[PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[Alex Dowad]

The 'stack_size' argument is never used to pass a stack size. It's only used when
forking a kernel thread, in which case it is an argument which should be passed
to the 'main' function which the kernel thread executes. Hence, rename it to
'kthread_arg'.

Signed-off-by: Alex Dowad <alexinbeijing@gmail.com>
---

Hi,

Please have a look at this patch. If this is accepted, I have a series of patches
ready for a similar cleanup to all the arch-specific implementations of copy_thread()
(as suggested by Andrew Morton in a private e-mail).

Thank you,
Alex Dowad

 kernel/fork.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index cf65139..b38a2ae 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
  * It copies the registers, and all the appropriate
  * parts of the process environment (as per the clone
  * flags). The actual kick-off is left to the caller.
+ *
+ * When copying a kernel thread, 'stack_start' is the function to run.
  */
 static struct task_struct *copy_process(unsigned long clone_flags,
 					unsigned long stack_start,
-					unsigned long stack_size,
+					unsigned long kthread_arg,
 					int __user *child_tidptr,
 					struct pid *pid,
 					int trace)
@@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	retval = copy_io(clone_flags, p);
 	if (retval)
 		goto bad_fork_cleanup_namespaces;
-	retval = copy_thread(clone_flags, stack_start, stack_size, p);
+	retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
 	if (retval)
 		goto bad_fork_cleanup_io;
 
@@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
  * it and waits for it to finish using the VM if required.
  */
 long do_fork(unsigned long clone_flags,
-	      unsigned long stack_start,
-	      unsigned long stack_size,
+	      unsigned long stack_start, /* or function for kthread to run */
+	      unsigned long kthread_arg,
 	      int __user *parent_tidptr,
 	      int __user *child_tidptr)
 {
@@ -1656,7 +1658,7 @@ long do_fork(unsigned long clone_flags,
 			trace = 0;
 	}
 
-	p = copy_process(clone_flags, stack_start, stack_size,
+	p = copy_process(clone_flags, stack_start, kthread_arg,
 			 child_tidptr, NULL, trace);
 	/*
 	 * Do this prior waking up the new thread - the thread pointer
@@ -1740,7 +1742,7 @@ SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
 		 int, tls_val)
 #elif defined(CONFIG_CLONE_BACKWARDS3)
 SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
-		int, stack_size,
+		int, ignored,
 		int __user *, parent_tidptr,
 		int __user *, child_tidptr,
 		int, tls_val)
-- 
2.0.0.GIT

Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[Kees Cook]

On Wed, Mar 4, 2015 at 4:40 AM, Alex Dowad <alexinbeijing@gmail.com> wrote:
> The 'stack_size' argument is never used to pass a stack size. It's only used when
> forking a kernel thread, in which case it is an argument which should be passed
> to the 'main' function which the kernel thread executes. Hence, rename it to
> 'kthread_arg'.
>
> Signed-off-by: Alex Dowad <alexinbeijing@gmail.com>

This looks correct to me. Thanks for cleaning it up!

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
>
> Hi,
>
> Please have a look at this patch. If this is accepted, I have a series of patches
> ready for a similar cleanup to all the arch-specific implementations of copy_thread()
> (as suggested by Andrew Morton in a private e-mail).
>
> Thank you,
> Alex Dowad
>
>  kernel/fork.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index cf65139..b38a2ae 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
>   * It copies the registers, and all the appropriate
>   * parts of the process environment (as per the clone
>   * flags). The actual kick-off is left to the caller.
> + *
> + * When copying a kernel thread, 'stack_start' is the function to run.
>   */
>  static struct task_struct *copy_process(unsigned long clone_flags,
>                                         unsigned long stack_start,
> -                                       unsigned long stack_size,
> +                                       unsigned long kthread_arg,
>                                         int __user *child_tidptr,
>                                         struct pid *pid,
>                                         int trace)
> @@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>         retval = copy_io(clone_flags, p);
>         if (retval)
>                 goto bad_fork_cleanup_namespaces;
> -       retval = copy_thread(clone_flags, stack_start, stack_size, p);
> +       retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
>         if (retval)
>                 goto bad_fork_cleanup_io;
>
> @@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
>   * it and waits for it to finish using the VM if required.
>   */
>  long do_fork(unsigned long clone_flags,
> -             unsigned long stack_start,
> -             unsigned long stack_size,
> +             unsigned long stack_start, /* or function for kthread to run */
> +             unsigned long kthread_arg,
>               int __user *parent_tidptr,
>               int __user *child_tidptr)
>  {
> @@ -1656,7 +1658,7 @@ long do_fork(unsigned long clone_flags,
>                         trace = 0;
>         }
>
> -       p = copy_process(clone_flags, stack_start, stack_size,
> +       p = copy_process(clone_flags, stack_start, kthread_arg,
>                          child_tidptr, NULL, trace);
>         /*
>          * Do this prior waking up the new thread - the thread pointer
> @@ -1740,7 +1742,7 @@ SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
>                  int, tls_val)
>  #elif defined(CONFIG_CLONE_BACKWARDS3)
>  SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
> -               int, stack_size,
> +               int, ignored,
>                 int __user *, parent_tidptr,
>                 int __user *, child_tidptr,
>                 int, tls_val)
> --
> 2.0.0.GIT
>



