Libguestfs - Feb 2023 - [libnbd PATCH v3 09/29] lib/utils: introduce async-signal-safe execvpe()

On 2/21/23 13:08, Daniel P. Berrang? wrote:
> On Wed, Feb 15, 2023 at 03:11:38PM +0100, Laszlo Ersek wrote:
>> execvp() [01] is powerful:
>>
>> - it performs PATH search [02] if necessary,
>>
>> - it falls back to executing the file with the shell as a shell
>>   script in case the underlying execv() or execve() fails with
>>   ENOEXEC.
>>
>> However, execvp() is not async-signal-safe [03], and so we
shouldn't
>> call it in a child process forked from a multi-threaded parent
>> process [04], which the libnbd client application may well be.
>
> Is that actually a problem in the real world ?  There are various
> things not technically listed as async-signal-safe by POSIX, which
> all sane impls none the less make safe, or are practically safe in
> any sane program using them.
By my count, it's virtually impossible to implement exec*p*() and
friends without calling malloc() internally. That's the case even if you
can get PATH in there without any problems. In turn, malloc() relies on
libc-internal mutex(es) or other synchronization mechanisms, so that
multiple threads can call malloc() / free() etc simultaneously, for
managing the one shared address space of the process.

The problem is with calling fork() in a multi-threaded process. The
child process created by fork() only has one thread -- the thread
returning from fork(). All other threads simply "disappear", as viewed
from the child process's perspective. This means that, if one of those
"other" (disappearing) threads was in the middle of malloc(), holding
for example a mutex, then in the child process, that mutex will appear
as acquired, but no thread will own it -- so no thread will ever release
it. Then, when the one thread returning from fork() in the child process
calls exec*p*(), the malloc() call internal to exec*p*() will deadlock
on that mutex.

Rich mentioned that libnbd had actually encountered a bug of this kind,
just not specifically in exec*p*().

So the "async-signal-safe" term is a bit misleading here; I think what
POSIX actually means is a kind of general reentrancy. Anyway, the POSIX
language does use "async-signal-safe" when discussing this topic of
threads, in the fork() specification.

Now, the exec*p*() manual at

  https://man7.org/linux/man-pages/man3/execvp.3.html

calls execlp(), execvp(), execvpe() "MT-Safe env", so you are right
about at least Linux/glibc. I've (intentionally) not looked at the glibc
implementation of execvp(), but I agree that, *if* we are happy with
getenv() just because the linux/glibc manual calls it "MT-Safe env",
*then* we could arguably be satisfied with execvp() as well.

I don't know if/how that applies to FreeBSD / OpenBSD though.

  https://man.freebsd.org/cgi/man.cgi?query=execvp
  https://man.openbsd.org/execvp.3

These man pages do not seem to contain any of the strings "mt",
"async",
"signal", "safe", "thread".
> IIUC with execvp the risk would be that setenv makes any use of the
> environment potentially unsafe,
to a smaller extent, yes
> and possibly execvp might use malloc which is also technically unsafe.
primarily this one, yes
> Both of these factors would technically make your replacement unsafe
> too.
No, they wouldn't. That's the whole point. Both getenv() and malloc()
are restricted to the _init API, which is called in the parent process,
before fork(). The necessary information is prepared in a context
structure, which the child only inherits. After fork(), the child calls
a second API -- the effective execvpe() replacement -- which operates on
the inherited context structure. The only functions called from this
second API are write() and abort() -- indirectly, in case the new
async-signal-safe assert() variant fails --, and execve(). All three of
these functions are async-signal-safe.
> Apps simply shouldn't ever call setenv once threads are created, and
> malloc() is safe in any impl that is relevant these days.
FWIW, the linux/glibc manual states, in
<https://man7.org/linux/man-pages/man2/fork.2.html>:
>        *  After a fork() in a multithreaded program, the child can
>           safely call only async-signal-safe functions (see
>           signal-safety(7)) until such time as it calls execve(2).
and <https://man7.org/linux/man-pages/man7/signal-safety.7.html> does
not list either execvp() or malloc().

The glibc manual is more verbose
<https://www.gnu.org/software/libc/manual/html_node/Creating-a-Process.html>:
> The _Fork function is similar to fork, but it does not invoke any
> callbacks registered with pthread_atfork, nor does it reset any
> internal state or locks (such as the malloc locks). In the new
> subprocess, only async-signal-safe functions may be called, such as
> dup2 or execve.
This implies that malloc() and fork() in glibc actually inter-operate,
so that fork() -- not _Fork() -- "reset malloc locks".

This confirms your point about latest glibc, but I don't know if/how the
same applies to the BSDs, or to earlier versions of glibc (for example
the one in RHEL7, which libnbd still cares about, IIUC).

(I'm unsure if it's wise for me to dig into the glibc commit history
with git-blame, for finding the earliest release that adds this extra
safety beyond POSIX.)

