Re: [Xen-devel] [PATCH RFC 05/20] libxc/xc_sr: factor out filter_pages()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/03/17 10:06, Joshua Otto wrote:
> diff --git a/tools/libxc/xc_sr_restore.c b/tools/libxc/xc_sr_restore.c
> index 481a904..8574ee8 100644
> --- a/tools/libxc/xc_sr_restore.c
> +++ b/tools/libxc/xc_sr_restore.c
> @@ -194,6 +194,68 @@ int populate_pfns(struct xc_sr_context *ctx, unsigned 
> count,
>      return rc;
>  }
>  
> +static void set_page_types(struct xc_sr_context *ctx, unsigned count,
> +                           xen_pfn_t *pfns, uint32_t *types)
> +{
> +    unsigned i;

Please use unsigned int rather than just "unsigned" throughout.

> +
> +    for ( i = 0; i < count; ++i )
> +        ctx->restore.ops.set_page_type(ctx, pfns[i], types[i]);
> +}
> +
> +/*
> + * Given count pfns and their types, allocate and fill in buffer bpfns with 
> only
> + * those pfns that are 'backed' by real page data that needs to be migrated.
> + * The caller must later free() *bpfns.
> + *
> + * Returns 0 on success and non-0 on failure.  *bpfns can be free()ed even 
> after
> + * failure.
> + */
> +static int filter_pages(struct xc_sr_context *ctx,
> +                        unsigned count,
> +                        xen_pfn_t *pfns,
> +                        uint32_t *types,
> +                        /* OUT */ unsigned *nr_pages,
> +                        /* OUT */ xen_pfn_t **bpfns)
> +{
> +    xc_interface *xch = ctx->xch;

Pointers to arrays are very easy to get wrong in C.  This code will be
less error if you use

xen_pfn_t *_pfns;  (variable name subject to improvement)

> +    unsigned i;
> +
> +    *nr_pages = 0;
> +    *bpfns = malloc(count * sizeof(*bpfns));

_pfns = *bfns = malloc(...).

Then use _pfns in place of (*bpfns) everywhere else.

However,  your sizeof has the wrong indirection.  It works on x86
because xen_pfn_t is the same size as a pointer, but it will blow up on
32bit ARM, where a pointer is 4 bytes but xen_pfn_t is 8 bytes.

> +    if ( !(*bpfns) )
> +    {
> +        ERROR("Failed to allocate %zu bytes to process page data",
> +              count * (sizeof(*bpfns)));
> +        return -1;
> +    }
> +
> +    for ( i = 0; i < count; ++i )
> +    {
> +        switch ( types[i] )
> +        {
> +        case XEN_DOMCTL_PFINFO_NOTAB:
> +
> +        case XEN_DOMCTL_PFINFO_L1TAB:
> +        case XEN_DOMCTL_PFINFO_L1TAB | XEN_DOMCTL_PFINFO_LPINTAB:
> +
> +        case XEN_DOMCTL_PFINFO_L2TAB:
> +        case XEN_DOMCTL_PFINFO_L2TAB | XEN_DOMCTL_PFINFO_LPINTAB:
> +
> +        case XEN_DOMCTL_PFINFO_L3TAB:
> +        case XEN_DOMCTL_PFINFO_L3TAB | XEN_DOMCTL_PFINFO_LPINTAB:
> +
> +        case XEN_DOMCTL_PFINFO_L4TAB:
> +        case XEN_DOMCTL_PFINFO_L4TAB | XEN_DOMCTL_PFINFO_LPINTAB:
> +
> +            (*bpfns)[(*nr_pages)++] = pfns[i];
> +            break;
> +        }
> +    }
> +
> +    return 0;
> +}
> +
>  /*
>   * Given a list of pfns, their types, and a block of page data from the
>   * stream, populate and record their types, map the relevant subset and copy
> @@ -203,7 +265,7 @@ static int process_page_data(struct xc_sr_context *ctx, 
> unsigned count,
>                               xen_pfn_t *pfns, uint32_t *types, void 
> *page_data)
>  {
>      xc_interface *xch = ctx->xch;
> -    xen_pfn_t *mfns = malloc(count * sizeof(*mfns));
> +    xen_pfn_t *mfns = NULL;

This shows a naming bug, which is my fault.  This should be named gfns,
not mfns.  (It inherits its name from the legacy migration code, but
that was also wrong.)

Please correct it, either in this patch or another; the memory
management terms are hard enough, even when all the code is correct.

~Andrew

>      int *map_errs = malloc(count * sizeof(*map_errs));
>      int rc;
>      void *mapping = NULL, *guest_page = NULL;
> @@ -211,11 +273,11 @@ static int process_page_data(struct xc_sr_context *ctx, 
> unsigned count,
>          j,         /* j indexes the subset of pfns we decide to map. */
>          nr_pages = 0;
>  
> -    if ( !mfns || !map_errs )
> +    if ( !map_errs )
>      {
>          rc = -1;
>          ERROR("Failed to allocate %zu bytes to process page data",
> -              count * (sizeof(*mfns) + sizeof(*map_errs)));
> +              count * sizeof(*map_errs));
>          goto err;
>      }
>  
>


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