-- 
Kees Cook
Chrome OS Security

Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[David Rientjes]

On Wed, 4 Mar 2015, Alex Dowad wrote:

> The 'stack_size' argument is never used to pass a stack size. It's only used when
> forking a kernel thread, in which case it is an argument which should be passed
> to the 'main' function which the kernel thread executes. Hence, rename it to
> 'kthread_arg'.
> 
> Signed-off-by: Alex Dowad <alexinbeijing@gmail.com>
> ---
> 
> Hi,
> 
> Please have a look at this patch. If this is accepted, I have a series of patches
> ready for a similar cleanup to all the arch-specific implementations of copy_thread()
> (as suggested by Andrew Morton in a private e-mail).
> 
> Thank you,
> Alex Dowad
> 
>  kernel/fork.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index cf65139..b38a2ae 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
>   * It copies the registers, and all the appropriate
>   * parts of the process environment (as per the clone
>   * flags). The actual kick-off is left to the caller.
> + *
> + * When copying a kernel thread, 'stack_start' is the function to run.
>   */
>  static struct task_struct *copy_process(unsigned long clone_flags,
>  					unsigned long stack_start,
> -					unsigned long stack_size,
> +					unsigned long kthread_arg,
>  					int __user *child_tidptr,
>  					struct pid *pid,
>  					int trace)
> @@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>  	retval = copy_io(clone_flags, p);
>  	if (retval)
>  		goto bad_fork_cleanup_namespaces;
> -	retval = copy_thread(clone_flags, stack_start, stack_size, p);
> +	retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
>  	if (retval)
>  		goto bad_fork_cleanup_io;
>  
> @@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
>   * it and waits for it to finish using the VM if required.
>   */
>  long do_fork(unsigned long clone_flags,
> -	      unsigned long stack_start,
> -	      unsigned long stack_size,
> +	      unsigned long stack_start, /* or function for kthread to run */
> +	      unsigned long kthread_arg,
>  	      int __user *parent_tidptr,
>  	      int __user *child_tidptr)
>  {

Looks fine, but I'm not sure about commenting functional formals.  Since 
copy_process() and do_fork() can have formals with different meanings, 
then why not just rename them "arg1" and "arg2" respectively and then 
define in the comment above the function what the possible combinations 
are?

> @@ -1656,7 +1658,7 @@ long do_fork(unsigned long clone_flags,
>  			trace = 0;
>  	}
>  
> -	p = copy_process(clone_flags, stack_start, stack_size,
> +	p = copy_process(clone_flags, stack_start, kthread_arg,
>  			 child_tidptr, NULL, trace);
>  	/*
>  	 * Do this prior waking up the new thread - the thread pointer
> @@ -1740,7 +1742,7 @@ SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
>  		 int, tls_val)
>  #elif defined(CONFIG_CLONE_BACKWARDS3)
>  SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
> -		int, stack_size,
> +		int, ignored,
>  		int __user *, parent_tidptr,
>  		int __user *, child_tidptr,
>  		int, tls_val)

Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[Alex Dowad]

