um: add a UML specific futex implementation

From: Anton Ivanov <anton.ivanov@cambridgegreys.com>

The generic asm futex implementation emulates atomic access to
memory by doing a get_user followed by put_user. These translate
to two mapping operations on UML with paging enabled in the
meantime. This, in turn may end up changing interrupts,
invoking the signal loop, etc.

This replaces the generic implementation by a mapping followed
by an operation on the mapped segment. It is for now limited
to 64 bit as the 32 bit may also need to take care of highmem,
etc.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
---
 arch/um/include/asm/futex.h   |  20 ++++++
 arch/um/kernel/skas/uaccess.c | 114 ++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100644 arch/um/include/asm/futex.h

On 12/11/2020 18:06, anton.ivanov@cambridgegreys.com wrote:
> From: Anton Ivanov <anton.ivanov@cambridgegreys.com>
>
> The generic asm futex implementation emulates atomic access to
> memory by doing a get_user followed by put_user. These translate
> to two mapping operations on UML with paging enabled in the
> meantime. This, in turn may end up changing interrupts,
> invoking the signal loop, etc.
>
> This replaces the generic implementation by a mapping followed
> by an operation on the mapped segment. It is for now limited
> to 64 bit as the 32 bit may also need to take care of highmem,

The other reason to keep it 64 bit is that for that, the actual ops may be replaced by inline asm, though frankly, with no preemption and paging disabled I do not see why bother. The gain will be marginal compared to the gain from not going twice through the page mapper.

A.

> etc.
>
> Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
> ---
>   arch/um/include/asm/futex.h   |  20 ++++++
>   arch/um/kernel/skas/uaccess.c | 114 ++++++++++++++++++++++++++++++++++
>   2 files changed, 134 insertions(+)
>   create mode 100644 arch/um/include/asm/futex.h
>
> diff --git a/arch/um/include/asm/futex.h b/arch/um/include/asm/futex.h
> new file mode 100644
> index 000000000000..9af35ab66b6e
> --- /dev/null
> +++ b/arch/um/include/asm/futex.h
> @@ -0,0 +1,20 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_GENERIC_FUTEX_H
> +#define _ASM_GENERIC_FUTEX_H
> +
> +#ifdef CONFIG_64BIT
> +
> +#include <linux/futex.h>
> +#include <linux/uaccess.h>
> +#include <asm/errno.h>
> +
> +
> +extern int arch_futex_atomic_op_inuser(int op, u32 oparg, int *oval, u32 __user *uaddr);
> +extern int futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
> +			      u32 oldval, u32 newval);
> +
> +#else
> +#include <"asm-generic/futex.h">
> +#endif /* CONFIG_64BIT */
> +
> +#endif
> diff --git a/arch/um/kernel/skas/uaccess.c b/arch/um/kernel/skas/uaccess.c
> index 2dec915abe6f..83bfaa018b3b 100644
> --- a/arch/um/kernel/skas/uaccess.c
> +++ b/arch/um/kernel/skas/uaccess.c
> @@ -11,6 +11,7 @@
>   #include <asm/current.h>
>   #include <asm/page.h>
>   #include <kern_util.h>
> +#include <asm/futex.h>
>   #include <os.h>
>   
>   pte_t *virt_to_pte(struct mm_struct *mm, unsigned long addr)
> @@ -248,3 +249,116 @@ long __strnlen_user(const void __user *str, long len)
>   	return 0;
>   }
>   EXPORT_SYMBOL(__strnlen_user);
> +
> +/**
> + * arch_futex_atomic_op_inuser() - Atomic arithmetic operation with constant
> + *			  argument and comparison of the previous
> + *			  futex value with another constant.
> + *
> + * @encoded_op:	encoded operation to execute
> + * @uaddr:	pointer to user space address
> + *
> + * Return:
> + * 0 - On success
> + * -EFAULT - User access resulted in a page fault
> + * -EAGAIN - Atomic operation was unable to complete due to contention
> + * -ENOSYS - Operation not supported
> + */
> +int arch_futex_atomic_op_inuser(int op, u32 oparg, int *oval, u32 __user *uaddr)
> +{
> +	int oldval, ret;
> +	struct page *page;
> +	pte_t *pte;
> +
> +	ret = -EFAULT;
> +	preempt_disable();
> +	pte = maybe_map((unsigned long) uaddr, 1);
> +	if (pte == NULL)
> +		goto out_inuser;
> +
> +	page = pte_page(*pte);
> +	pagefault_disable();
> +	uaddr = page_address(page) + (((unsigned long) uaddr) & ~PAGE_MASK);
> +
> +	oldval = *uaddr;
> +
> +	ret = 0;
> +
> +	switch (op) {
> +	case FUTEX_OP_SET:
> +		*uaddr = oparg;
> +		break;
> +	case FUTEX_OP_ADD:
> +		*uaddr += oparg;
> +		break;
> +	case FUTEX_OP_OR:
> +		*uaddr |= oparg;
> +		break;
> +	case FUTEX_OP_ANDN:
> +		*uaddr &= ~oparg;
> +		break;
> +	case FUTEX_OP_XOR:
> +		*uaddr ^= oparg;
> +		break;
> +	default:
> +		ret = -ENOSYS;
> +	}
> +	pagefault_enable();
> +
> +out_inuser:
> +	preempt_enable();
> +
> +	if (ret == 0)
> +		*oval = oldval;
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(arch_futex_atomic_op_inuser);
> +
> +/**
> + * futex_atomic_cmpxchg_inatomic() - Compare and exchange the content of the
> + *				uaddr with newval if the current value is
> + *				oldval.
> + * @uval:	pointer to store content of @uaddr
> + * @uaddr:	pointer to user space address
> + * @oldval:	old value
> + * @newval:	new value to store to @uaddr
> + *
> + * Return:
> + * 0 - On success
> + * -EFAULT - User access resulted in a page fault
> + * -EAGAIN - Atomic operation was unable to complete due to contention
> + * -ENOSYS - Function not implemented (only if !HAVE_FUTEX_CMPXCHG)
> + */
> +
> +int futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr,
> +			      u32 oldval, u32 newval)
> +{
> +	u32 val;
> +	struct page *page;
> +	pte_t *pte;
> +	int ret = -EFAULT;
> +
> +	preempt_disable();
> +	pte = maybe_map((unsigned long) uaddr, 1);
> +	if (pte == NULL)
> +		goto out_inatomic;
> +
> +	page = pte_page(*pte);
> +	pagefault_disable();
> +	uaddr = page_address(page) + (((unsigned long) uaddr) & ~PAGE_MASK);
> +
> +	val = *uaddr;
> +
> +	if (val == oldval)
> +		*uaddr = newval;
> +
> +	*uval = val;
> +	pagefault_enable();
> +	ret = 0;
> +
> +out_inatomic:
> +	preempt_enable();
> +	return ret;
> +}
> +EXPORT_SYMBOL(futex_atomic_cmpxchg_inatomic);