[src/trunk]: src/sys/uvm mmap(2): If we fail with a hint, try again without it.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/12f1e6818059
branches:  trunk
changeset: 366648:12f1e6818059
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sat Jun 04 20:54:03 2022 +0000

description:
mmap(2): If we fail with a hint, try again without it.

`Hint' here means nonzero addr, but no MAP_FIXED or MAP_TRYFIXED.

This is suboptimal -- we could teach uvm_mmap to do a fancier search
using the address as a hint.  But this should do for now.

Candidate fix for PR kern/55533.

diffstat:

 sys/uvm/uvm_mmap.c |  41 +++++++++++++++++++++++++++++++++++------
 1 files changed, 35 insertions(+), 6 deletions(-)

diffs (88 lines):

diff -r fa1f6123531c -r 12f1e6818059 sys/uvm/uvm_mmap.c
--- a/sys/uvm/uvm_mmap.c        Sat Jun 04 20:32:49 2022 +0000
+++ b/sys/uvm/uvm_mmap.c        Sat Jun 04 20:54:03 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: uvm_mmap.c,v 1.179 2022/04/19 22:53:34 riastradh Exp $ */
+/*     $NetBSD: uvm_mmap.c,v 1.180 2022/06/04 20:54:03 riastradh Exp $ */
 
 /*
  * Copyright (c) 1997 Charles D. Cranor and Washington University.
@@ -46,7 +46,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: uvm_mmap.c,v 1.179 2022/04/19 22:53:34 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: uvm_mmap.c,v 1.180 2022/06/04 20:54:03 riastradh Exp $");
 
 #include "opt_compat_netbsd.h"
 #include "opt_pax.h"
@@ -277,7 +277,8 @@
        vsize_t size, pageoff, newsize;
        vm_prot_t prot, maxprot, extraprot;
        int flags, fd, advice;
-       vaddr_t defaddr;
+       vaddr_t defaddr = 0;    /* XXXGCC */
+       bool addrhint = false;
        struct file *fp = NULL;
        struct uvm_object *uobj;
        int error;
@@ -349,6 +350,12 @@
                        addr = MAX(addr, defaddr);
                else
                        addr = MIN(addr, defaddr);
+
+               /*
+                * If addr is nonzero and not the default, then the
+                * address is a hint.
+                */
+               addrhint = (addr != 0 && addr != defaddr);
        }
 
        /*
@@ -399,11 +406,30 @@
        pax_aslr_mmap(l, &addr, orig_addr, flags);
 
        /*
-        * now let kernel internal function uvm_mmap do the work.
+        * Now let kernel internal function uvm_mmap do the work.
+        *
+        * If the user provided a hint, take a reference to uobj in
+        * case the first attempt to satisfy the hint fails, so we can
+        * try again with the default address.
         */
-
+       if (addrhint) {
+               if (uobj)
+                       (*uobj->pgops->pgo_reference)(uobj);
+       }
        error = uvm_mmap(&p->p_vmspace->vm_map, &addr, size, prot, maxprot,
            flags, advice, uobj, pos, p->p_rlimit[RLIMIT_MEMLOCK].rlim_cur);
+       if (addrhint) {
+               if (error) {
+                       addr = defaddr;
+                       pax_aslr_mmap(l, &addr, orig_addr, flags);
+                       error = uvm_mmap(&p->p_vmspace->vm_map, &addr, size,
+                           prot, maxprot, flags, advice, uobj, pos,
+                           p->p_rlimit[RLIMIT_MEMLOCK].rlim_cur);
+               } else if (uobj) {
+                       /* Release the exta reference we took.  */
+                       (*uobj->pgops->pgo_detach)(uobj);
+               }
+       }
 
        /* remember to add offset */
        *retval = (register_t)(addr + pageoff);
@@ -818,9 +844,12 @@
  * - used by sys_mmap and various framebuffers
  * - uobj is a struct uvm_object pointer or NULL for MAP_ANON
  * - caller must page-align the file offset
+ *
+ * XXX This appears to leak the uobj in various error branches?  Need
+ * to clean up the contract around uobj reference.
  */
 
-int
+static int
 uvm_mmap(struct vm_map *map, vaddr_t *addr, vsize_t size, vm_prot_t prot,
     vm_prot_t maxprot, int flags, int advice, struct uvm_object *uobj,
     voff_t foff, vsize_t locklimit)