib/utils: introduce async-signal-safe execvpe()

[Laszlo Ersek]

On 3/22/23 15:45, Laszlo Ersek wrote:
> On 3/21/23 18:28, Eric Blake wrote:
> 
>> it is indeed a bug in busybox now that POSIX is moving towards
>> standardizing realpath, so I've filed it:
>> https://bugs.busybox.net/show_bug.cgi?id=15466
> 
> I've found another busybox bug.
> 
> The "/bin/sh" utility is provided by busybox as well (via the usual symlinking).
> 
> Per POSIX, if
> 
>   execvp(file, { argv[0], argv[1], ..., NULL })
> 
> were to fail with -1/ENOEXEC, then execvp() must retry "as if" with
> 
>   execv(<shell path>, { argv[0], file, argv[1], ..., NULL })
> 
> In other words, if direct execution of "file" failed because "file" "has the appropriate access permission but has an unrecognized format", then execvp() is required to try executing "file" as a shell script. For that, <shell path> is left unspecified by POSIX, but the arguments of the shell are specified:
> 
> - Argv[0] remains the same. That is, what we wanted "file" to know itself as, is what we now want *the shell executable* to know itself as.
> 
> - argv[1] becomes "file" -- this is the script that the shell is supposed to run.
> 
> - argv[2] and onwards become positional parameters $1, $2, ... for the shell script.
> 
> And the argv[0] specification is what's violated by busybox, because if argv[0] is anything other than "sh", then the busybox binary doesn't recognize itself as the shell!
> 
> The simplest way to demonstrate the bug is this:
> 
> bash-5.2$ ( exec -a foobar /bin/sh <<< "echo hello" )
> foobar: applet not found
> 
> 
> And then, another way to demonstrate the same busybox issue... lets us, in fact, discover a musl bug in turn!!!
> 
> Consider the following C program (called "test-execvp.c"; the binary is called "test-execvp"):
> 
> -------------
> #include <stdio.h>
> #include <unistd.h>
> 
> int main(void)
> {
>   char *args[] = { "foobar", NULL };
> 
>   execvp("hello.sh", args);
>   perror("execvp");
>   return 1;
> }
> -------------
> 
> The file "hello.sh" resides in the current directory (same directory where "test-execvp" resides). Furthermore it has execute permission, and the following contents:
> 
> -------
> echo hello
> -------
> 
> Now consider the following command (from bash):
> 
> $ PATH=.:$PATH test-execvp
> 
> What is supposed to happen is this:
> 
> (1) bash shall find test-execvp in the current directory per PATH,
> (2) execvp() shall find "hello.sh" in the current directory per PATH,
> (3) execvp() shall hit an internal failure -1/ENOEXEC,
> (4) execvp() shall then invoke the shell (under an unspecified pathname),
> (5) the shell shall get "foobar" for its argv[0], and "hello.sh" for its argv[1]
> (6) we shall see "hello" on the standard output.
> 
> That's exactly what happens on Linux/glibc. (Note: this result has absolutely nothing to do with my execvpe() implementation, or libnbd in the first place.)
> 
> Now, according to my above description of the busybox bug, we're tempted to believe that step (6) fails on Alpine Linux (using musl + busybox). We expect the busybox binary to be launched, via the /bin/sh symlink, in step (4), and we expect it to fail after step (5), due to it not recognizing "foobar" as an "applet name".
> 
> It turns out however that step (4) does not happen. musl does not handle ENOEXEC:

It's getting crazier by the hour.

I thought to create a reproducer for busybox, in spite of musl breaking down at an earlier point (see above). For that, I *statically linked* "test-execvp" on RHEL-9.1 (using glibc), and then executed the binary in the Alpine Linux container. This should eliminate musl from the picture, and exercise the ENOEXEC fallback (from glibc), invoking /bin/sh (aka busybox) under argv[0] "foobar", and trigger the "unknown applet" bug in busybox.

However, this does not happen. Instead, I get "hello" printed. How is that possible?

The solution is that glibc *too* has a bug, and that bug hides the busybox bug. Namely, in glibc, going back to historical commit

commit 6a032d81581978187f562e5533a32e0a6a3d352b (tag: cvs/libc-960210)
Author: Roland McGrath <roland at gnu.org>
Date:   Sat Feb 10 10:00:27 1996 +0000

    Sat Feb 10 04:18:48 1996  Roland McGrath  <roland at churchy.gnu.ai.mit.edu>
    
            * posix/execvp.c: If execv fails with ENOEXEC, run the shell on
            the file.
    
    Fri Feb  9 11:46:45 1996  Roland McGrath  <roland at churchy.gnu.ai.mit.edu>
    
            * time/Makefile (CFLAGS-zdump.c, CFLAGS-zic.c, CFLAGS-ialloc.c,
            CFLAGS-scheck.c): Use -DNOID instead of -Wno-unused.
    
            * hurd/Makefile (user-interfaces): Added hurd/tioctl.

(note the date: 1996!), the POSIX-mandated fallback

  execv(<shell path>, { argv[0], file, argv[1], ..., NULL })

is not being done. Instead, the following is done:

  execv(<shell path>, { <shell path>, file, argv[1], ..., NULL })

In other words, the original argv[0] is not preserved, but is replaced by <shell path>. (Look for _PATH_BSHELL in said historical glibc commit, and also in today's glibc file "posix/execvpe.c".)

This can be demonstrated with:

$ PATH=.:$PATH strace -etrace=execve test-execvp

execve("./test-execvp", ["test-execvp"], 0x7ffc0d1e5248 /* 85 vars */) = 0
execve("./hello.sh", ["foobar"], 0x7ffc528e14a8 /* 85 vars */) = -1 ENOEXEC (Exec format error)
execve("/bin/sh", ["/bin/sh", "./hello.sh"], 0x7ffc528e14a8 /* 85 vars */) = 0
hello
+++ exited with 0 +++

The third execve() call should be:

execve("/bin/sh", ["foobar", "./hello.sh"], 0x7ffc528e14a8 /* 85 vars */) = 0
                   ^^^^^^^^

Laszlo