github-mirror/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-09-28 20:12:01 +02:00
Code Issues Actions Packages Projects Releases Wiki Activity Graph

kprobes: Fix permanent hang when flushing the kprobe optimizer

Browse Source 
Writing 0 to /proc/sys/debug/kprobes-optimization while a kprobe is
jump-optimized never returns. The writer sleeps in D state forever with
kprobe_sysctl_mutex held, so any later read or write of that sysctl
hangs as well. For example, with vfs_read+9 as an optimizable address
in this build:

  # cd /sys/kernel/tracing
  # echo 'p:myprobe vfs_read+9' >> kprobe_events
  # echo 1 > events/kprobes/myprobe/enable
  # # wait until /sys/kernel/debug/kprobes/list shows [OPTIMIZED]
  # echo 0 > /proc/sys/debug/kprobes-optimization

  INFO: task sh:246 blocked for more than 10 seconds.
  Call Trace:
   <TASK>
   __schedule+0x1176/0x4f70
   schedule+0xdc/0x2c0
   schedule_timeout+0x17b/0x260
   wait_for_completion+0x173/0x3c0
   wait_for_kprobe_optimizer_locked+0xbc/0x130
   proc_kprobes_optimization_handler+0x156/0x1b0
   proc_sys_call_handler+0x324/0x490
   vfs_write+0x52d/0xfe0
   ksys_write+0xff/0x200
   do_syscall_64+0x106/0x630
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
   </TASK>
  ...
  INFO: task cat:265 is blocked on a mutex likely owned by task sh:246.

wait_for_kprobe_optimizer_locked() reinitializes optimizer_completion,
asks the optimizer thread to flush and sleeps in wait_for_completion().
The thread drains the (un)optimizing lists, but calls complete() only
if completion_done() is true, i.e. if the completion is already done,
which never happens while someone waits. disarm_all_kprobes() and
kprobe_trace_self_tests_init() wait the same way.

Calling complete() unconditionally would not be enough: the waiter
drops kprobe_mutex while it sleeps, and nothing else serializes the
sysctl handler against the debugfs "enabled" file. A second flusher
that still finds the lists non-empty, e.g. because a disabled probe is
queued for unoptimizing, reinitializes the completion under the first:

  sysctl write                      debugfs "enabled" write
  unoptimize_all_kprobes()
    wait_for_kprobe_optimizer_locked()
      init_completion(c)
      mutex_unlock(&kprobe_mutex)
      wait_for_completion(c)
                                    disarm_all_kprobes()
                                      wait_for_kprobe_optimizer_locked()
                                        init_completion(c)
                                          // c->wait is reset, the first
                                          // waiter is off the queue
                                        mutex_unlock(&kprobe_mutex)
                                        wait_for_completion(c)
  kprobe_optimizer()
    complete(c)
      // wakes the debugfs writer only

where c is &optimizer_completion. Lining up the two writes during an
optimizer pass loses the sysctl writer this way.

Replace the completion with a counter of optimizer passes, bumped at the
end of each pass and signalled with wake_up_var_locked(), both under
kprobe_mutex. A flusher samples the count and waits with
wait_var_event_mutex(), which drops kprobe_mutex only while sleeping, so
a new count means a whole pass ran in the meantime. Nothing is
reinitialized, so several flushers can sleep in the wait at once.

Link: https://lore.kernel.org/all/20260924092142.199198-1-parri.andrea@gmail.com/

Fixes: 73c12f2094 ("kprobes: Use dedicated kthread for kprobe optimizer")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
This commit is contained in:
Andrea Parri 2026-09-24 11:21:39 +02:00 committed by Masami Hiramatsu (Google)
parent 1d653a1839
commit 5bfa9f1a9d
1 changed files with 14 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

22
kernel/kprobes.c
View File

@ -42,6 +42,7 @@
#include <linux/execmem.h>
#include <linux/cleanup.h>
#include <linux/wait.h>
#include <linux/wait_bit.h>
#include <asm/sections.h>
#include <asm/cacheflush.h>
@ -526,7 +527,8 @@ enum {
OPTIMIZER_ST_FLUSHING = 2,
};
static DECLARE_COMPLETION(optimizer_completion);
/* Bumped at the end of each kprobe_optimizer() pass, under 'kprobe_mutex' */
static unsigned long optimizer_passes;
#define OPTIMIZE_DELAY 5
@ -654,9 +656,9 @@ static void kprobe_optimizer(void)
do_free_cleaned_kprobes();
}
/* Step 5: Kick optimizer again if needed. But if there is a flush requested, */
if (completion_done(&optimizer_completion))
complete(&optimizer_completion);
/* Step 5: Wake up flushers, and kick optimizer again if needed. */
optimizer_passes++;
wake_up_var_locked(&optimizer_passes, &kprobe_mutex);
if (!list_empty(&optimizing_list) || !list_empty(&unoptimizing_list))
kick_kprobe_optimizer(); /*normal kick*/
@ -708,7 +710,8 @@ static void wait_for_kprobe_optimizer_locked(void)
lockdep_assert_held(&kprobe_mutex);
while (!list_empty(&optimizing_list) || !list_empty(&unoptimizing_list)) {
init_completion(&optimizer_completion);
unsigned long passes = optimizer_passes;
/*
* Set state to OPTIMIZER_ST_FLUSHING and wake up the thread if it's
* idle. If it's already kicked, it will see the state change.
@ -717,9 +720,12 @@ static void wait_for_kprobe_optimizer_locked(void)
OPTIMIZER_ST_FLUSHING) != OPTIMIZER_ST_FLUSHING)
wake_up(&kprobe_optimizer_wait);
mutex_unlock(&kprobe_mutex);
wait_for_completion(&optimizer_completion);
mutex_lock(&kprobe_mutex);
/*
* kprobe_optimizer() holds 'kprobe_mutex' for a whole pass, which
* this drops while sleeping, so a new count means a full pass ran.
*/
wait_var_event_mutex(&optimizer_passes,
optimizer_passes != passes, &kprobe_mutex);
}
}