[PATCH] arm64: Fix 'lock held when returning to user space' lockdep warning

[PATCH] arm64: Fix 'lock held when returning to user space' lockdep warning

[Eric Chan]

The arm64 architecture lacks support for CONFIG_GENERIC_ENTRY, resulting
in the failure to report the lockdep warning
"lock held when returning to user space" when lockdep is enabled.

Rename the function to align with exit_to_user_mode_prepare in
kernel/entry/common.c to improve readability.

Signed-off-by: Eric Chan <ericchancf@google.com>
---
 arch/arm64/kernel/entry-common.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
index 3af3c01c93a6..9d6827201b6c 100644
--- a/arch/arm64/kernel/entry-common.c
+++ b/arch/arm64/kernel/entry-common.c
@@ -126,7 +126,7 @@ static __always_inline void __exit_to_user_mode(void)
 	lockdep_hardirqs_on(CALLER_ADDR0);
 }
 
-static __always_inline void prepare_exit_to_user_mode(struct pt_regs *regs)
+static __always_inline void exit_to_user_mode_prepare(struct pt_regs *regs)
 {
 	unsigned long flags;
 
@@ -135,11 +135,13 @@ static __always_inline void prepare_exit_to_user_mode(struct pt_regs *regs)
 	flags = read_thread_flags();
 	if (unlikely(flags & _TIF_WORK_MASK))
 		do_notify_resume(regs, flags);
+
+	lockdep_sys_exit();
 }
 
 static __always_inline void exit_to_user_mode(struct pt_regs *regs)
 {
-	prepare_exit_to_user_mode(regs);
+	exit_to_user_mode_prepare(regs);
 	mte_check_tfsr_exit();
 	__exit_to_user_mode();
 }
-- 
2.41.0.rc0.172.g3f132b7071-goog

Re: [PATCH] arm64: Fix 'lock held when returning to user space' lockdep warning

[Mark Rutland]

Hi Eric,

The subject line for this patch is misleading, as it makes it sound like we're
currently hitting a warning of the form:

  lock held when returning to user space

... when in actuality this patch is *adding* support for that warning.

This is a missing feature, not a fix (and e.g. doesn't require backporting to
stable).

On Fri, May 26, 2023 at 06:48:23PM +0000, Eric Chan wrote:
> The arm64 architecture lacks support for CONFIG_GENERIC_ENTRY, resulting
> in the failure to report the lockdep warning
> "lock held when returning to user space" when lockdep is enabled.
> 
> Rename the function to align with exit_to_user_mode_prepare in
> kernel/entry/common.c to improve readability.

Considering the point about the commit title, could you please reword this to
be clear that we're adding support for a feature, rather than fixing something
that's broken, e.g.

| arm64: lockdep: enable checks for held locks when returning to userspace
|
| Currently arm64 doesn't use CONFIG_GENERIC_ENTRY and doesn't call
| lockdep_sys_exit() when returning to userspace. This means that lockdep won't
| check for held locks when returning to userspace, which would be useful to
| detect kernel bugs.
|
| Call lockdep_sys_exit() when returning to userspace, enabling checking for
| held locks.
|
| At the same time, rename arm64's prepare_exit_to_user_mode() to
| exit_to_user_mode_prepare() to more clearly align with the naming in the
| generic entry code.

With that wording:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.

> Signed-off-by: Eric Chan <ericchancf@google.com>
> ---
>  arch/arm64/kernel/entry-common.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kernel/entry-common.c b/arch/arm64/kernel/entry-common.c
> index 3af3c01c93a6..9d6827201b6c 100644
> --- a/arch/arm64/kernel/entry-common.c
> +++ b/arch/arm64/kernel/entry-common.c
> @@ -126,7 +126,7 @@ static __always_inline void __exit_to_user_mode(void)
>  	lockdep_hardirqs_on(CALLER_ADDR0);
>  }
>  
> -static __always_inline void prepare_exit_to_user_mode(struct pt_regs *regs)
> +static __always_inline void exit_to_user_mode_prepare(struct pt_regs *regs)
>  {
>  	unsigned long flags;
>  
> @@ -135,11 +135,13 @@ static __always_inline void prepare_exit_to_user_mode(struct pt_regs *regs)
>  	flags = read_thread_flags();
>  	if (unlikely(flags & _TIF_WORK_MASK))
>  		do_notify_resume(regs, flags);
> +
> +	lockdep_sys_exit();
>  }
>  
>  static __always_inline void exit_to_user_mode(struct pt_regs *regs)
>  {
> -	prepare_exit_to_user_mode(regs);
> +	exit_to_user_mode_prepare(regs);
>  	mte_check_tfsr_exit();
>  	__exit_to_user_mode();
>  }
> -- 
> 2.41.0.rc0.172.g3f132b7071-goog
> 

Re: [PATCH] arm64: Fix 'lock held when returning to user space' lockdep warning

[Eric Chan]

Hi Mark,

Thank you for your detailed response and guidance. I will prepare
PATCH v2 promptly and resubmit it for review.

Thanks,
Eric Chan