* [PATCH] futex: Use user_write_access_begin() in futex_put_value()
@ 2025-08-09 21:24 Waiman Long
2025-08-11 14:02 ` Waiman Long
0 siblings, 1 reply; 2+ messages in thread
From: Waiman Long @ 2025-08-09 21:24 UTC (permalink / raw)
To: Thomas Gleixner, Ingo Molnar, Peter Zijlstra, Darren Hart,
Davidlohr Bueso, André Almeida, Sebastian Andrzej Siewior
Cc: linux-kernel, Waiman Long
Commit cec199c5e39b ("futex: Implement FUTEX2_NUMA") introduces a new
futex_put_value() helper function to write a value to the given user
address. However, it uses user_read_access_begin() before the write.
For arches that differentiate between read and write accesses, like
powerpc, futex_put_value() fails with a -EFAULT return value. Fix that
by using user_write_access_begin().
Fixes: cec199c5e39b ("futex: Implement FUTEX2_NUMA")
Signed-off-by: Waiman Long <longman@redhat.com>
---
kernel/futex/futex.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
index c74eac572acd..2b6ae6a2b2a2 100644
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -319,7 +319,7 @@ static __always_inline int futex_put_value(u32 val, u32 __user *to)
{
if (can_do_masked_user_access())
to = masked_user_access_begin(to);
- else if (!user_read_access_begin(to, sizeof(*to)))
+ else if (!user_write_access_begin(to, sizeof(*to)))
return -EFAULT;
unsafe_put_user(val, to, Efault);
user_read_access_end();
--
2.50.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] futex: Use user_write_access_begin() in futex_put_value()
2025-08-09 21:24 [PATCH] futex: Use user_write_access_begin() in futex_put_value() Waiman Long
@ 2025-08-11 14:02 ` Waiman Long
0 siblings, 0 replies; 2+ messages in thread
From: Waiman Long @ 2025-08-11 14:02 UTC (permalink / raw)
To: Thomas Gleixner, Ingo Molnar, Peter Zijlstra, Darren Hart,
Davidlohr Bueso, André Almeida, Sebastian Andrzej Siewior
Cc: linux-kernel
On 8/9/25 5:24 PM, Waiman Long wrote:
> Commit cec199c5e39b ("futex: Implement FUTEX2_NUMA") introduces a new
> futex_put_value() helper function to write a value to the given user
> address. However, it uses user_read_access_begin() before the write.
> For arches that differentiate between read and write accesses, like
> powerpc, futex_put_value() fails with a -EFAULT return value. Fix that
> by using user_write_access_begin().
>
> Fixes: cec199c5e39b ("futex: Implement FUTEX2_NUMA")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
> kernel/futex/futex.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
> index c74eac572acd..2b6ae6a2b2a2 100644
> --- a/kernel/futex/futex.h
> +++ b/kernel/futex/futex.h
> @@ -319,7 +319,7 @@ static __always_inline int futex_put_value(u32 val, u32 __user *to)
> {
> if (can_do_masked_user_access())
> to = masked_user_access_begin(to);
> - else if (!user_read_access_begin(to, sizeof(*to)))
> + else if (!user_write_access_begin(to, sizeof(*to)))
> return -EFAULT;
> unsafe_put_user(val, to, Efault);
> user_read_access_end();
Sorry, I forgot to use a matching user_write_access_end(). Will send out
a v2.
Cheers,
Longman
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