On 05/03/15 01:07, David Rientjes wrote:
> On Wed, 4 Mar 2015, Alex Dowad wrote:
>
>> The 'stack_size' argument is never used to pass a stack size. It's only used when
>> forking a kernel thread, in which case it is an argument which should be passed
>> to the 'main' function which the kernel thread executes. Hence, rename it to
>> 'kthread_arg'.
>>
>> Signed-off-by: Alex Dowad <alexinbeijing@gmail.com>
>> ---
>>
>> Hi,
>>
>> Please have a look at this patch. If this is accepted, I have a series of patches
>> ready for a similar cleanup to all the arch-specific implementations of copy_thread()
>> (as suggested by Andrew Morton in a private e-mail).
>>
>> Thank you,
>> Alex Dowad
>>
>>   kernel/fork.c | 14 ++++++++------
>>   1 file changed, 8 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/fork.c b/kernel/fork.c
>> index cf65139..b38a2ae 100644
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum pid_type type, struct pid *pid)
>>    * It copies the registers, and all the appropriate
>>    * parts of the process environment (as per the clone
>>    * flags). The actual kick-off is left to the caller.
>> + *
>> + * When copying a kernel thread, 'stack_start' is the function to run.
>>    */
>>   static struct task_struct *copy_process(unsigned long clone_flags,
>>   					unsigned long stack_start,
>> -					unsigned long stack_size,
>> +					unsigned long kthread_arg,
>>   					int __user *child_tidptr,
>>   					struct pid *pid,
>>   					int trace)
>> @@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
>>   	retval = copy_io(clone_flags, p);
>>   	if (retval)
>>   		goto bad_fork_cleanup_namespaces;
>> -	retval = copy_thread(clone_flags, stack_start, stack_size, p);
>> +	retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
>>   	if (retval)
>>   		goto bad_fork_cleanup_io;
>>   
>> @@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
>>    * it and waits for it to finish using the VM if required.
>>    */
>>   long do_fork(unsigned long clone_flags,
>> -	      unsigned long stack_start,
>> -	      unsigned long stack_size,
>> +	      unsigned long stack_start, /* or function for kthread to run */
>> +	      unsigned long kthread_arg,
>>   	      int __user *parent_tidptr,
>>   	      int __user *child_tidptr)
>>   {
> Looks fine, but I'm not sure about commenting functional formals.  Since
> copy_process() and do_fork() can have formals with different meanings,
> then why not just rename them "arg1" and "arg2" respectively and then
> define in the comment above the function what the possible combinations
> are?

The second argument is *only* ever used for one thing: an argument 
passed to a kernel thread. That's why I would like to rename it to 
"kthread_arg". The previous argument (currently named "stack_start") is 
indeed used for 2 different things: a new stack pointer for a user 
thread, or a function to be executed by a kernel thread. Rather than 
"arg1", what would you think of something like "sp_or_fn", or "usp_or_fn"?

Thanks for your feedback!

>
>> @@ -1656,7 +1658,7 @@ long do_fork(unsigned long clone_flags,
>>   			trace = 0;
>>   	}
>>   
>> -	p = copy_process(clone_flags, stack_start, stack_size,
>> +	p = copy_process(clone_flags, stack_start, kthread_arg,
>>   			 child_tidptr, NULL, trace);
>>   	/*
>>   	 * Do this prior waking up the new thread - the thread pointer
>> @@ -1740,7 +1742,7 @@ SYSCALL_DEFINE5(clone, unsigned long, newsp, unsigned long, clone_flags,
>>   		 int, tls_val)
>>   #elif defined(CONFIG_CLONE_BACKWARDS3)
>>   SYSCALL_DEFINE6(clone, unsigned long, clone_flags, unsigned long, newsp,
>> -		int, stack_size,
>> +		int, ignored,
>>   		int __user *, parent_tidptr,
>>   		int __user *, child_tidptr,
>>   		int, tls_val)

Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[David Rientjes]

On Thu, 5 Mar 2015, Alex Dowad wrote:

> > > diff --git a/kernel/fork.c b/kernel/fork.c
> > > index cf65139..b38a2ae 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum
> > > pid_type type, struct pid *pid)
> > >    * It copies the registers, and all the appropriate
> > >    * parts of the process environment (as per the clone
> > >    * flags). The actual kick-off is left to the caller.
> > > + *
> > > + * When copying a kernel thread, 'stack_start' is the function to run.
> > >    */
> > >   static struct task_struct *copy_process(unsigned long clone_flags,
> > >   					unsigned long stack_start,
> > > -					unsigned long stack_size,
> > > +					unsigned long kthread_arg,
> > >   					int __user *child_tidptr,
> > >   					struct pid *pid,
> > >   					int trace)
> > > @@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned
> > > long clone_flags,
> > >   	retval = copy_io(clone_flags, p);
> > >   	if (retval)
> > >   		goto bad_fork_cleanup_namespaces;
> > > -	retval = copy_thread(clone_flags, stack_start, stack_size, p);
> > > +	retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
> > >   	if (retval)
> > >   		goto bad_fork_cleanup_io;
> > >   @@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
> > >    * it and waits for it to finish using the VM if required.
> > >    */
> > >   long do_fork(unsigned long clone_flags,
> > > -	      unsigned long stack_start,
> > > -	      unsigned long stack_size,
> > > +	      unsigned long stack_start, /* or function for kthread to run */
> > > +	      unsigned long kthread_arg,
> > >   	      int __user *parent_tidptr,
> > >   	      int __user *child_tidptr)
> > >   {
> > Looks fine, but I'm not sure about commenting functional formals.  Since
> > copy_process() and do_fork() can have formals with different meanings,
> > then why not just rename them "arg1" and "arg2" respectively and then
> > define in the comment above the function what the possible combinations
> > are?
> 
> The second argument is *only* ever used for one thing: an argument passed to a
> kernel thread. That's why I would like to rename it to "kthread_arg". The
> previous argument (currently named "stack_start") is indeed used for 2
> different things: a new stack pointer for a user thread, or a function to be
> executed by a kernel thread. Rather than "arg1", what would you think of
> something like "sp_or_fn", or "usp_or_fn"?
> 

