Re: [PATCH 1/2] dma-buf: improve sg_table debugging hack v2

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

On 12/4/25 16:51, Matthew Auld wrote:
> On 04/12/2025 14:59, Christian König wrote:
>> This debugging hack is important to enforce the rule that importers
>> should *never* touch the underlying struct page of the exporter.
>>
>> Instead of just mangling the page link create a copy of the sg_table
>> but only copy over the DMA addresses and not the pages.
>>
>> This will cause a NULL pointer de-reference if the importer tries to
>> touch the struct page. Still quite a hack but this at least allows the
>> exporter to properly keeps it's sg_table intact while allowing the
>> DMA-buf maintainer to find and fix misbehaving importers and finally
>> switch over to using a different data structure in the future.
>>
>> v2: improve the hack further by using a wrapper structure and explaining
>> the background a bit more in the commit message.
>>
>> Signed-off-by: Christian König <christian.koenig@amd.com>
>> Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com> (v1)
>> ---
>>   drivers/dma-buf/dma-buf.c | 72 +++++++++++++++++++++++++++++++--------
>>   1 file changed, 58 insertions(+), 14 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>> index 2305bb2cc1f1..8c4afd360b72 100644
>> --- a/drivers/dma-buf/dma-buf.c
>> +++ b/drivers/dma-buf/dma-buf.c
>> @@ -35,6 +35,12 @@
>>     #include "dma-buf-sysfs-stats.h"
>>   +/* Wrapper to hide the sg_table page link from the importer */
>> +struct dma_buf_sg_table_wrapper {
>> +    struct sg_table *original;
>> +    struct sg_table wrapper;
>> +};
>> +
>>   static inline int is_dma_buf_file(struct file *);
>>     static DEFINE_MUTEX(dmabuf_list_mutex);
>> @@ -828,21 +834,57 @@ void dma_buf_put(struct dma_buf *dmabuf)
>>   }
>>   EXPORT_SYMBOL_NS_GPL(dma_buf_put, "DMA_BUF");
>>   -static void mangle_sg_table(struct sg_table *sg_table)
>> +static int dma_buf_mangle_sg_table(struct sg_table **sg_table)
>>   {
>> -#ifdef CONFIG_DMABUF_DEBUG
>> -    int i;
>> -    struct scatterlist *sg;
>> -
>> -    /* To catch abuse of the underlying struct page by importers mix
>> -     * up the bits, but take care to preserve the low SG_ bits to
>> -     * not corrupt the sgt. The mixing is undone on unmap
>> -     * before passing the sgt back to the exporter.
>> +    struct scatterlist *to_sg, *from_sg;
>> +    struct sg_table *from = *sg_table;
>> +    struct dma_buf_sg_table_wrapper *to;
>> +    int i, ret;
>> +
>> +    if (!IS_ENABLED(CONFIG_DMABUF_DEBUG))
>> +        return 0;
>> +
>> +    /*
>> +     * To catch abuse of the underlying struct page by importers copy the
>> +     * sg_table without copying the page_link and give only the copy back to
>> +     * the importer.
>>        */
>> -    for_each_sgtable_sg(sg_table, sg, i)
>> -        sg->page_link ^= ~0xffUL;
>> -#endif
>> +    to = kzalloc(sizeof(*to), GFP_KERNEL);
>> +    if (!to)
>> +        return -ENOMEM;
>> +
>> +    ret = sg_alloc_table(&to->wrapper, from->nents, GFP_KERNEL);
>> +    if (ret)
>> +        goto free_to;
>> +
>> +    to_sg = to->wrapper.sgl;
>> +    for_each_sgtable_dma_sg(from, from_sg, i) {
>> +        sg_set_page(to_sg, NULL, 0, 0);
> 
> Are we still allowed to pass NULL page here? There looks to be the recently added:
> 
> VM_WARN_ON_ONCE(!page_range_contiguous(page, ALIGN(len + offset, PAGE_SIZE) / PAGE_SIZE));
> 
> And if page_range_contiguous() does not just return true, it potentially wants to dereference the page, like with page_to_pfn()?

Good point.

It doesn't crash at the moment because page_to_pfn() also works with NULL as page, but it is clearly not the nicest thing to do.

I will switch over to using sg_assign_page() instead.

> 
> 
>> +                sg_dma_address(to_sg) = sg_dma_address(from_sg);
>> +                sg_dma_len(to_sg) = sg_dma_len(from_sg);
> 
> Nit: formatting looks off here.

Oh, indeed.

Thanks,
Christian.

> 
>> +        to_sg = sg_next(to_sg);
>> +    }
>>   +    to->original = from;
>> +    *sg_table = &to->wrapper;
>> +    return 0;
>> +
>> +free_to:
>> +    kfree(to);
>> +    return ret;
>> +}
>> +
>> +static void dma_buf_demangle_sg_table(struct sg_table **sg_table)
>> +{
>> +    struct dma_buf_sg_table_wrapper *copy;
>> +
>> +    if (!IS_ENABLED(CONFIG_DMABUF_DEBUG))
>> +        return;
>> +
>> +    copy = container_of(*sg_table, typeof(*copy), wrapper);
>> +    *sg_table = copy->original;
>> +    sg_free_table(&copy->wrapper);
>> +    kfree(copy);
>>   }
>>     static inline bool
>> @@ -1139,7 +1181,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
>>           if (ret < 0)
>>               goto error_unmap;
>>       }
>> -    mangle_sg_table(sg_table);
>> +    ret = dma_buf_mangle_sg_table(&sg_table);
>> +    if (ret)
>> +        goto error_unmap;
>>         if (IS_ENABLED(CONFIG_DMA_API_DEBUG)) {
>>           struct scatterlist *sg;
>> @@ -1220,7 +1264,7 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
>>         dma_resv_assert_held(attach->dmabuf->resv);
>>   -    mangle_sg_table(sg_table);
>> +    dma_buf_demangle_sg_table(&sg_table);
>>       attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
>>         if (dma_buf_pin_on_map(attach))
>