Re: [Xen-devel] [PATCH v3 21/23] xsplice: Add support for shadow variables

[Martin Pohlack <mpohlack@xxxxxxxxxx>]

On 12.02.2016 19:05, Konrad Rzeszutek Wilk wrote:
> From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> 
> Shadow variables are a piece of infrastructure to be used by xsplice
> modules. They are used to attach a new piece of data to an existing
> structure in memory.
> 
> Signed-off-by: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
> ---
>  xen/common/Makefile             |   1 +
>  xen/common/xsplice_shadow.c     | 105 
> ++++++++++++++++++++++++++++++++++++++++
>  xen/include/xen/xsplice_patch.h |  39 +++++++++++++++
>  3 files changed, 145 insertions(+)
>  create mode 100644 xen/common/xsplice_shadow.c
>  create mode 100644 xen/include/xen/xsplice_patch.h
> 
> diff --git a/xen/common/Makefile b/xen/common/Makefile
> index a8ceaff..f4d54ad 100644
> --- a/xen/common/Makefile
> +++ b/xen/common/Makefile
> @@ -75,3 +75,4 @@ subdir-$(CONFIG_HAS_DEVICE_TREE) += libfdt
>  
>  obj-$(CONFIG_XSPLICE) += xsplice.o
>  obj-$(CONFIG_XSPLICE) += xsplice_elf.o
> +obj-$(CONFIG_XSPLICE) += xsplice_shadow.o
> diff --git a/xen/common/xsplice_shadow.c b/xen/common/xsplice_shadow.c
> new file mode 100644
> index 0000000..619cdee
> --- /dev/null
> +++ b/xen/common/xsplice_shadow.c
> @@ -0,0 +1,105 @@
> +#include <xen/init.h>
> +#include <xen/kernel.h>
> +#include <xen/lib.h>
> +#include <xen/list.h>
> +#include <xen/spinlock.h>
> +#include <xen/xsplice_patch.h>
> +
> +#define SHADOW_SLOTS 256

Using something very round here will give you lot's of hash collisions
at the price of a very fast hash computation as compilers, linkers, and
memory allocators tend to align starting addresses.  I would suggest to
use a small prime here, e.g., 257 or 251, to have a first approximation
of a simple hash function.

Or use existing hash infrastructure (see below).

> +struct hlist_head shadow_tbl[SHADOW_SLOTS];
> +static DEFINE_SPINLOCK(shadow_lock);
> +
> +struct shadow_var {
> +    struct hlist_node list;         /* Linked to 'shadow_tbl' */
> +    void *data;
> +    const void *obj;
> +    char var[16];
> +};
> +
> +void *xsplice_shadow_alloc(const void *obj, const char *var, size_t size)
> +{
> +    struct shadow_var *shadow;
> +    unsigned int slot;
> +
> +    shadow = xmalloc(struct shadow_var);
> +    if ( !shadow )
> +        return NULL;
> +
> +    shadow->obj = obj;
> +    strlcpy(shadow->var, var, sizeof shadow->var);
> +    shadow->data = xmalloc_bytes(size);
> +    if ( !shadow->data )
> +    {
> +        xfree(shadow);
> +        return NULL;
> +    }
> +
> +    slot = (unsigned long)obj % SHADOW_SLOTS;

hash.h has an earlier import from Linux and provides hash_long().  That
looks like it would not suffer from direct hash collisions.

(also for all other occurrences of "obj % SHADOW_SLOTS" below)

> +    spin_lock(&shadow_lock);
> +    hlist_add_head(&shadow->list, &shadow_tbl[slot]);
> +    spin_unlock(&shadow_lock);
> +
> +    return shadow->data;
> +}
> +
> +void xsplice_shadow_free(const void *obj, const char *var)
> +{
> +    struct shadow_var *entry, *shadow = NULL;
> +    unsigned int slot;
> +    struct hlist_node *next;
> +
> +    slot = (unsigned long)obj % SHADOW_SLOTS;
> +
> +    spin_lock(&shadow_lock);
> +    hlist_for_each_entry(entry, next, &shadow_tbl[slot], list)
> +    {
> +        if ( entry->obj == obj &&
> +             !strcmp(entry->var, var) )
> +        {
> +            shadow = entry;
> +            break;
> +        }
> +    }
> +    if (shadow)
> +    {
> +        hlist_del(&shadow->list);
> +        xfree(shadow->data);
> +        xfree(shadow);
> +    }
> +    spin_unlock(&shadow_lock);
> +}
> +
> +void *xsplice_shadow_get(const void *obj, const char *var)
> +{
> +    struct shadow_var *entry;
> +    unsigned int slot;
> +    struct hlist_node *next;
> +    void *ret = NULL;
> +
> +    slot = (unsigned long)obj % SHADOW_SLOTS;
> +
> +    spin_lock(&shadow_lock);
> +    hlist_for_each_entry(entry, next, &shadow_tbl[slot], list)
> +    {
> +        if ( entry->obj == obj &&
> +             !strcmp(entry->var, var) )
> +        {
> +            ret = entry->data;
> +            break;
> +        }
> +    }
> +
> +    spin_unlock(&shadow_lock);
> +    return ret;
> +}
> +
> +static int __init xsplice_shadow_init(void)
> +{
> +    int i;
> +
> +    for ( i = 0; i < SHADOW_SLOTS; i++ )
> +        INIT_HLIST_HEAD(&shadow_tbl[i]);
> +
> +    return 0;
> +}
> +__initcall(xsplice_shadow_init);
> diff --git a/xen/include/xen/xsplice_patch.h b/xen/include/xen/xsplice_patch.h
> new file mode 100644
> index 0000000..e3f344b
> --- /dev/null
> +++ b/xen/include/xen/xsplice_patch.h
> @@ -0,0 +1,39 @@
> +#ifndef __XEN_XSPLICE_PATCH_H__
> +#define __XEN_XSPLICE_PATCH_H__
> +
> +/*
> + * The following definitions are to be used in patches. They are taken
> + * from kpatch.
> + */
> +
> +/*
> + * xsplice shadow variables
> + *
> + * These functions can be used to add new "shadow" fields to existing data
> + * structures.  For example, to allocate a "newpid" variable associated with 
> an
> + * instance of task_struct, and assign it a value of 1000:
> + *
> + * struct task_struct *tsk = current;
> + * int *newpid;
> + * newpid = xsplice_shadow_alloc(tsk, "newpid", sizeof(int));
> + * if (newpid)
> + *   *newpid = 1000;
> + *
> + * To retrieve a pointer to the variable:
> + *
> + * struct task_struct *tsk = current;
> + * int *newpid;
> + * newpid = xsplice_shadow_get(tsk, "newpid");
> + * if (newpid)
> + *   printk("task newpid = %d\n", *newpid); // prints "task newpid = 1000"
> + *
> + * To free it:
> + *
> + * xsplice_shadow_free(tsk, "newpid");
> + */
> +
> +void *xsplice_shadow_alloc(const void *obj, const char *var, size_t size);
> +void xsplice_shadow_free(const void *obj, const char *var);
> +void *xsplice_shadow_get(const void *obj, const char *var);
> +
> +#endif /* __XEN_XSPLICE_PATCH_H__ */
> 

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