3 files changed, 20 insertions, 10 deletions

diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
index d411ca63c8b6..3a9d65c912e7 100644
--- a/Documentation/filesystems/proc.txt
+++ b/Documentation/filesystems/proc.txt
@@ -140,7 +140,8 @@ Table 1-1: Process specific entries in /proc
  stat           Process status
  statm          Process memory status information
  status         Process status in human readable form
- wchan          If CONFIG_KALLSYMS is set, a pre-decoded wchan
+ wchan          Present with CONFIG_KALLSYMS=y: it shows the kernel function
+                symbol the task is blocked in - or "0" if not blocked.
  pagemap        Page table
  stack          Report full stack trace, enable via CONFIG_STACKTRACE
  smaps          a extension based on maps, showing the memory consumption of
@@ -310,7 +311,7 @@ Table 1-4: Contents of the stat files (as of 2.6.30-rc7)
   blocked       bitmap of blocked signals
   sigign        bitmap of ignored signals
   sigcatch      bitmap of caught signals
-  wchan         address where process went to sleep
+  0             (place holder, used to be the wchan address, use /proc/PID/wchan instead)
   0             (place holder)
   0             (place holder)
   exit_signal   signal to send to parent thread on exit
diff --git a/fs/proc/array.c b/fs/proc/array.c
index f60f0121e331..eed2050db9be 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -375,7 +375,7 @@ int proc_pid_status(struct seq_file *m, struct pid_namespace *ns,
 static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
                         struct pid *pid, struct task_struct *task, int whole)
 {
-        unsigned long vsize, eip, esp, wchan = ~0UL;
+        unsigned long vsize, eip, esp, wchan = 0;
         int priority, nice;
         int tty_pgrp = -1, tty_nr = 0;
         sigset_t sigign, sigcatch;
@@ -507,7 +507,19 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
         seq_put_decimal_ull(m, ' ', task->blocked.sig[0] & 0x7fffffffUL);
         seq_put_decimal_ull(m, ' ', sigign.sig[0] & 0x7fffffffUL);
         seq_put_decimal_ull(m, ' ', sigcatch.sig[0] & 0x7fffffffUL);
-        seq_put_decimal_ull(m, ' ', wchan);
+
+        /*
+         * We used to output the absolute kernel address, but that's an
+         * information leak - so instead we show a 0/1 flag here, to signal
+         * to user-space whether there's a wchan field in /proc/PID/wchan.
+         *
+         * This works with older implementations of procps as well.
+         */
+        if (wchan)
+                seq_puts(m, " 1");
+        else
+                seq_puts(m, " 0");
+
         seq_put_decimal_ull(m, ' ', 0);
         seq_put_decimal_ull(m, ' ', 0);
         seq_put_decimal_ll(m, ' ', task->exit_signal);
diff --git a/fs/proc/base.c b/fs/proc/base.c
index b25eee4cead5..29595af32866 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -430,13 +430,10 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
 
         wchan = get_wchan(task);
 
-        if (lookup_symbol_name(wchan, symname) < 0) {
-                if (!ptrace_may_access(task, PTRACE_MODE_READ))
-                        return 0;
-                seq_printf(m, "%lu", wchan);
-        } else {
+        if (wchan && ptrace_may_access(task, PTRACE_MODE_READ) && !lookup_symbol_name(wchan, symname))
                 seq_printf(m, "%s", symname);
-        }
+        else
+                seq_putc(m, '0');
 
         return 0;
 }