Re: [PATCH 5/6] xen/dt-overlay: support phandle-based targeting in overlay_get_nodes_info

[Luca Fancellu <Luca.Fancellu@xxxxxxx>]

>>> 
>>> diff --git a/xen/common/device-tree/dt-overlay.c 
>>> b/xen/common/device-tree/dt-overlay.c
>>> index d3d4669718ac..a0dee7edb7e5 100644
>>> --- a/xen/common/device-tree/dt-overlay.c
>>> +++ b/xen/common/device-tree/dt-overlay.c
>>> @@ -286,6 +286,63 @@ static unsigned int overlay_node_count(const void 
>>> *overlay_fdt)
>>>    return num_overlay_nodes;
>>> }
>>> 
>>> +/*
>>> + * Resolve the target path for an overlay fragment.
>>> + *
>>> + * This is called before fdt_overlay_apply(), so phandle-based targets
>>> + * (target = <&label>) are still unresolved (compiled as 0xffffffff by 
>>> DTC).
>>> + * Handle the two cases that actually occur:
>>> + *  - target-path property: the path string is used directly,
>>> + *  - target = <&label>: the label is looked up in the overlay's __fixups__
>>> + *    node, then resolved to a path via the base DTB's __symbols__ node.
>>> + *
>>> + * Returns a pointer into the FDT on success, NULL on failure.
>>> + */
>>> +static const char *overlay_get_target_path(const void *fdt, const void 
>>> *fdto,
>>> +                                           int fragment)
>>> +{
>>> +    const char *path, *fragment_name;
>>> +    int fixups_off, symbols_off, property;
>>> +    int fragment_name_len;
>>> +
>>> +    /* Try target-path first (string-based targeting) */
>>> +    path = fdt_getprop(fdto, fragment, "target-path", NULL);
>>> +    if ( path )
>>> +        return path;
>>> +
>>> +    /* Phandle-based target: resolve via __fixups__ and __symbols__ */
>>> +    fixups_off = fdt_path_offset(fdto, "/__fixups__");
>>> +    if ( fixups_off < 0 )
>>> +        return NULL;
>>> +
>>> +    symbols_off = fdt_path_offset(fdt, "/__symbols__");
>>> +    if ( symbols_off < 0 )
>>> +        return NULL;
>>> +
>>> +    fragment_name = fdt_get_name(fdto, fragment, &fragment_name_len);
>>> +    if ( !fragment_name )
>>> +        return NULL;
>>> +
>>> +    fdt_for_each_property_offset(property, fdto, fixups_off)
>>> +    {
>>> +        const char *val, *label, *p;
>>> +        int val_len;
>>> +
>>> +        val = fdt_getprop_by_offset(fdto, property, &label, &val_len);
>>> +        if ( !val )
>>> +            continue;
>>> +
>>> +        /* Match entries of the form "/<fragment_name>:target:0" */
>>> +        for ( p = val; p < (val + val_len); p += (strlen(p) + 1) )
>> 
>> what guarantees us that p will be null terminated, if a malformed overlay
>> is passed this strlen can read past the string, we can use strnlen having as
>> upper bound a counter=val_len? decreasing counter each iteration.
>> 
>> Or do you think it can never happen?
> In theory it can happen, in practice this is something not usually taken into
> account. But we can surely stay on the defensive side and do a single check
> right after fdt_getprop_by_offset to catch not-NUL terminated stringlist:
> if ( !val || !val_len || val[val_len - 1] != '\0' )

Ok, do we also want to print a warning in case of malformed properties instead
of ignoring and continue?

> 
> In case of no other remarks, I'd do that on commit.
> 
> ~Michal
> 

Cheers,
Luca