[patch 2/2] Sort module list by pointer address to get coherent sleepable seq_file iterators

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

A race that appears both in /proc/modules and in kallsyms: if, between the
seq file reads, the process is put to sleep and at this moment a module is
or removed from the module list, the listing will skip an amount of
modules/symbols corresponding to the amount of elements present in the unloaded
module, but at the current position in the list if the iteration is located
after the removed module.

The cleanest way I found to deal with this problem is to sort the module list.
We can then keep the old struct module * as the old iterator, knowing the it may
be removed between the seq file reads, but we only use it as "get next". If it
is not present in the module list, the next pointer will be used.

By doing this, removing a given module will now only fuzz the output related to
this specific module, not any random module anymore. Since modprobe uses
/proc/modules, it might be important to make sure multiple concurrent running
modprobes won't interfere with each other.


Small test program for this:

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#define BUFSIZE 1024

int main()
{
	int fd = open("/proc/modules", O_RDONLY);
	char buf[BUFSIZE];
	ssize_t size;

	do {
		size = read(fd, buf, 1);
		printf("%c", buf[0]);
		usleep(100000);
	} while(size > 0);

	close(fd);
	return 0;
}


Actual test (kernel 2.6.23-rc3):

dijkstra:~# lsmod
Module                  Size  Used by
pl2303                 18564  0
usbserial              29032  1 pl2303
ppdev                   7844  0
sky2                   37476  0
skge                   36368  0
rtc                    10104  0
snd_hda_intel         265628  0

  (here, while we are printing the 2nd line, I rmmod pl2303)
compudj@dijkstra:~/test$ ./module
pl2303 18564 0 - Live 0xf886e000
usbserial 29032 1 pl2303, Live 0xf8865000
sky2 37476 0 - Live 0xf884f000
skge 36368 0 - Live 0xf8838000
rtc 10104 0 - Live 0xf8825000

We see the the 2nd line is garbage.

Now, with my patch applied:
  (here, while we are printing the rtc module, I rmmod rtc)
nd_hda_intel 268708 0 - Live 0xf8820000
ltt_control 2372 0 - Live 0xf8866000
rtc 10392 0 - Live 0xf886d000
skge 36768 0 - Live 0xf8871000
ltt_statedump 8516 0 - Live 0xf887b000

We see that since the rtc line was already in the buffer, it has been
printed completely.


  (here, while we are printing the skge module, I rmmod rtc)
snd_hda_intel 268708 0 - Live 0xf8820000
ltt_control 2372 0 - Live 0xf8866000
rtc 10392 0 - Live 0xf886d000
skge 36768 0 - Live 0xf8871000
ltt_statedump 8516 0 - Live 0xf887b000
sky2 38420 0 - Live 0xf88cd000

We see that the iteration continued at the same position even though the
rtc module, located in earlier addresses, was removed.

Changelog:

When reading the data by small chunks (i.e. byte by byte), the index (ppos) is
incremented by seq_read() directly and no "next" callback is called when going
to the next module.

Therefore, use ppos instead of m->private to deal with the fact that this index
is incremented directly to pass to the next module in seq_read() after the
buffer has been emptied.

Before fix, it prints the first module indefinitely. The patch fixes
this.

Changelog:
- Remove module_mutex usage: depend on functions implemented in module.c for
  that.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
---
 kernel/module.c |   23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

Index: linux-2.6-lttng/kernel/module.c
===================================================================
--- linux-2.6-lttng.orig/kernel/module.c	2007-09-13 17:24:46.000000000 -0400
+++ linux-2.6-lttng/kernel/module.c	2007-09-13 17:25:26.000000000 -0400
@@ -64,7 +64,8 @@ extern int module_sysfs_initialized;
 #define INIT_OFFSET_MASK (1UL << (BITS_PER_LONG-1))
 
 /* List of modules, protected by module_mutex or preempt_disable
- * (add/delete uses stop_machine). */
+ * (add/delete uses stop_machine). Sorted by ascending list node address.
+ */
 static DEFINE_MUTEX(module_mutex);
 static LIST_HEAD(modules);
 static DECLARE_MUTEX(notify_mutex);
@@ -2130,10 +2131,24 @@ nomodsectinfo:
 /*
  * link the module with the whole machine is stopped with interrupts off
  * - this defends against kallsyms not taking locks
+ * We sort the modules by struct module pointer address to permit correct
+ * iteration over modules of, at least, kallsyms for preemptible operations,
+ * such as read(). Sorting by struct module pointer address is equivalent to
+ * sort by list node address.
  */
 static int __link_module(void *_mod)
 {
-	struct module *mod = _mod;
+	struct module *mod = _mod, *iter;
+
+	list_for_each_entry_reverse(iter, &modules, list) {
+		BUG_ON(iter == mod);	/* Should never be in the list twice */
+		if (iter < mod) {
+			/* We belong to the location right after iter. */
+			list_add(&mod->list, &iter->list);
+			return 0;
+		}
+	}
+	/* We should be added at the head of the list */
 	list_add(&mod->list, &modules);
 	return 0;
 }
@@ -2398,12 +2413,12 @@ unsigned long module_kallsyms_lookup_nam
 static void *m_start(struct seq_file *m, loff_t *pos)
 {
 	mutex_lock(&module_mutex);
-	return seq_list_start(&modules, *pos);
+	return seq_sorted_list_start(&modules, pos);
 }
 
 static void *m_next(struct seq_file *m, void *p, loff_t *pos)
 {
-	return seq_list_next(p, &modules, pos);
+	return seq_sorted_list_next(p, &modules, pos);
 }
 
 static void m_stop(struct seq_file *m, void *p)

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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