Re: [PATCH v8 1/8] Get rid of __get_task_comm()

[Alejandro Colomar <alx@kernel.org>]

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On Wed, Aug 28, 2024 at 12:15:40PM GMT, Alejandro Colomar wrote:
> Hi Yafang,
> 
> On Wed, Aug 28, 2024 at 11:03:14AM 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 | 32 ++++++++++++++++++++++++++------
> >  kernel/kthread.c      |  2 +-
> >  4 files changed, 28 insertions(+), 18 deletions(-)
> > 
> 
> [...]
> 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index f8d150343d42..c40b95a79d80 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> 
> [...]
> 
> > @@ -1914,10 +1917,27 @@ 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);
> > +/*
> 
> [...]
> 
> > + * - 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));	\
> 
> I see that there's a two-argument macro
> 
> 	#define strscpy(dst, src)	sized_strscpy(dst, src, sizeof(dst))
> 
> which is used in patch 2/8
> 
> 	diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> 	index 6f0d6fb6523f..e4ef5e57dde9 100644
> 	--- a/kernel/auditsc.c
> 	+++ b/kernel/auditsc.c
> 	@@ -2730,7 +2730,7 @@ void __audit_ptrace(struct task_struct *t)
> 		context->target_uid = task_uid(t);
> 		context->target_sessionid = audit_get_sessionid(t);
> 		security_task_getsecid_obj(t, &context->target_sid);
> 	-       memcpy(context->target_comm, t->comm, TASK_COMM_LEN);
> 	+       strscpy(context->target_comm, t->comm);
> 	 }
> 
> 	 /**

Ahh, the actual generic definition is in <include/linux/string.h>.
You could do

	diff --git i/include/linux/string.h w/include/linux/string.h
	index 9edace076ddb..060504719904 100644
	--- i/include/linux/string.h
	+++ w/include/linux/string.h
	@@ -76,11 +76,11 @@ ssize_t sized_strscpy(char *, const char *, size_t);
	  * known size.
	  */
	 #define __strscpy0(dst, src, ...)      \
	-       sized_strscpy(dst, src, sizeof(dst) + __must_be_array(dst))
	+       sized_strscpy(dst, src, ARRAY_SIZE(dst))
	 #define __strscpy1(dst, src, size)     sized_strscpy(dst, src, size)
	 
	 #define __strscpy_pad0(dst, src, ...)  \
	-       sized_strscpy_pad(dst, src, sizeof(dst) + __must_be_array(dst))
	+       sized_strscpy_pad(dst, src, ARRAY_SIZE(dst))
	 #define __strscpy_pad1(dst, src, size) sized_strscpy_pad(dst, src, size)
	 
	 /**

> 
> I propose modifying that macro to use ARRAY_SIZE() instead of sizeof(),
> and then calling that macro here too.  That would not only make sure
> that this is an array, but make sure that every call to that macro is an
> array.  An if there are macros for similar string functions that reduce
> the argument with a usual sizeof(), the same thing could be done to
> those too.
> 
> Have a lovley day!
> Alex
> 
> > +	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;
> >  	}
> >  
> > -- 
> > 2.43.5
> > 
> 
> -- 
> <https://www.alejandro-colomar.es/>



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<https://www.alejandro-colomar.es/>

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