[PATCH] Race between sys_swapon and /proc/swaps (2.4)

[Paul Menage]

sys_swapon() sets SWP_USED in p->flags when it begins to set up a swap
area, and then calls vmalloc() to allocate p->swap_map[], which may
sleep. Most other users of the swap info structures either traverse the
swap list (to which the new swap area hasn't yet been added) or check
SWP_WRITEOK (which hasn't yet been set), but get_swaparea_info() only
checks for SWP_USED, and then proceeds to dereference ptr->swap_map. So
reading /proc/swaps whilst doing a swapon() can Oops. 

This could either be solved by making get_swaparea_info() check for 
ptr->swap_map, or check for ((ptr->flags & SWP_WRITEOK) == SWP_WRITEOK).
The patch below (applicable to 2.4.5 - patch for 2.2 in following email)
checks for the former on the grounds that that is what's causing the
immediate problem, and some people might want to be able to use 
/proc/swaps to track the progress of a swapoff().

Paul

diff -Naur linux/mm/swapfile.c linux.new/mm/swapfile.c
--- linux/mm/swapfile.c        Wed Apr 11 21:59:56 2001
+++ linux.new/mm/swapfile.c Fri Jun  8 17:18:32 2001
@@ -503,7 +503,7 @@
 
 	len += sprintf(buf, "Filename\t\t\tType\t\tSize\tUsed\tPriority\n");
 	for (i = 0 ; i < nr_swapfiles ; i++, ptr++) {
-		if (ptr->flags & SWP_USED) {
+		if ((ptr->flags & SWP_USED) && ptr->swap_map) {
 			char * path = d_path(ptr->swap_file, ptr->swap_vfsmnt,
 				page, PAGE_SIZE);