Laszlo

On Tue, Feb 21, 2023 at 05:03:23PM +0100, Laszlo Ersek
wrote:
> On 2/21/23 13:08, Daniel P. Berrang? wrote:
> > On Wed, Feb 15, 2023 at 03:11:38PM +0100, Laszlo Ersek wrote:
> >> execvp() [01] is powerful:
> >>
> >> - it performs PATH search [02] if necessary,
> >>
> >> - it falls back to executing the file with the shell as a shell
> >>   script in case the underlying execv() or execve() fails with
> >>   ENOEXEC.
> >>
> >> However, execvp() is not async-signal-safe [03], and so we
shouldn't
> >> call it in a child process forked from a multi-threaded parent
> >> process [04], which the libnbd client application may well be.
> >
> > Is that actually a problem in the real world ?  There are various
> > things not technically listed as async-signal-safe by POSIX, which
> > all sane impls none the less make safe, or are practically safe in
> > any sane program using them.
> 
> By my count, it's virtually impossible to implement exec*p*() and
> friends without calling malloc() internally. That's the case even if
you
> can get PATH in there without any problems. In turn, malloc() relies on
> libc-internal mutex(es) or other synchronization mechanisms, so that
> multiple threads can call malloc() / free() etc simultaneously, for
> managing the one shared address space of the process.
> 
> The problem is with calling fork() in a multi-threaded process. The
> child process created by fork() only has one thread -- the thread
> returning from fork(). All other threads simply "disappear", as
viewed
> from the child process's perspective. This means that, if one of those
> "other" (disappearing) threads was in the middle of malloc(),
holding
> for example a mutex, then in the child process, that mutex will appear
> as acquired, but no thread will own it -- so no thread will ever release
> it. Then, when the one thread returning from fork() in the child process
> calls exec*p*(), the malloc() call internal to exec*p*() will deadlock
> on that mutex.
> 
> Rich mentioned that libnbd had actually encountered a bug of this kind,
> just not specifically in exec*p*().
> 
> So the "async-signal-safe" term is a bit misleading here; I think
what
> POSIX actually means is a kind of general reentrancy. Anyway, the POSIX
> language does use "async-signal-safe" when discussing this topic
of
> threads, in the fork() specification.
> 
> Now, the exec*p*() manual at
> 
>   https://man7.org/linux/man-pages/man3/execvp.3.html
> 
> calls execlp(), execvp(), execvpe() "MT-Safe env", so you are
right
> about at least Linux/glibc. I've (intentionally) not looked at the
glibc
> implementation of execvp(), but I agree that, *if* we are happy with
> getenv() just because the linux/glibc manual calls it "MT-Safe
env",
> *then* we could arguably be satisfied with execvp() as well.
> 
> I don't know if/how that applies to FreeBSD / OpenBSD though.
> 
>   https://man.freebsd.org/cgi/man.cgi?query=execvp
>   https://man.openbsd.org/execvp.3
> 
> These man pages do not seem to contain any of the strings "mt",
"async",
> "signal", "safe", "thread".
> 
> > IIUC with execvp the risk would be that setenv makes any use of the
> > environment potentially unsafe,
> 
> to a smaller extent, yes
> 
> > and possibly execvp might use malloc which is also technically unsafe.
> 
> primarily this one, yes
> 
> > Both of these factors would technically make your replacement unsafe
> > too.
> 
> No, they wouldn't. That's the whole point. Both getenv() and
malloc()
> are restricted to the _init API, which is called in the parent process,
> before fork(). The necessary information is prepared in a context
> structure, which the child only inherits. After fork(), the child calls
> a second API -- the effective execvpe() replacement -- which operates on
> the inherited context structure. The only functions called from this
> second API are write() and abort() -- indirectly, in case the new
> async-signal-safe assert() variant fails --, and execve(). All three of
> these functions are async-signal-safe.
Oh I missed that subtlety.
> 
> > Apps simply shouldn't ever call setenv once threads are created,
and
> > malloc() is safe in any impl that is relevant these days.
> 
> FWIW, the linux/glibc manual states, in
> <https://man7.org/linux/man-pages/man2/fork.2.html>:
> 
> >        *  After a fork() in a multithreaded program, the child can
> >           safely call only async-signal-safe functions (see
> >           signal-safety(7)) until such time as it calls execve(2).
> 
> and <https://man7.org/linux/man-pages/man7/signal-safety.7.html> does
> not list either execvp() or malloc().
> 
> The glibc manual is more verbose
>
<https://www.gnu.org/software/libc/manual/html_node/Creating-a-Process.html>:
Yes, the docs are written by language lawyers, but the reality is
that malloc is safe in glibc for 15+ years and that's not going to
change.
> 
> > The _Fork function is similar to fork, but it does not invoke any
> > callbacks registered with pthread_atfork, nor does it reset any
> > internal state or locks (such as the malloc locks). In the new
> > subprocess, only async-signal-safe functions may be called, such as
> > dup2 or execve.
> 
> This implies that malloc() and fork() in glibc actually inter-operate,
> so that fork() -- not _Fork() -- "reset malloc locks".
> 
> This confirms your point about latest glibc, but I don't know if/how
the
> same applies to the BSDs, or to earlier versions of glibc (for example
> the one in RHEL7, which libnbd still cares about, IIUC).
macOS, Linux glib, Linux musl, FreeBSD are all malloc safe after fork
with threads. Windows doesn't have fork.  I'm fairly certain that I
validated the OpenBSD/NetBSD malloc impl too, but it was a while ago.

Every single non-trivial program today uses threads, so you can
more or less guarantee that malloc is going to be used between
fork & exec on a app that uses threads. Any platform that doesn't
make malloc safe is going to be on the receiving end of many bug
reports.  Musl tried to be a hold out in this respect, refusing
to make their malloc safe, but eventually they just accepted the
modern reality and made it safe.

IMHO it just isn't worth spending time avoiding malloc between
fork/exec, and apps which call setenv()/unsetenv from outside
their initial main() method are almost certainly already broken,
because so many library APIs create threads behind your back.

With regards,
Daniel
-- 
|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|

On Tue, Feb 21, 2023 at 05:03:23PM +0100, Laszlo Ersek
wrote:
> Rich mentioned that libnbd had actually encountered a bug of this kind,
> just not specifically in exec*p*().
Probably this one?

https://github.com/libguestfs/libguestfs/commit/e1c9bbb3d1d5ef81490977060120dda0963eb567

glibc was pretty tolerant of this code bug, and the error only
manifested itself when we used glibc.malloc.check=1

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
nbdkit - Flexible, fast NBD server with plugins
https://gitlab.com/nbdkit/nbdkit