I would recommend exactly "arg" since it can be used for multiple purposes 
and if the formal could ever be used for a third purpose we don't want to 
go through another re-naming patch to change it from sp_or_fn or 
usp_or_fn.

If that's done, then the comment above the function could define what arg 
can represent.

Re: [PATCH] do_fork(): Rename 'stack_size' argument to reflect actual use

[Alex Dowad]

On 05/03/15 22:29, David Rientjes wrote:
> On Thu, 5 Mar 2015, Alex Dowad wrote:
>
>>>> diff --git a/kernel/fork.c b/kernel/fork.c
>>>> index cf65139..b38a2ae 100644
>>>> --- a/kernel/fork.c
>>>> +++ b/kernel/fork.c
>>>> @@ -1186,10 +1186,12 @@ init_task_pid(struct task_struct *task, enum
>>>> pid_type type, struct pid *pid)
>>>>     * It copies the registers, and all the appropriate
>>>>     * parts of the process environment (as per the clone
>>>>     * flags). The actual kick-off is left to the caller.
>>>> + *
>>>> + * When copying a kernel thread, 'stack_start' is the function to run.
>>>>     */
>>>>    static struct task_struct *copy_process(unsigned long clone_flags,
>>>>    					unsigned long stack_start,
>>>> -					unsigned long stack_size,
>>>> +					unsigned long kthread_arg,
>>>>    					int __user *child_tidptr,
>>>>    					struct pid *pid,
>>>>    					int trace)
>>>> @@ -1401,7 +1403,7 @@ static struct task_struct *copy_process(unsigned
>>>> long clone_flags,
>>>>    	retval = copy_io(clone_flags, p);
>>>>    	if (retval)
>>>>    		goto bad_fork_cleanup_namespaces;
>>>> -	retval = copy_thread(clone_flags, stack_start, stack_size, p);
>>>> +	retval = copy_thread(clone_flags, stack_start, kthread_arg, p);
>>>>    	if (retval)
>>>>    		goto bad_fork_cleanup_io;
>>>>    @@ -1629,8 +1631,8 @@ struct task_struct *fork_idle(int cpu)
>>>>     * it and waits for it to finish using the VM if required.
>>>>     */
>>>>    long do_fork(unsigned long clone_flags,
>>>> -	      unsigned long stack_start,
>>>> -	      unsigned long stack_size,
>>>> +	      unsigned long stack_start, /* or function for kthread to run */
>>>> +	      unsigned long kthread_arg,
>>>>    	      int __user *parent_tidptr,
>>>>    	      int __user *child_tidptr)
>>>>    {
>>> Looks fine, but I'm not sure about commenting functional formals.  Since
>>> copy_process() and do_fork() can have formals with different meanings,
>>> then why not just rename them "arg1" and "arg2" respectively and then
>>> define in the comment above the function what the possible combinations
>>> are?
>> The second argument is *only* ever used for one thing: an argument passed to a
>> kernel thread. That's why I would like to rename it to "kthread_arg". The
>> previous argument (currently named "stack_start") is indeed used for 2
>> different things: a new stack pointer for a user thread, or a function to be
>> executed by a kernel thread. Rather than "arg1", what would you think of
>> something like "sp_or_fn", or "usp_or_fn"?
>>
> I would recommend exactly "arg" since it can be used for multiple purposes
> and if the formal could ever be used for a third purpose we don't want to
> go through another re-naming patch to change it from sp_or_fn or
> usp_or_fn.
>
> If that's done, then the comment above the function could define what arg
> can represent.
Do others concur with this idea? Personally, I feel the code will be 
more readable/maintainable if the naming of args/variables/etc reflects 
what they are actually used for.

(Case in point: on IA64, copy_thread() adds the kernel thread arg to the 
user stack pointer. The kernel thread arg is always 0 when forking a 
user process, so this "works", but it's certainly not what the author 
intended. Good names make it harder to write buggy code!)

For readability, using the same arg for 2 different purposes is a bad 
practice (though it might be good for keeping the object code small). I 
hate to think that "arg" might be co-opted for another purpose again.