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

On Tue, Feb 21, 2023 at 05:03:23PM +0100, Laszlo Ersek wrote: > On 2/21/23 13:08, Daniel P. Berrangé 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.htm...;: 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.