Re: [PATCH v6 1/9] Get rid of __get_task_comm()

[Yafang Shao <laoar.shao@gmail.com>]

On Mon, Aug 12, 2024 at 4:05 PM Alejandro Colomar <alx@kernel.org> wrote:
>
> Hi Yafang,
>
> On Mon, Aug 12, 2024 at 10:29:25AM GMT, Yafang Shao wrote:
> > We want to eliminate the use of __get_task_comm() for the following
> > reasons:
> >
> > - The task_lock() is unnecessary
> >   Quoted from Linus [0]:
> >   : Since user space can randomly change their names anyway, using locking
> >   : was always wrong for readers (for writers it probably does make sense
> >   : to have some lock - although practically speaking nobody cares there
> >   : either, but at least for a writer some kind of race could have
> >   : long-term mixed results
> >
> > - The BUILD_BUG_ON() doesn't add any value
> >   The only requirement is to ensure that the destination buffer is a valid
> >   array.
> >
> > - Zeroing is not necessary in current use cases
> >   To avoid confusion, we should remove it. Moreover, not zeroing could
> >   potentially make it easier to uncover bugs. If the caller needs a
> >   zero-padded task name, it should be explicitly handled at the call site.
> >
> > Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> > Link: https://lore.kernel.org/all/CAHk-=wivfrF0_zvf+oj6==Sh=-npJooP8chLPEfaFV0oNYTTBA@mail.gmail.com [0]
> > Link: https://lore.kernel.org/all/CAHk-=whWtUC-AjmGJveAETKOMeMFSTwKwu99v7+b6AyHMmaDFA@mail.gmail.com/
> > Suggested-by: Alejandro Colomar <alx@kernel.org>
> > Link: https://lore.kernel.org/all/2jxak5v6dfxlpbxhpm3ey7oup4g2lnr3ueurfbosf5wdo65dk4@srb3hsk72zwq
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> > Cc: Christian Brauner <brauner@kernel.org>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: Eric Biederman <ebiederm@xmission.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
> > Cc: Matus Jokay <matus.jokay@stuba.sk>
> > Cc: Alejandro Colomar <alx@kernel.org>
> > Cc: "Serge E. Hallyn" <serge@hallyn.com>
> > ---
> >  fs/exec.c             | 10 ----------
> >  fs/proc/array.c       |  2 +-
> >  include/linux/sched.h | 31 +++++++++++++++++++++++++------
> >  kernel/kthread.c      |  2 +-
> >  4 files changed, 27 insertions(+), 18 deletions(-)
> >
> > diff --git a/fs/exec.c b/fs/exec.c
> > index a47d0e4c54f6..2e468ddd203a 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1264,16 +1264,6 @@ static int unshare_sighand(struct task_struct *me)
> >       return 0;
> >  }
> >
> > -char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> > -{
> > -     task_lock(tsk);
> > -     /* Always NUL terminated and zero-padded */
> > -     strscpy_pad(buf, tsk->comm, buf_size);
>
> This comment is correct (see other comments below).
>
> (Except that pedantically, I'd write it as NUL-terminated with a hyphen,
>  just like zero-padded.)
>
> > -     task_unlock(tsk);
> > -     return buf;
> > -}
> > -EXPORT_SYMBOL_GPL(__get_task_comm);
> > -
> >  /*
> >   * These functions flushes out all traces of the currently running executable
> >   * so that a new one can be started
> > diff --git a/fs/proc/array.c b/fs/proc/array.c
> > index 34a47fb0c57f..55ed3510d2bb 100644
> > --- a/fs/proc/array.c
> > +++ b/fs/proc/array.c
> > @@ -109,7 +109,7 @@ void proc_task_name(struct seq_file *m, struct task_struct *p, bool escape)
> >       else if (p->flags & PF_KTHREAD)
> >               get_kthread_comm(tcomm, sizeof(tcomm), p);
> >       else
> > -             __get_task_comm(tcomm, sizeof(tcomm), p);
> > +             get_task_comm(tcomm, p);
>
> LGTM.  (This would have been good even if not removing the helper.)
>
> >
> >       if (escape)
> >               seq_escape_str(m, tcomm, ESCAPE_SPACE | ESCAPE_SPECIAL, "\n\\");
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 33dd8d9d2b85..e0e26edbda61 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -1096,9 +1096,11 @@ struct task_struct {
> >       /*
> >        * executable name, excluding path.
> >        *
> > -      * - normally initialized setup_new_exec()
> > -      * - access it with [gs]et_task_comm()
> > -      * - lock it with task_lock()
> > +      * - normally initialized begin_new_exec()
> > +      * - set it with set_task_comm()
> > +      *   - strscpy_pad() to ensure it is always NUL-terminated
>
> The comment above is inmprecise.
> It should say either
> "strscpy() to ensure it is always NUL-terminated", or
> "strscpy_pad() to ensure it is NUL-terminated and zero-padded".

will change it.

>
> > +      *   - task_lock() to ensure the operation is atomic and the name is
> > +      *     fully updated.
> >        */
> >       char                            comm[TASK_COMM_LEN];
> >
> > @@ -1912,10 +1914,27 @@ static inline void set_task_comm(struct task_struct *tsk, const char *from)
> >       __set_task_comm(tsk, from, false);
> >  }
> >
> > -extern char *__get_task_comm(char *to, size_t len, struct task_struct *tsk);
> > +/*
> > + * - Why not use task_lock()?
> > + *   User space can randomly change their names anyway, so locking for readers
> > + *   doesn't make sense. For writers, locking is probably necessary, as a race
> > + *   condition could lead to long-term mixed results.
> > + *   The strscpy_pad() in __set_task_comm() can ensure that the task comm is
> > + *   always NUL-terminated.
>
> This comment has the same imprecission that I noted above.

will change it.

>
> > Therefore the race condition between reader and
> > + *   writer is not an issue.
> > + *
> > + * - Why not use strscpy_pad()?
> > + *   While strscpy_pad() prevents writing garbage past the NUL terminator, which
> > + *   is useful when using the task name as a key in a hash map, most use cases
> > + *   don't require this. Zero-padding might confuse users if it’s unnecessary,
> > + *   and not zeroing might even make it easier to expose bugs. If you need a
> > + *   zero-padded task name, please handle that explicitly at the call site.
> > + *
> > + * - ARRAY_SIZE() can help ensure that @buf is indeed an array.
> > + */
> >  #define get_task_comm(buf, tsk) ({                   \
> > -     BUILD_BUG_ON(sizeof(buf) != TASK_COMM_LEN);     \
> > -     __get_task_comm(buf, sizeof(buf), tsk);         \
> > +     strscpy(buf, (tsk)->comm, ARRAY_SIZE(buf));     \
> > +     buf;                                            \
> >  })
> >
> >  #ifdef CONFIG_SMP
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index f7be976ff88a..7d001d033cf9 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -101,7 +101,7 @@ void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> >       struct kthread *kthread = to_kthread(tsk);
> >
> >       if (!kthread || !kthread->full_name) {
> > -             __get_task_comm(buf, buf_size, tsk);
> > +             strscpy(buf, tsk->comm, buf_size);
> >               return;
> >       }
>
> Other than that, LGTM.

Thanks for your review.

-- 
Regards
Yafang