Auto merge of #89924 - cuviper:beta-clone3, r=Mark-Simulacrum

Only use `clone3` when needed for pidfd

In #89522 we learned that `clone3` is interacting poorly with Gentoo's
`sandbox` tool. We only need that for the unstable pidfd extensions, so
otherwise avoid that and use a normal `fork`.

r? `@Mark-Simulacrum`

library/std/src/sys/unix/process/process_unix.rs

@@ -166,14 +166,20 @@ impl Command {
             fn clone3(cl_args: *mut clone_args, len: libc::size_t) -> libc::c_long
         }
 
+        // Bypassing libc for `clone3` can make further libc calls unsafe,
+        // so we use it sparingly for now. See #89522 for details.
+        // Some tools (e.g. sandboxing tools) may also expect `fork`
+        // rather than `clone3`.
+        let want_clone3 = self.get_create_pidfd();
+
         // If we fail to create a pidfd for any reason, this will
         // stay as -1, which indicates an error.
         let mut pidfd: pid_t = -1;
 
         // Attempt to use the `clone3` syscall, which supports more arguments
         // (in particular, the ability to create a pidfd). If this fails,
         // we will fall through this block to a call to `fork()`
-        if HAS_CLONE3.load(Ordering::Relaxed) {
+        if want_clone3 && HAS_CLONE3.load(Ordering::Relaxed) {
             let mut flags = 0;
             if self.get_create_pidfd() {
                 flags |= CLONE_PIDFD;
@@ -212,8 +218,8 @@ impl Command {
             }
         }
 
-        // If we get here, the 'clone3' syscall does not exist
-        // or we do not have permission to call it
+        // Generally, we just call `fork`. If we get here after wanting `clone3`,
+        // then the syscall does not exist or we do not have permission to call it.
         cvt(libc::fork()).map(|res| (res, pidfd))
     }