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

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.

> 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...;:

> 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).