Re: [PATCH v4 1/1] of: unittest: overlay: ensure proper alignment of copied FDT

[Frank Rowand <frowand.list@gmail.com>]

On 4/8/21 4:28 PM, Rob Herring wrote:
> On Thu, Apr 8, 2021 at 3:45 PM <frowand.list@gmail.com> wrote:
>>
>> From: Frank Rowand <frank.rowand@sony.com>
>>
>> The Devicetree standard specifies an 8 byte alignment of the FDT.
>> Code in libfdt expects this alignment for an FDT image in memory.
>> kmemdup() returns 4 byte alignment on openrisc.  Replace kmemdup()
>> with kmalloc(), align pointer, memcpy() to get proper alignment.
>>
>> The 4 byte alignment exposed a related bug which triggered a crash
>> on openrisc with:
>> commit 79edff12060f ("scripts/dtc: Update to upstream version v1.6.0-51-g183df9e9c2b9")
>> as reported in:
>> https://lore.kernel.org/lkml/20210327224116.69309-1-linux@roeck-us.net/
>>
>> Reported-by: Guenter Roeck <linux@roeck-us.net>
>> Signed-off-by: Frank Rowand <frank.rowand@sony.com>
>>
>> ---
>>
>> changes since version 1:
>>   - use pointer from kmalloc() for kfree() instead of using pointer that
>>     has been modified for FDT alignment
>>
>> changes since version 2:
>>   - version 1 was a work in progress version, I failed to commit the following
>>     final changes
>>   - reorder first two arguments of of_overlay_apply()
>>
>> changes since version 3:
>>   - size of memory allocation and size of copy after pointer alignment
>>     differ, use separate variables with correct values for each case
>>   - edit comment to more clearly describe that ovcs->fdt is the allocated
>>     memory region, which may be different than where the aligned pointer points
>>   - remove unused parameter from of_overlay_apply()
>>
>>  drivers/of/of_private.h |  2 ++
>>  drivers/of/overlay.c    | 27 +++++++++++++++++----------
>>  drivers/of/unittest.c   | 13 ++++++++++---
>>  3 files changed, 29 insertions(+), 13 deletions(-)
>>
>> diff --git a/drivers/of/of_private.h b/drivers/of/of_private.h
>> index d9e6a324de0a..d717efbd637d 100644
>> --- a/drivers/of/of_private.h
>> +++ b/drivers/of/of_private.h
>> @@ -8,6 +8,8 @@
>>   * Copyright (C) 1996-2005 Paul Mackerras.
>>   */
>>
>> +#define FDT_ALIGN_SIZE 8
>> +
>>  /**
>>   * struct alias_prop - Alias property in 'aliases' node
>>   * @link:      List node to link the structure in aliases_lookup list
>> diff --git a/drivers/of/overlay.c b/drivers/of/overlay.c
>> index 50bbe0edf538..ecf967c57900 100644
>> --- a/drivers/of/overlay.c
>> +++ b/drivers/of/overlay.c
>> @@ -57,7 +57,7 @@ struct fragment {
>>   * struct overlay_changeset
>>   * @id:                        changeset identifier
>>   * @ovcs_list:         list on which we are located
>> - * @fdt:               FDT that was unflattened to create @overlay_tree
>> + * @fdt:               base of memory allocated to hold aligned FDT that was unflattened to create @overlay_tree
>>   * @overlay_tree:      expanded device tree that contains the fragment nodes
>>   * @count:             count of fragment structures
>>   * @fragments:         fragment nodes in the overlay expanded device tree
>> @@ -719,8 +719,8 @@ static struct device_node *find_target(struct device_node *info_node)
>>  /**
>>   * init_overlay_changeset() - initialize overlay changeset from overlay tree
>>   * @ovcs:      Overlay changeset to build
>> - * @fdt:       the FDT that was unflattened to create @tree
>> - * @tree:      Contains all the overlay fragments and overlay fixup nodes
>> + * @fdt:       base of memory allocated to hold aligned FDT that was unflattened to create @tree
>> + * @tree:      Contains the overlay fragments and overlay fixup nodes
>>   *
>>   * Initialize @ovcs.  Populate @ovcs->fragments with node information from
>>   * the top level of @tree.  The relevant top level nodes are the fragment
>> @@ -873,7 +873,7 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
>>   * internal documentation
>>   *
>>   * of_overlay_apply() - Create and apply an overlay changeset
>> - * @fdt:       the FDT that was unflattened to create @tree
>> + * @fdt:       base of memory allocated to hold the aligned FDT
>>   * @tree:      Expanded overlay device tree
>>   * @ovcs_id:   Pointer to overlay changeset id
>>   *
>> @@ -913,7 +913,7 @@ static void free_overlay_changeset(struct overlay_changeset *ovcs)
>>   */
>>
>>  static int of_overlay_apply(const void *fdt, struct device_node *tree,
>> -               int *ovcs_id)
>> +                           int *ovcs_id)
>>  {
>>         struct overlay_changeset *ovcs;
>>         int ret = 0, ret_revert, ret_tmp;
>> @@ -953,7 +953,9 @@ static int of_overlay_apply(const void *fdt, struct device_node *tree,
>>         /*
>>          * after overlay_notify(), ovcs->overlay_tree related pointers may have
>>          * leaked to drivers, so can not kfree() tree, aka ovcs->overlay_tree;
>> -        * and can not free fdt, aka ovcs->fdt
>> +        * and can not free memory containing aligned fdt.  The aligned fdt
>> +        * is contained within the memory at ovcs->fdt, possibly at an offset
>> +        * from ovcs->fdt.
>>          */
>>         ret = overlay_notify(ovcs, OF_OVERLAY_PRE_APPLY);
>>         if (ret) {
>> @@ -1014,9 +1016,10 @@ static int of_overlay_apply(const void *fdt, struct device_node *tree,
>>  int of_overlay_fdt_apply(const void *overlay_fdt, u32 overlay_fdt_size,
>>                          int *ovcs_id)
>>  {
>> -       const void *new_fdt;
>> +       void *new_fdt;
>> +       void *new_fdt_align;
>>         int ret;
>> -       u32 size;
>> +       u32 size, size_alloc;
>>         struct device_node *overlay_root;
>>
>>         *ovcs_id = 0;
>> @@ -1036,11 +1039,15 @@ int of_overlay_fdt_apply(const void *overlay_fdt, u32 overlay_fdt_size,
>>          * Must create permanent copy of FDT because of_fdt_unflatten_tree()
>>          * will create pointers to the passed in FDT in the unflattened tree.
>>          */
>> -       new_fdt = kmemdup(overlay_fdt, size, GFP_KERNEL);
>> +       size_alloc = size + FDT_ALIGN_SIZE;
>> +       new_fdt = kmalloc(size_alloc, GFP_KERNEL);
> 
> As size_alloc is only used once, you can just do:
> 
> new_fdt = kmalloc(size + FDT_ALIGN_SIZE, GFP_KERNEL);
> 
> Same for the unittest. I can fix up.
> 
> Applying now so this gets into linux-next this week.

Thanks, that change looks like an improvement to me.

> 
> Rob
>