[src/trunk]: src/sys/kern ksyms(4): Allow multiple concurrent opens of /dev/k...

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

details:   https://anonhg.NetBSD.org/src/rev/9978d78b04a6
branches:  trunk
changeset: 983723:9978d78b04a6
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Thu Jun 03 09:22:47 2021 +0000

description:
ksyms(4): Allow multiple concurrent opens of /dev/ksyms.

First one takes a snapshot; others all agree with the snapshot.

Previously this code path was just broken (could fail horribly if
modules were unloaded after one of the opens is closed), so I just
blocked it off in an earlier commit, but that broke crash(8).  So
let's continue allowing multiple opens seeing the same snapshot, but
without the horrible bugs.

diffstat:

 sys/kern/kern_ksyms.c |  26 ++++++++++++--------------
 1 files changed, 12 insertions(+), 14 deletions(-)

diffs (85 lines):

diff -r 6b3268277e7b -r 9978d78b04a6 sys/kern/kern_ksyms.c
--- a/sys/kern/kern_ksyms.c     Thu Jun 03 09:09:22 2021 +0000
+++ b/sys/kern/kern_ksyms.c     Thu Jun 03 09:22:47 2021 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_ksyms.c,v 1.96 2021/06/03 01:00:24 riastradh Exp $        */
+/*     $NetBSD: kern_ksyms.c,v 1.97 2021/06/03 09:22:47 riastradh Exp $        */
 
 /*-
  * Copyright (c) 2008 The NetBSD Foundation, Inc.
@@ -73,7 +73,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_ksyms.c,v 1.96 2021/06/03 01:00:24 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_ksyms.c,v 1.97 2021/06/03 09:22:47 riastradh Exp $");
 
 #if defined(_KERNEL) && defined(_KERNEL_OPT)
 #include "opt_copy_symtab.h"
@@ -112,7 +112,7 @@
 #endif
 
 static int ksyms_maxlen;
-static bool ksyms_isopen;
+static uint64_t ksyms_opencnt;
 static struct ksyms_symtab *ksyms_last_snapshot;
 static bool ksyms_initted;
 static bool ksyms_loaded;
@@ -791,7 +791,7 @@
                        continue;
                st->sd_gone = true;
                ksyms_sizes_calc();
-               if (!ksyms_isopen) {
+               if (ksyms_opencnt == 0) {
                        /*
                         * Ensure ddb never witnesses an inconsistent
                         * state of the queue, unless memory is so
@@ -1004,14 +1004,12 @@
                return ENXIO;
 
        /*
-        * Create a "snapshot" of the kernel symbol table.  Setting
-        * ksyms_isopen will prevent symbol tables from being freed.
+        * Create a "snapshot" of the kernel symbol table.  Bumping
+        * ksyms_opencnt will prevent symbol tables from being freed.
         */
        mutex_enter(&ksyms_lock);
-       if (ksyms_isopen) {
-               mutex_exit(&ksyms_lock);
-               return EBUSY;
-       }
+       if (ksyms_opencnt++)
+               goto out;
        ksyms_hdr.kh_shdr[SYMTAB].sh_size = ksyms_symsz;
        ksyms_hdr.kh_shdr[SYMTAB].sh_info = ksyms_symsz / sizeof(Elf_Sym);
        ksyms_hdr.kh_shdr[STRTAB].sh_offset = ksyms_symsz +
@@ -1020,9 +1018,8 @@
        ksyms_hdr.kh_shdr[SHCTF].sh_offset = ksyms_strsz +
            ksyms_hdr.kh_shdr[STRTAB].sh_offset;
        ksyms_hdr.kh_shdr[SHCTF].sh_size = ksyms_ctfsz;
-       ksyms_isopen = true;
        ksyms_last_snapshot = TAILQ_LAST(&ksyms_symtabs, ksyms_symtab_queue);
-       mutex_exit(&ksyms_lock);
+out:   mutex_exit(&ksyms_lock);
 
        return 0;
 }
@@ -1036,7 +1033,8 @@
 
        /* Discard references to symbol tables. */
        mutex_enter(&ksyms_lock);
-       ksyms_isopen = false;
+       if (--ksyms_opencnt)
+               goto out;
        ksyms_last_snapshot = NULL;
        TAILQ_FOREACH_SAFE(st, &ksyms_symtabs, sd_queue, next) {
                if (st->sd_gone) {
@@ -1053,7 +1051,7 @@
        }
        if (!TAILQ_EMPTY(&to_free))
                ksyms_sizes_calc();
-       mutex_exit(&ksyms_lock);
+out:   mutex_exit(&ksyms_lock);
 
        TAILQ_FOREACH_SAFE(st, &to_free, sd_queue, next) {
                kmem_free(st->sd_nmap, st->sd_nmapsize * sizeof(uint32_t));