On Mon, Oct 26, 2026 at 3:30 PM <[email protected]> wrote: > > > stacktrace: Don't unwind the user stack without an mm > > TL;DR: Commit message only: the Fixes: tag may point at the wrong commit; > 7e8e6816c649, which dropped the !current->mm check, looks like the real > culprit rather than 214d8ca6ee85. > > > stacktrace: Don't unwind the user stack without an mm > > > > stack_trace_save_user() only refuses to unwind the user stack of tasks > > with PF_KTHREAD set. That doesn't cover all tasks without an mm: > > - A user task that has already gone through exit_mm(), e.g. when an > > event fires from exit_files(), exit_task_work() or the final context > > switch in do_exit(). > > - PID 1 before it execs init, and usermode helper children before they > > exec, which are created with user_mode_thread() and don't have > > PF_KTHREAD set. E.g. booting with trace_options=userstacktrace and > > an event enabled with trace_event= unwinds PID 1's "user stack" > > during initcalls. > > With the ftrace userstacktrace option enabled, the architecture > > unwinders then read the user stack without an mm. That access does not > > reliably fail: the user half of the address space belongs to whatever > > mm happens to be lazily loaded, if any, so the unwinder may read > > another task's memory. > > Use is_user_task() instead, which also checks current->mm, as perf > > already does for the same reason since commit 76ed27608f7d ("perf: > > sched: Fix perf crash with new is_user_task() helper"). This also stops > > unwinding the user stack of PF_USER_WORKER threads, which never run in > > user space, matching perf. > > Fixes: 214d8ca6ee85 ("stacktrace: Provide common infrastructure") > > Signed-off-by: James Houghton <[email protected]> > > Is the Fixes: tag pointing at the right commit? > > When 214d8ca6ee85 added the ARCH_STACKWALK version of > stack_trace_save_user(), it guarded the unwind with: > > /* Trace user stack if not a kernel thread */ > if (!current->mm) > return 0; > > That check already covers both cases described above (a task past exit_mm(), > and PID 1 or usermode helpers before exec, which have mm == NULL). > > The check was replaced by commit 7e8e6816c649 ("stacktrace: Use PF_KTHREAD to > check for kernel threads"), which switched it to: > > if (current->flags & PF_KTHREAD) > return 0; > > That dropped the no-mm protection, which looks like what this patch restores > via is_user_task(). > > Should this be: > > Fixes: 7e8e6816c649 ("stacktrace: Use PF_KTHREAD to check for kernel > threads")
Thank you, yes that appears to be correct. Updated in v2.
