Make linux(4) set the openfiles soft resource limit to 1024 for Linux

applications, which often depend on this being the case.  There's a new
sysctl, compat.linux.default_openfiles, to control this behaviour.

Reviewed by:	kevans, emaste, bcr (manpages)
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D25177

share/man/man4/linux.4

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 16, 2019
+.Dd June 10, 2020
 .Dt LINUX 4
 .Os
 .Sh NAME
@@ -95,6 +95,10 @@ variables and
 .Xr loader 8
 tunables:
 .Bl -tag -width indent
+.It Va compat.linux.default_openfiles
+Default soft openfiles resource limit for Linux applications.
+Set to -1 to disable the limit.
+Defaults to 1024.
 .It Va compat.linux.emul_path
 Path to the Linux run-time environment.
 Defaults to

sys/compat/linux/linux_emul.c

@@ -42,10 +42,12 @@ __FBSDID("$FreeBSD$");
 #include <sys/mutex.h>
 #include <sys/sx.h>
 #include <sys/proc.h>
+#include <sys/resourcevar.h>
 #include <sys/syscallsubr.h>
 #include <sys/sysent.h>
 
 #include <compat/linux/linux_emul.h>
+#include <compat/linux/linux_mib.h>
 #include <compat/linux/linux_misc.h>
 #include <compat/linux/linux_persona.h>
 #include <compat/linux/linux_util.h>
@@ -87,6 +89,32 @@ pem_find(struct proc *p)
 	return (pem);
 }
 
+/*
+ * Linux apps generally expect the soft open file limit to be set
+ * to 1024, often iterating over all the file descriptors up to that
+ * limit instead of using closefrom(2).  Give them what they want,
+ * unless there already is a resource limit in place.
+ */
+static void
+linux_set_default_openfiles(struct thread *td, struct proc *p)
+{
+	struct rlimit rlim;
+	int error;
+
+	if (linux_default_openfiles < 0)
+		return;
+
+	PROC_LOCK(p);
+	lim_rlimit_proc(p, RLIMIT_NOFILE, &rlim);
+	PROC_UNLOCK(p);
+	if (rlim.rlim_cur != rlim.rlim_max ||
+	    rlim.rlim_cur <= linux_default_openfiles)
+		return;
+	rlim.rlim_cur = linux_default_openfiles;
+	error = kern_proc_setrlimit(td, p, RLIMIT_NOFILE, &rlim);
+	KASSERT(error == 0, ("kern_proc_setrlimit failed"));
+}
+
 void
 linux_proc_init(struct thread *td, struct thread *newtd, int flags)
 {
@@ -115,6 +143,8 @@ linux_proc_init(struct thread *td, struct thread *newtd, int flags)
 			p->p_emuldata = pem;
 		}
 		newtd->td_emuldata = em;
+
+		linux_set_default_openfiles(td, p);
 	} else {
 		p = td->td_proc;
 

sys/compat/linux/linux_mib.c

@@ -63,6 +63,11 @@ static unsigned linux_osd_jail_slot;
 SYSCTL_NODE(_compat, OID_AUTO, linux, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
     "Linux mode");
 
+int linux_default_openfiles = 1024;
+SYSCTL_INT(_compat_linux, OID_AUTO, default_openfiles, CTLFLAG_RWTUN,
+    &linux_default_openfiles, 0,
+    "Default soft openfiles resource limit, or -1 for unlimited");
+
 int linux_ignore_ip_recverr = 1;
 SYSCTL_INT(_compat_linux, OID_AUTO, ignore_ip_recverr, CTLFLAG_RWTUN,
     &linux_ignore_ip_recverr, 0, "Ignore enabling IP_RECVERR");

sys/compat/linux/linux_mib.h

@@ -62,6 +62,7 @@ int	linux_kernver(struct thread *td);
 
 #define	linux_use26(t)		(linux_kernver(t) >= LINUX_KERNVER_2006000)
 
+extern int linux_default_openfiles;
 extern int linux_ignore_ip_recverr;
 extern int linux_preserve_vstatus;
 extern bool linux_map_sched_prio;